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Point-of-care assessment of antiplatelet agents in the perioperative period: a review.

An increasing number of patients presenting for surgery are taking regular antiplatelet agents, ranging from simple analgesics with antiplatelet side-effects to potent anti-thrombotic drugs with specific antiplatelet activity. In the past, most patients were advised to cease their antiplatelet medication seven to ten days preoperatively, in order to reduce the risk of increased bleeding. However, this is no longer the case. Many patients are now advised to continue their antiplatelet drugs throughout the perioperative period, in order to avoid an unacceptably high-risk of perioperative thrombosis (1). This is particularly so for patients with drug-eluting coronary artery stents (2,3).

The perioperative management of patients receiving antiplatelet drugs will vary depending on the drug taken, the planned procedure and the planned anaesthetic technique. For many patients, no additional interventions or changes in management are necessary (4,5). However, in others, there may be a need for allogeneic platelets, modification of the anaesthetic technique (e.g. avoidance of neuroaxial techniques) or even postponement of surgery (2-6). One investigation that would assist in the management of these patients is an accurate contemporaneous assessment of the effect of the antiplatelet agent on their platelet function. This is because patients vary in their response to antiplatelet drugs (7,8). An accurate assessment would provide an opportunity to better balance the risks and benefits of various management strategies. It would also provide an opportunity to monitor recovery of platelet function over time or its response to specific therapy.

Unfortunately, the laboratory assessment of platelet function is labour intensive and time consuming, making it impractical for most perioperative patients (9-11). Laboratory turbidometric (light transmission) platelet aggregometry is the most widely used technique, although more advanced techniques are available in specialised centres (11,12). What is needed is a simple test providing rapid and accurate information at the point-of-care. There are now a large number of commercial devices available for this purpose. However, in contrast to the measurement of platelet count, the assessment of platelet function is complex and the choice of an appropriate device may not be obvious.

There have been a large number of studies and review articles on various aspects of point-of-care assessment of platelet function. However, there have been few that have specifically reviewed methods of point-of-care assessment of antiplatelet drug effects. The aim of this paper is to review the strengths and limitations of the 'point-of-care' methods currently available for the detection of antiplatelet drug effects in the perioperative period.

METHODS

In order to identify the platelet function tests that are currently available at the 'point-of-care', a Medline search was performed using the key words "platelet function tests" and "point-of-care". A series of further Medline searches was then performed using the point-of-care platelet function tests identified as a key word with both "platelet aggregometry" and "anesthesia" or "anaesthesia". The searches were limited to review articles in English published between January 1996 and September 2008.

The abstracts of these articles were then assessed to identify their primary aim and methodology. Only those articles in which the primary aim was to assess or compare the ability of a point-of-care platelet function test to detect an antiplatelet drug were reviewed. If the article was a review article, the original references to the tests described were also reviewed.

PHYSIOLOGY OF PLATELET FUNCTION

The physiology of platelets has been reviewed extensively elsewhere (13-15) and only a brief summary is provided here. Platelets circulate as smooth anucleate biconcave discs. The normal platelet count is 150 to 400x[10.sup.9]/l and the normal lifespan is seven to ten days. Platelets adhere to exposed subendothelial collagen fibrils via surface glycoprotein ([G.sub.p]) complexes (16). This is facilitated by von Willebrand Factor (vWF) bridges, especially in areas of high shear. Sufficient binding of vWF to [G.sub.p][I.sub.b] and collagen to [G.sub.p]VI activates platelets by stimulating phospholipase C (PLC), resulting in release of a granule and dense body contents, shape change and the expression of [G.sub.p][II.sub.b]/[III.sub.a] receptors (i.e. the platelet release reaction, Figure 1). The a granules contain adenosine diphosphate (ADP), which when released, binds to both [P.sub.2][Y.sub.1] and the [P.sub.2][Y.sub.12] receptors on adjacent platelets. Stimulation of these receptors inhibits adenylcyclase via a G protein, thereby reducing intracellular cyclic-adenosinemono-phosphate (c-AMP) concentrations (17). Intracellular c-AMP inhibits the activity of platelet protein kinase C, which is required for the platelet 'release reaction'. In this way, ADP promotes the release reaction, resulting in local positive feedback, and amplification of the initial stimulus.

At the same time, phospholipase [A.sub.2] is stimulated through another membrane G protein, resulting in release of arachidonic acid (AA) from membrane phospholipids (18,19) (Figure 1). This pathway is stimulated not only by collagen, but also by ADP, platelet activating factor (PAF), thromboxane [A.sub.2] ([TXA.sub.2]) and thrombin, but in lower concentrations than those required for inhibition of adenylcyclase or stimulation of PLC. It is also stimulated by adrenaline and serotonin. The AA is a substrate for cycloxygenase (COX), which converts it to prostaglandin [G.sub.2] and [H.sub.2]. These form a substrate for thromboxane synthetase, which rapidly converts them to [TXA.sub.2] (20). Thromboxane [A.sub.2] is a powerful calcium ionophore, causing release of calcium from the dense tubular system. The increase in calcium is required for protein kinase C to promote the 'release' reaction. In addition to the intracellular effects, the [TXA.sub.2] diffuses out of the platelet and stimulates adjacent platelets through a specific thromboxane receptor. The contribution of ADP and [TXA.sub.2] is required to amplify the initial response to collagen. However, once thrombin is formed, the role of these agonists becomes secondary.

Thrombin, like collagen, PAF and [TXA.sub.2], if present in sufficient concentration, stimulates (again through a specific receptor involving a G protein) platelet PLC. This promotes the production of inositol triphosphate and diacylglycerol. Inositol triphosphate is another powerful calcium ionophore and diacylglycerol stimulates protein kinase C (Figure 1). The combination results in the platelet 'release' reaction independently of collagen, ADP or [TXA.sub.2].

Once [G.sub.p][II.sub.b]/[III.sub.a] receptors are expressed, fibrinogen is able to bind to receptors on adjacent platelets (Figure 1). This is the mechanism of platelet aggregation. At the same time, platelet shape change exposes phospholipid on the outside of the platelet membrane. This phospholipid is used in association with calcium ions and activated coagulation factors in the production of thromboplastin. This function of platelets is known as platelet procoagulant activity or 'platelet factor 3'.

[FIGURE 1 OMITTED]

The amount of platelet aggregation is limited by factors released by adjacent normal endothelium. Prostacyclin, released by intact endothelium, inhibits further platelet activation by stimulating adenylcyclase, thereby increasing platelet c-AMP levels. Similarly, nitric oxide stimulates platelet guanylcyclase, thereby increasing cyclic guanosine-3,5-monophosphate levels. Both c-AMP and cyclic guanosine-3,5-monophosphate inhibit protein kinase C and limit further platelet activation.

MECHANISM OF ACTION OF ANTIPLATELET AGENTS

Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDS) inhibit platelets by inactivating platelet cyclooxygenase-1 (COX-1), the constitutive form of the enzyme (20). Active COX-1 is required for conversion of AA residues to prostaglandin [G.sub.2] and [H.sub.2], which in platelets become the substrate for thromboxane synthetase and are converted rapidly to [TXA.sub.2] (21). The effect of aspirin on COX-1 is irreversible for the remaining life of the platelet, because without a nucleus, platelets do not have the capacity to regenerate new COX-1. Therefore, the offset of an aspirin effect can occur only by the generation of new platelets from megakaryocytes. About 10 to 15% of platelets are turned over each day, so it will take seven to ten days for all affected platelets to be replaced. However, about 10 or 15% of platelets are replaced each day, so up to 50% of platelets will be normal in three to four days.

Other NSAIDS can be classified as selective or non-selective, depending on whether they affect both COX-1 and COX-2 (the inducible form of the enzyme) or are selective mostly for COX-2 (22). Non-selective NSAIDS inactivate COX-1 in the same way as aspirin (23). However, their effect is reversible, and dissipates as the plasma level of the drug falls. Therefore, the effect of non-aspirin NSAIDs on platelets will be less than 24 hours. Selective NSAIDS (COX-2 inhibitors) have little effect on COX-1 and therefore have little or no effect on platelets.

Aspirin and other COX-1 inhibitors are weak antiplatelet drugs, because they affect only one of several pathways of platelet activation (23). Even if all the COX-1 is inactivated, platelets will still respond to higher concentrations of other agonists through separate pathways involving adenylcyclase or PLC (Figure 1).

The thienopyridine derivatives, ticlopidine and clopidogrel, inhibit ADP-induced platelet activation by binding covalently to the [P.sub.2][Y.sub.12] receptor (24,25). Both ticlopidine and clopidogrel are inactive prodrugs that are converted to active metabolites by cytochrome [P.sub.450]. They take several days to produce their maximum effect. The covalent binding to the [P.sub.2][Y.sub.12] receptor means that their effect persists for the life of the platelet. Stimulation of both the [P.sub.2][Y.sub.1] and [P.sub.2][Y.sub.12] receptors is required for the normal inhibition of adenylcyclase. By blocking the [P.sub.2][Y.sub.12] receptor, these agents prevent the inhibition of adenylcyclase by ADP, thereby reducing c-AMP levels and inhibiting protein kinase C. These agents are not only more potent than aspirin and NSAIDS, they are synergistic with them. However, they still cannot prevent the activation of platelets by higher concentrations of agonists acting through PLC.

Irrespective of the pathway of platelet activation, or the agonists involved, the final step in platelet aggregation involves the binding of fibrinogen to newly exposed [G.sub.p][II.sub.b]/[III.sub.a] receptors on adjacent platelets. In the absence of this binding, platelet aggregation cannot occur. The [G.sub.p][II.sub.b]/[III.sub.a] inhibitors block these receptors, thereby preventing platelet aggregation (26). [G.sub.p][II.sub.b]/[III.sub.a] inhibitors vary in their affinity for the [G.sub.p][II.sub.b]/[III.sub.a] receptor and their plasma half-life, but otherwise have a similar mechanism of action. Abciximab has high affinity and slow dissociation resulting in a biological half-life of 12 to 24 hours. Eptifibatide and tirofiban are much smaller molecules with lower affinity and rapid dissociation. They have a half-life of only two to four hours. The effect of [G.sub.p][II.sub.b]/[III.sub.a] inhibitors is dose-dependent, and is related to both the numbers of receptors expressed and the percentage occupied.

POINT-OF-CARE ASSESSMENTS OF PLATELET FUNCTION

Tests involving global assessments of 'haemostasis'

Standard Thrombelastography[R]

Thrombelastography[R] (TEG[R] 5000, Haemoscope, Niles, IL, USA) provides a real time assessment of the viscoelastic properties of a clot as it forms ex vivo (27). Thrombelastography can be performed using whole blood at the point-of-care, and its 'global' nature is appealing to many clinicians, although completion of the test may take 30 to 60 minutes. The aspect of the TEG that relates most closely to platelet function is the maximum amplitude (MA) (27). unfortunately, standard TEG has many limitations in relation to the assessment of platelet function.

The TEG MA is not specific for platelet function, because it is affected by both platelet function and fibrin formation (27). Therefore, if fibrin formation is impaired, it is not possible to assess platelet function using standard TEG. There are many potential causes of impaired fibrin formation in the perioperative period, including anticoagulant drugs, dilution of coagulation factor levels and a range of disease processes. Moreover, the adequacy or otherwise of fibrin formation may not be known. Another problem is that in the presence of impaired fibrin formation, the time to achieve MA may exceed 60 minutes, making the MA impossible to interpret. For example, in the presence of even minimal amounts of heparin, the TEG trace will be grossly abnormal despite normal platelet function (28). While a heparin effect can be reversed by the addition of heparinase in vitro (29), there are no mechanisms to reverse the effect of other causes of impaired fibrin formation during a TEG run.

While standard TEG may be sensitive to many causes of impaired platelet function, it is insensitive to most drug-induced causes platelet dysfunction. This is due to the presence of thrombin in standard TEG cups, which is produced through the intrinsic pathway of coagulation. Therefore, standard TEG is insensitive to the antiplatelet effects of COX-1 inhibitors (aspirin and other NSAIDS) and [P.sub.2][Y.sub.12] antagonists (ticlopidine and clopidogrel). However, it is sensitive to the effect of [G.sub.p][II.sub.b]/[III.sub.a] inhibitors, because their action is independent of thrombin.

The inability of aspirin to affect the TEG has been known for many years. This was demonstrated in volunteers by Trentalange and Walts (30), and in patients by Orlikowski et al (31). In both these studies, the TEG was unaffected by aspirin in vitro, despite an increase in bleeding times. It has also been shown that the addition of ketorolac (a non-selective NSAID) to blood samples in vitro has no effect on the TEG MA (28). This limitation of the TEG was emphasised in an editorial by Samama. He pointed out the danger of using a normal TEG to exclude an aspirin effect on platelets (32).

Thienopyridines have also been shown to have no effect on standard TEG. Tanaka et al found that the TEG MA was not affected by a high concentration of an ADP receptor antagonist in vitro, despite complete inhibition of platelet aggregation as determined by laboratory platelet aggregometry (33). In the same study, TEG MA was normal or increased in vitro in 18 clopidogrel-treated cardiac surgical patients, many of whom had excessive blood loss postoperatively (33). In a recent case report, a patient had a normal TEG despite receiving both aspirin and clopidogrel (34). The same patient had abnormal ADP and adrenaline-induced platelet aggregation. Craft et al have also shown that standard TEG is insensitive to the effects of clopidogrel (35).

There have been mixed results with the use of standard TEG to assess the effect of [G.sub.p][II.sub.b]/[III.sub.a] inhibitors. Bowbrick et al found that the MA showed a statistically significant reduction with tirofiban 0.4 mg/ml in vitro, but that the TEG was not as sensitive as (ADP-induced) laboratory platelet aggregometry (36). They felt that the TEG does not provide a comprehensive or sensitive reflection of tirofiban-induced impaired platelet function (36). In another in vitro model, Bailey et al found that clinical concentrations of abciximab, eptifibatide and tirofiban had minimal and inconsistent effects on the TEG (37). They concluded that the TEG was unable to demonstrate clinically significant differences in platelet function related to these agents. Katori et al found that standard TEG was relatively insensitive to eptifibatide in vitro (38).

Rotation thrombelastography

The ROTEM[R] (rotation thrombelastometry, Pentapharm, Munich, Germany) is based on the same principle as standard thrombelastography (39), so the same limitations in relation to the assessment of antiplatelet agents should apply.

Sonoclot[R]

The Sonoclot[R] coagulation analyser (Sienco[R], Morrison, CO, USA) is an automated dynamic viscometer that was developed in the 1970s (40). The mechanism involves the insertion of a plastic probe, vibrating at an ultrasonic frequency, into a sample of whole blood in a glass cuvette. The vibration of the probe is altered as the blood in the sample clots, producing a characteristic trace or 'signature'. Various aspects of the signature relate to clotting events, such as the onset of fibrin formation, the rate of fibrin formation, and platelet-fibrin interaction. The 'time to peak' viscosity is considered an index of platelet function.

Like the standard TEG, the Sonoclot is not specific for platelet function, being influenced to a large extent by fibrin formation. It is also insensitive to the effects of aspirin and other NSAIDS (41,42). Horlocker et al found that the Sonoclot was influenced also by patient age, gender and haematocrit (43). More recently, the new Sonoclot glass bead-activated test was found to have a high correlation with platelet laboratory aggregometry in patients receiving tirofiban, but only so long as heparinase was used (44). These considerations, along with the subjective nature of the interpretation of the clot 'signature', have limited the use of the Sonoclot for the assessment of platelet function (45,46). Recent reviews of point-of-care platelet function monitors have not included the Sonoclot (47-49).

Hemodyne[TM]

The Hemodyne[TM] (Hemodyne[TM], Richmond, VA, uSA) was developed in the early 1990s as a device to measure the force generated by platelets during clot retraction. In the current instrument, known as the Hemostasis Analysis System, thrombin and calcium are added to a small sample of whole blood. The clot elastic modulus is a measure of the degree of deformation of blood as it clots. The platelet contractile force is a measure of the force generated by clot during its retraction phase. Both are plotted as a function of time. The thrombin generation time is an index of the time to the commencement of clot formation, identified by the initial change in the clot elastic modulus.

The Hemodyne is another device that assesses coagulation as a whole (45,46). Like other global assessments, various aspects of the output relate to different aspects of the clotting process. The platelet contractile force relates most closely to platelet function, but is not specific because it requires adequate fibrin generation. It should also be insensitive to the effects of NSAIDS and ADP antagonists, as a result of having thrombin present in the sample. However, Grielich et al found that it is sensitive to the effects of [G.sub.p][II.sub.b]/[III.sub.a] inhibitors (47). Like the Sonoclot, the Hemodyne has not been included in most recent reviews of point-of-care platelet function monitors (48-50).

Clot Signature Analyser[R]

The Clot Signature Analyser[R] (CSA[R], Xylum, Scarsdale, Ny, USA), which was based on the Haemostatometer[R], provides a global assessment of haemostasis under simulated physiological conditions (45-47,51). Non-anticoagulated whole blood is forced through a tube under conditions of high shear. The time taken for a small hole in the wall of the tube to be occluded by a platelet plug is known as the 'platelet haemostasis time' (PHT). The time taken for fibrin formation and occlusion of the entire tube lumen is known as the clot time. In another tube, blood is exposed to collagen under similar conditions. The time taken for this tube to be occluded is known as the collagen-induced thrombus formation test (CITF).

Several studies have shown the CSA to be a sensitive screening test for a range of bleeding disorders, including von Willebrand's disease, coagulation factor deficiencies and platelet function defects (52,53). It has also been used to predict blood loss and transfusion requirements after cardiac surgery (51). However, the global nature of the test reduces its specificity for platelet function. For example, the PHT and CITF may be prolonged by heparin, protamine and warfarin (54). There is also little correlation between the PHT or clot time and the bleeding time (55).

The few studies on the sensitivity of the CSA for the detection of antiplatelet drug effects have not been encouraging. Igawa et al found that in 53 normal volunteers, the PHT increased following the ingestion of low doses of aspirin (56). However, the coefficient of variation for the PHT was very wide (49 to 65%) and the CITF showed little change. Simon et al measured PHT and laboratory platelet aggregometry in 36 patients taking either abciximab, eptifibatide or tirofiban for percutaneous coronary interventions (57). All three agents increased both the PHT and CITF, although there was poor correlation between both the PHT and CITF and laboratory platelet aggregometry, and most measurements 'timed out' at 30 minutes (the maximum prolongation).

In comparison with other methods of platelet function assessment, the CSA device is relatively large and requires a larger blood sample (up to 6 ml) (58,59). It also takes longer for a result to be available, especially for the clot time. The signal is also more complex to interpret (59).

HemoSTATuS[TM]

The Platelet Activated Clotting Test (HemoSTATUS[TM], Medtronic Inc, Parker, IL, USA) was developed as a whole blood test of platelet responsiveness (platelet procoagulant activity) in patients undergoing cardiac surgery (60). The test was performed on a modified heparin management system device, with a purpose designed six-channel cartridge. The first channel of the cartridge contained kaolin as activator. The remaining channels, in addition to the kaolin, contained increasing concentrations of PAF. The activated clotting time was measured simultaneously on all six channels. Platelet responsiveness was assessed as a shortening of the activated clotting time with increasing concentrations of PAF. Initial trials suggested that the Platelet Activated Clotting Test was predictive of increased blood loss and response to 1-deamino-D-arginine vasopressin following cardiac surgery. However, these findings were not supported in subsequent studies (61,62) and the device is no longer commercially available (47).

Tests involving global assessments of 'platelet function'

Platelet Function Analyser (PFA-100[R])

The PFA-100[R] (Dade-Behring, Dudingen, Switzerland) is an automated device for the assessment of platelet function using whole blood (63,64). Its simplicity and speed of assessment (<10 minutes) make it easy to use at the point-of-care. It measures the time taken for blood to occlude an aperture (147 [micro]M) in a biologically active membrane. The membrane is made active by a choice of either collagen (COL) and ADP, or COL and epinephrine (adrenaline) (EPI). The membranes are presented in pre-prepared disposable cartridges, to which a sample of whole blood (0.8 ml) is added. When the test is initiated, the blood is aspirated at high shear (4000/s) through the aperture in the membrane. The time taken for a platelet plug to occlude the aperture is recorded (closure time, CT). The PFA-100 was introduced as a screening test for a range of platelet function defects, including von Willebrand's disease, inherent platelet disorders and antiplatelet drug effects (45-47,58,59).

One of the main advantages of the PFA-100 is that it does not require fibrin formation and is therefore not affected by anticoagulants. However, it is affected by thrombocytopenia, anaemia and low vWF levels (50). Moreover, the normal ranges for both cartridges are very wide (COL-ADP 71 to 118 seconds; COL-EPI 94 to 193 seconds) (46). This makes many values difficult to interpret. For example, a COL-ADP CT of 106 seconds could be 'normal', or could represent a 50% prolongation of 'normal'. In the absence of a baseline for comparison, it is not possible to determine which is correct. The same applies to the COL-EPI cartridges. There is also a large 10 to 14% coefficient of variation in CT within individuals (46).

Early reports suggested that a prolonged PFA-100 COL-EPI CT, particularly in association with a normal COL-ADP CT, had a high sensitivity for the detection of aspirin (45-47,49,65,66). Blaicher et al also found the PFA-100 useful for discriminating between the effects of COX-1 and COX-2 inhibitors (67). However, the PFA-100 has proved far less sensitive in symptomatic patients receiving aspirin as part of their antithrombotic therapy. For example Coakley et al found that 52% of 92 patients taking aspirin 75 mg/day were classified as 'non-responders' when assessed using the PFA-100 (68). This is a much larger percentage than typically observed using more sensitive techniques (e.g. laboratory platelet aggregometry in response to AA). In 36 cardiac surgical patients who had been taking regular aspirin, Gibbs et al found that all had impaired platelet aggregation responses to AA, but only 33% had a prolonged COL-EPI CT (69). More recently, Agarwal et al found that five of 20 patients taking aspirin had a PFA-100 COL-EPI CT within the normal range (70). None of these patients had normal laboratory platelet aggregometry. Paniccia et al found that the sensitivity and specificity of the PFA-100 for the detection of aspirin in a study of 484 consecutive percutaneous coronary interventions patients, using laboratory platelet aggregometry in response to AA as a reference standard, were only 62% and 80.2% respectively (71). These findings indicate that PFA-100 has low sensitivity for the detection of aspirin.

Although it was initially assumed that the COLADP CT would detect the effect of platelet ADP receptor antagonists, this has proved not to be the case. Several studies have shown that the PFA-100 COL-ADP CT is not consistently prolonged by ticlopidine or clopidogrel. Van der Plancken et al found that clopidogrel 450 mg significantly increased the PFA-100 COL-ADP CT, but there was a wide variation in response, with many patients' CT remaining within the normal range72. Mueller et al found that clopidogrel 75 mg/day had no effect on COL-ADP CT (73). More recently, Agarwal et al found that in 20 patients taking clopidogrel, 90% had a COL-ADP CT in the normal range (70). Alstrom et al found no effect of 300 mg clopidogrel on COL-ADP CT in 26 patients undergoing coronary angiography (74). Similar findings have been reported by Jaremo et al (75).

Both the COL-ADP CT and the COL-EPI CT are prolonged by [G.sub.p][II.sub.b]/[III.sub.a] inhibitors (76,77). Therefore, values within the normal range exclude a clinically relevant [G.sub.p][II.sub.b]/[III.sub.a] effect. However, due to the sensitivity of the PFA-100 to these agents, a quantitative assay to allow titration of dose is not possible. Even low levels increase the CT >300 s (76,77). For this reason, although the PFA-100 is prolonged by [G.sub.p][II.sub.b]/[III.sub.a] inhibitors, it is not recommended for monitoring the activity of these agents (50,64).

These findings indicate that, while the PFA-100 may be a useful screening test for a range of other platelet defects, it is of limited use for the detection of antiplatelet agent effects.

PlateletWorks[R]

The ICHOR PlateletWorks[R] (Helena Laboratories, Beaumont, TX, USA) platelet count ratio assay was developed to provide a simple bedside monitor of platelet function using whole blood (78). The platelet count in a 'control' ethylenediamine tetra-acetic acid blood sample is compared to the platelet count in a similar 'test' sample that has been exposed to a platelet agonist. In patients without platelet dysfunction, the presence of an agonist reduces the platelet count to close to zero, due to the aggregation of most of the platelets. In contrast, the platelet count in the control sample, without an agonist remains 'normal'. However, in patients with platelet dysfunction, platelets will be refractory to the agonist, reducing the difference in platelet count between the test and control. In this way, the 'platelet count ratio' provides an index of platelet dysfunction. An additional advantage of PlateletWorks is that platelet count as well as function is assessed.

Two separate test samples were originally available; containing either ADP or COL as agonists (50). An early prototype found good agreement between the platelet count ratio and laboratory platelet aggregometry on platelet rich plasma. Lau et al found a high correlation (0.88 to 0.93) between PlateletWorks and laboratory platelet aggregometry in a range of patients (78). Carville et al found correlations of 0.90 between PlateletWorks ADP and laboratory aggregometry, and 0.97 between PlateletWorks COL and laboratory aggregometry in patients undergoing cardiopulmonary bypass (79). On the basis of these reports, it was suggested that PlateletWorks could be used to monitor the effect of various antiplatelet drugs. However, clinical experience has been mixed.

Lennon et al found that the area under the receiver operator curve (a plot of sensitivity vs 1-specificity) for PlateletWorks detection of recent aspirin ingestion in cardiac surgical patients using the COL cartridges was only 0.58 (80). In the same patients, the area under the receiver operator curve for laboratory aggregometry was 0.77 (80). This finding is not surprising, given that COL can act independently of the COX-1 pathway. An AA cartridge has since been developed. Theoretically, this cartridge should be both more sensitive and more specific for the assessment of aspirin and other NSAID effects (50). However, there have been no published reports evaluating its use for this purpose.

Lau et al evaluated platelet aggregation using the PlateletWorks ADP cartridge in patients before and after ingestion of clopidogrel 300 mg (78). Most patients were already receiving aspirin. They found that when using a concentration of 5 [micro]M ADP, there were significant reductions in platelet aggregation within two hours of clopidogrel ingestion. Mobley et al assessed the frequency of non-response to clopidogrel using laboratory platelet aggregometry, PlateletWorks ADP and reptilase-[FXIII.sub.a] modified TEG (81). Non-response was defined as <10% inhibition averaged across all three techniques. The PlateletWorks ADP agreement with this averaged assessment was 79%. Craft et al35 assessed the reversal of clopidogrel using the same three techniques as Mobley et al. They found that PlateletWorks ADP had less inhibition of platelet function (both absolute and relative) compared to laboratory platelet aggregometry and modified TEG.

Lau et al also evaluated PlateletWorks ADP response to abciximab (78). They found marked reductions in platelet aggregation post abciximab, but with a heterogeneous response. Similarly, Tanaka et al found dose-related reductions in platelet aggregation induced by tirofiban using PlateletWorks ADP (82). White et al found that PlateletWorks ADP, when modified by the use of phenylalanyl-prolyl-arginine-chloromethyl ketone rather than ethylenediamine tetra-acetic acid as the anticoagulant, mirrored results obtained using laboratory aggregometry for the detection of eptifibatide, tirofiban and abciximab ([r.sup.2]=0.92 to 0.98) (83). However, the samples had to be measured within five minutes to limit disaggregation. They also showed that ethylenediamine tetraacetic acid is not as effective as phenylalanyl-prolylarginine-chloromethyl ketone in inhibiting platelet aggregation, even in control samples, thereby making results less accurate.

These findings indicate that the PlateletWorks COL cartridge cannot be recommended for the assessment of COX-1 inhibitors. However, PlateletWorks ADP cartridge appears useful for the assessment of both [P.sub.2][Y.sub.12] inhibitors and [G.sub.p][II.sub.b]/[III.sub.a] antagonists. Further work with the AA cartridge is required.

Platelet function tests involving specific assessments of antiplatelet agents

VerifyNow[R]

The VerifyNow[R] System (Accumetrics, San Diego, CA, uSA) is based on the ultegra rapid platelet function assay (RPFA). The RPFA was introduced in 1998, primarily for monitoring [G.sub.p][II.sub.b]/[III.sub.a] antagonist activity at the point-of-care84. The device is automated, simple to use and provides a result in one to two minutes, using less than 1 ml of citrated whole blood. The principle is similar to laboratory platelet aggregometry, being based on an increase in light transmission as platelets aggregate. The disposable cartridges contain fibrinogen-coated beads and specific platelet activators depending on the antiplatelet drug being tested (85,86).

The original Ultegra used iso-TRAP (iso-thrombin receptor activating peptide) to stimulate platelets without producing fibrin (84). Several early studies found a high correlation between the RPFA and laboratory platelet aggregometry for the detection of abciximab. For example, Smith et al found a correlation of 0.99 between RPFA and laboratory aggregometry for the detection of [G.sub.p][II.sub.b]/[III.sub.a] receptor inhibition by abciximab in vitro (84). Wheeler et al found that a correlation of 0.89 between the RPFA and laboratory aggregometry in serial samples from 192 patients before, during and after abciximab therapy (87). Sotia et al found that in 59 patients with acute coronary syndromes, tirofiban caused an 86% inhibition of platelet aggregation as assessed by laboratory aggregometry, while the Ultegra TRAP cartridge showed a 93% inhibition (88). White et al found a close correlation between laboratory aggregometry and RPFA ([r.sup.2]=0.68 to 0.82) when eptifibatide, tirofiban and abciximab were added to the blood of healthy donors in vitro, although this was not as high as the correlation between laboratory aggregometry and a modified PlateletWorks assay (83). The Ultegra RPFA cartridges for the detection [G.sub.p][II.sub.b]/[III.sub.a] receptor antagonists are now marketed as the VerifyNow [G.sub.p][II.sub.b]/[III.sub.a] assay.

More recently, VerifyNow has developed cartridges for the detection of COX-1 inhibitors (aspirin assay) and thienopyridines ([P.sub.2][Y.sub.12] assay). The aspirin assay uses AA as the agonist. Gurbel et al found the VerifyNow aspirin assay showed significantly lower aggregation in patients who had received various doses of aspirin when compared to healthy patients who had not received aspirin (85). However, the VerifyNow was not as sensitive as laboratory aggregometry in detecting aspirin resistance (11% vs 2%). Lordkipanidze et al also found a low correlation between the VerifyNow aspirin assay and laboratory aggregometry (r=0.13) for the detection of aspirin resistance (89). Harrison et al found that the incidence of aspirin resistance in a group of 100 patients with cerebrovascular disease was 17% with the VerifyNow, but only 5% with laboratory aggregometry (90). Paniccia et al, in a study of 484 percutaneous coronary intervention patients found that the sensitivity of the VerifyNow aspirin assay in relation to laboratory aggregometry with AA was only 39.3%, although it had high specificity (96.4%) (71). These findings suggest that the RPFA aspirin assay is an improvement on other methods of assessing an aspirin effect, but is still not as sensitive as laboratory aggregometry.

The VerifyNow [P.sub.2][Y.sub.12] assay uses ADP as the agonist. Prostaglandin [E.sub.1] is added to prevent ADP activation of the [P.sub.2][Y.sub.1] receptor. In volunteers (n=10), Malinin et al found that the VerifyNow [P.sub.2][Y.sub.12] assay demonstrated similar inhibition to laboratory platelet aggregometry (using ADP and prostaglandin [E.sub.1]) in response to a specific [P.sub.2][Y.sub.12] receptor antagonist (93 vs 95%), with a test coefficient of variation of <8% (91). In a further 131 patients, the VerifyNow did not appear to be affected by platelet count, haematocrit or fibrinogen level (91). In a separate study, Malinin et al measured the effect of clopidogrel ingestion on the inhibition of [P.sub.2][Y.sub.12] reaction units in 147 patients presenting for coronary artery stenting (86). They found an average of about 62% inhibition after a 450 mg dose and 68% inhibition after a 75 mg dose. However, they did not assess laboratory platelet aggregometry. Jakubowski et al found that the VerifyNow [P.sub.2][Y.sub.12] assay showed similar inhibitions to laboratory aggregometry to ADP with both clopidogrel and prasugrel, although the VerifyNow had a more limited range (92). Paniccia et al compared PFA-100 COL-ADP CT, the VerifyNow assay, and laboratory aggregometry (ADP) in 1267 patients on dual therapy for coronary artery disease (93). Platelet residual reactivity was found in 25.1% of patients by laboratory aggregometry, 24.4% by PFA-100 and 24.7% by VerifyNow. The correlation coefficient between the VerifyNow and laboratory aggregometry was 0.62.

Not all studies have supported the use of the VerifyNow [P.sub.2][Y.sub.12] assay. Hochholzer et al found, in a study of 27 patients, that clopidogrel reduced laboratory platelet aggregometry to ADP from 65 to 42%, but the VerifyNow showed only minimal change94. More recently, Lordkipanidze et al found in 68 patients with coronary artery disease, the VerifyNow [P.sub.2][Y.sub.12] did not accurately quantify clopidogrel induced platelet inhibition, unless there was a baseline for comparison (95).

As with the aspirin assay, these findings suggest that the VerifyNow [P.sub.2][Y.sub.12] assay may be an improvement on previous methods, but is not as sensitive or specific as laboratory aggregometry. Moreover, as experience with both these assays is limited, further studies have been recommended.

Impact[TM] Cone and Platelet Analyser

The original Cone and Plate(let) analyser was based on the adhesion of platelets to an extracellular matrix (96). The Impact[TM] device (Diamed, Switzerland) substitutes polystyrene plates instead of the extracellular matrix, which facilitates automation and makes it more suitable for use at the point-of-care. A small amount of whole blood (0.12 ml) is exposed to a spinning cup producing a uniform shear. Platelets adhering to the plates are stained and imaged automatically by an inbuilt microscope. The amount of staining (surface coverage, SC) is dependent on platelet function, vWF and fibrinogen levels, and the availability of [G.sub.p][I.sub.b] and [G.sub.p][II.sub.b]/[III.sub.a] receptors. The whole process takes less than six minutes.

The device has been used to monitor [G.sub.p][II.sub.b]/[III.sub.a] antagonist activity (98). using an in vitro technique, in which blood from healthy volunteers was treated with specific [G.sub.p][II.sub.b]/[III.sub.a] receptor antibodies, there was high correlation between flow cytometric assessment of the receptor blockade and Cone and Platelet Analyser percentage of SC ([r.sup.2]=0.9, P <0.01). In 11 patients receiving abciximab therapy, SC was reduced within minutes in all patients, but with considerable individual variation in the rates of recovery (96). In the same study, SC was shown to be influenced by both platelet count and haematocrit (96).

More recently, the device has been modified to assess the effects of aspirin and clopidogrel. This involves the addition of AA or ADP to the test samples in vitro. In a study of 20 volunteers who were given aspirin, the SC of the standard test was unchanged (97). However, in the AA modified test, aspirin reduced the SC significantly, similar to the reduction in AA-induced laboratory platelet aggregometry (97).

Most of the studies using this device have been reported from the group of investigators involved in its development. Although the device is now commercially available, there is limited independent experience with its use.

Modified Thrombelastography[R] (mTEG[R], Platelet Mapping[TM])

In view of the very low sensitivity of the standard TEG for the detection of COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists, a novel modification was recently developed to overcome the 'problem of thrombin' within the TEG cup (Haemoscope, Niles, IL, USA) (99,100). The modification uses heparinised samples to avoid thrombin formation, thereby preventing the effect of thrombin on platelets. In the absence of thrombin, another mechanism for cross-linked fibrin generation is required. This is achieved by adding reptilase (batroxobin), which generates fibrin without platelet activation, and factor [XIII.sub.a] which cross-links fibrin monomers. The combination produces a relatively weak clot. Platelet activation is then achieved by adding a specific platelet agonist such as AA or ADP. If AA is added, then platelets are activated though the COX-1 pathway, and the effect of COX-1 inhibitors can be assessed. Similarly, if ADP is added, platelets are activated through the ADP [P.sub.2][Y.sub.12] receptor and the effect of platelet ADP antagonists can be assessed.

Platelet Mapping[TM] involves the use of up to four TEG channels simultaneously (100). In the first channel, a standard TEG (without heparin) is run using kaolin as an activator. In the second channel, a mTEG[R] is run (using a heparinised sample with reptilase and factor [XIII.sub.a]), but with no platelet activator. In the third and fourth channels, mTEGs are run with the addition of AA or ADP. The standard TEG MA provides an index of maximum clot strength in response to thrombin, while the mTEG MA provides an index of maximum clot strength with the reptilase/[FXIII.sub.a] combination. In the absence of a platelet activator, this will represent the strength of cross-linked fibrin alone. This is then compared to the MA of the AA or ADP activated channels. Any difference is considered to be due to the contribution of platelets. In this way the percentage inhibition of COX-1 inhibitors and ADP antagonists can be measured. Bochsen et al have reported low analytical coefficients of variation ([CV.sub.a] <6.6%) for all four assays in normal controls (101).

Craft et al compared AA 1 mmol/l activated mTEG MA ([MA.sub.AA]) in 30 patients taking aspirin and 14 controls (35). The [MA.sub.AA] was 11 [+ or -] 11% of maximum (median[+ or -]median absolute deviation) in the aspirin-treated patients and 94 [+ or -] 4% in the controls. Agarwal et al found that in 20 patients taking aspirin, 18 had >50% reduction in [MA.sub.AA] and only two had <30% inhibition (70). Swallow et al investigated the effect of aspirin on [MA.sub.AA] in patients taking aspirin 75 mg/day for a week (n=10) or a single dose of aspirin 300 mg (n=10), and found significant reductions (>60%) in both groups compared to baseline (102). Tantry et al found that the [MA.sub.AA] of six healthy subjects decreased from 86 [+ or -] 14 to 5 [+ or -] 7% 24 hours after receiving aspirin 325 mg, and in 203 patients who claimed compliance with regular aspirin therapy, the MAAA was 7 [+ or -] 17% (103). Alstrom et al found that the median reduction in [MA.sub.AA] for patients taking aspirin was 83% (interquartile range 65 to 90, n=28) (74). Lev et al found that only 13 of 100 patients taking aspirin had <20% inhibition of [MA.sub.AA] (104). These patients were considered 'low responders' to aspirin. In contrast, Bochsen et al found that MA was normal (64.6 [+ or -] 4.7 mm; mean [+ or -] SD) in 43 blood donors who had not taken aspirin, with an inter-individual coefficient of variation ([CV.sub.g]) of only 4.5% (101).

Craft et al also used mTEG MA (in response to ADP 1 [micro]mol/l, [MA.sub.ADP]) to determine the absolute and relative reversal of platelet inhibition by clopidogrel in 43 surgical patients (35). The MAADP was 28 [+ or -] 17% (mean [+ or -] SD) in patients who had received clopidogrel 75 mg/day for >30 days. This increased to 48 [+ or -] 29% after an average of 11 days after stopping clopidogrel. Bochsen et al found a wide range of range of ADP receptor inhibition in normal volunteers (0 to 58%) with an average of 18.6% (101). Swallow et al found significant reductions in [MA.sub.ADP] in patients ingesting clopidogrel 600 mg (n=10) (102). The [MA.sub.ADP] fell from 59.4 [+ or -] 4.5 mm to 39.1 [+ or -] 7.7 mm within six hours. A similar reduction was observed for another group (n=10) taking both aspirin 325 mg and clopidogrel 600 mg. Alstrom et al found that in patients taking aspirin, the ingestion of clopidogrel 300 mg increased the percentage inhibition of the [MA.sub.AA] from a median of 83 to a median of 92 (n=28) (74). However, the percentage inhibition of the [MA.sub.ADP] increased from a median of 10 (interquartile range 11 to 25) to only 30 (20 to 36). Lev et al found that 34 of 100 patients taking clopidogrel (75 mg/day for at least three days) were low responders (<20% inhibition of [MA.sub.ADP]) (104). Argawal et al found that in patients taking clopidogrel 75 mg/day, the percentage inhibition of [MA.sub.ADP] was a median of 33.3 (interquartile range 20.7 to 58.2, n=28) (70).

Hobson et al developed a novel method to more rapidly assess individual patients' time-dependent responses to aspirin and clopidogrel using mTEG (105). They used purpose-designed software to measure the area under the curve of the TEG amplitude over the first 15 minutes of a mTEG run (AUC15). The difference in the AuC15 before and after treatment could then be assessed as the percentage clotting inhibition. using this method they found considerable heterogeneity in the response to aspirin in 20 volunteers, and to clopidogrel in both volunteers (n=10) and patients (n=10). However, they did not compare their method to a reference standard or to an outcome measure.

Katori et al compared the effect of eptifibatide on conventional and batroxobin-mTEG in response to ADP 2 [micro]M (38). Increasing the eptifibatide concentration from 0 to 8 [micro]g/ml had little effect on the MA of conventional TEG (kaolin activated recalcified citrated samples) in vitro. In contrast, there were dose-related reductions in the reptilase mTEG MA from 44.6 [+ or -] 7.1 mm (control, mean [+ or -] SD) to 6.6 [+ or -] 2 mm. They noted that their reptilase-mTEG did not expose platelets to the very low calcium levels present in citrated samples. Craft et al found that in patients taking aspirin and clopidogrel, treatment with eptifibatide (180 [micro]g/kg bolus and 2 [micro]g/kg/min infusion) had no effect on the MA in response to ADP 1 [micro]mol/l or 2 [micro]mol/l, because the values were already very low (35). Increasing the ADP concentration to 100 [micro]mol/l overcame the inhibitory effects of aspirin and clopidogrel, exposing a significant inhibitory effect of eptifibatide.

These findings indicate that the mTEG Platelet Mapping assay should be suitable for the assessment of all three classes of antiplatelet agent. However, it is unclear whether it is as sensitive or specific as laboratory platelet aggregometry, and experience with its use is currently limited.

DISCUSSION

The findings of this review indicate that there is a wide range in the ability of current point-of-care platelet function monitors to assess the effect of antiplatelet agents (Table 1). Global assessments of haemostasis are essentially insensitive to COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists, and are not specific for platelet function. Global assessments of 'platelet function' also have limited sensitivity to COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists, but are more specific for platelet function. The newer devices developed specifically for the assessment of antiplatelet drug effects are more promising, but are not as sensitive as laboratory platelet aggregometry. There is also limited experience with these newer devices, especially for the assessment of COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists. Nevertheless, they have the potential to provide more meaningful information in the perioperative period and warrant further investigation. The detection of [G.sub.p][II.sub.b]/[III.sub.a] antagonist activity appears more straightforward, although not all techniques provide quantitative assays.

It is not surprising that point-of-care devices have limitations in the assessment of platelet function, because even laboratory techniques require careful attention to the choice and concentration of platelet agonist and the correct interpretation of platelet responses (9-11). There are also inherent difficulties that relate to the lack of a standard definition of platelet function, the multiple pathways of platelet activation, the wide interpatient variability in platelet responses and the interaction between platelets and other components of the clotting process (Table 2).

The most generally accepted definition of platelet function relates to the ability of platelets to aggregate (9-12). However, platelets have many other functions unrelated to aggregation and aggregation is influenced by many factors extraneous to platelets (13-14). It appears that platelet function is so complex that no one definition can capture all its components.

The multiple pathways of platelet activation are such that the choice of agonist affects both sensitivity and specificity (18). The presence of thrombin is a complicating factor in any fresh whole blood method of assessing platelet function. Thrombin is a potent agonist that activates platelets, even if they have been affected by COX-1 inhibitors or [P.sub.2][Y.sub.12] receptor antagonists (9,99). This explains why global assessments of haemostasis are insensitive to COX-1 inhibitors and [P.sub.2][Y.sub.12] receptor antagonists (Figure 1). The choice of agonist is less important for the assessment of [G.sub.p][II.sub.b]/[III.sub.a] receptor antagonists, because [G.sub.p][II.sub.b]/[III.sub.a] receptor expression occurs at a later stage of platelet activation and is independent of activation pathways (Figure 1).

The large interpatient variability in platelet function means that the population normal ranges are wide (10). With a wide population normal range, it is often difficult to determine whether an individual value is normal or abnormal (46). Therefore, any individual 'snapshot' value, without reference to a baseline value, must be interpreted with caution, especially if it is near the extremes of the population normal range (10,46). Moreover, the normal ranges are determined by obtaining the means and standard deviations of a population sample, not by comparing values to a biological standard. This means that slightly 'abnormal' values must also be interpreted with caution, because their biological significance is not known.

A further complicating factor is the issue of responders vs non-responders (7,8,90). It is evident that some patients' platelet function is unaffected by aspirin and a proportion of patients do not to respond to clopidogrel. The response to [G.sub.p][II.sub.b]/[III.sub.a] inhibitors also varies between patients (26). Therefore, the absence of an antiplatelet effect may not necessarily indicate that an assessment device is insensitive to the antiplatelet drug. Independent confirmation of non-responder status requires more sensitive techniques such as laboratory platelet aggregometry, which is often impractical.

Even if all the above factors are controlled, platelet function can never be assessed entirely in isolation. This is because assessments involving platelet aggregation are affected by fibrinogen level (9-11), assessments involving flow by haematocrit, and assessments involving clot formation by the integrity of fibrin formation. Therefore, any assessment of platelet function is based on the assumption that these extraneous factors are normal, or at least held constant. When assessing the effect of antiplatelet agents, there are other assumptions, such as an adequate platelet count, and the absence of other platelet defects unrelated to the antiplatelet drug (e.g. effects of cardiopulmonary bypass).

Another limitation in the assessment of platelet function is the inability to adequately assess the impact of any impairment on a clinical outcome measure. Clinical interest is related to the effect of platelets on bleeding or thrombosis. However, bleeding is dependent on such a large number of factors that it is rarely possible to attribute any increase to the effects of impaired platelet function alone. Without accurate information on the primary impact of impaired platelet function on bleeding, it is difficult to assess the secondary impact of these point-of-care devices on outcomes. For this reason, there has been no attempt to do so in this review.

It is clear from the findings of this review that global assessments of haemostasis, such as the standard TEG, Sonoclot, CSA and Hemodyne cannot be used for the assessment of antiplatelet agents. This does not mean that they cannot be used for other assessments of coagulation or for the assessment of other aspects of platelet function. Similarly, the findings of this review indicate that the PFA-100 is of limited use for the assessment of COX-1 and P2y12 antagonists, and it is not suitable for quantitative assays of [G.sub.p][II.sub.b]/[III.sub.a] antagonists. The PlateletWorks device, another global measure of platelet function, can be used to monitor both [P.sub.2][Y.sub.12] antagonists and [G.sub.p][II.sub.b]/[III.sub.a] antagonists. The PlateletWorks COL cartridge cannot be recommended for the assessment of COX-1 inhibitors, but the new AA cartridge may be an improvement.

The newer techniques such as Platelet Mapping and VerifyNow appear to be more sensitive and specific than previous methods, but are still not as sensitive as laboratory platelet aggregometry. The VerifyNow is the simpler of the two techniques. It has a single channel and provides a result within a few minutes using a single cartridge. However, if more than one test is required (e.g. AA and [P.sub.2][Y.sub.12]), they must be performed in series. In contrast, mTEG (Platelet Mapping) is more complex requiring two TEG 5000 devices. This is because four channels are required for complete interpretation. Given the specialised nature of the reagents, this increases the total cost of the assay. The TEG runs are not as rapid as the VerifyNow, taking a minimum of 10 to 15 minutes. On the other hand, the use of multiple channels permits simultaneous assessment of all three classes of antiplatelet agent.

At present, there is little independent experience with the Impact Cone and Plate(let) Analyser, although preliminary reports suggest that it is able to detect all three classes of antiplatelet agent.

Appreciation of the strengths and limitations of the platelet function test being used is important if the information provided is to be interpreted correctly. For example, using a global test of haemostasis to exclude an aspirin or clopidogrel effect will result in a false negative. If this is not appreciated, normal platelet function may be assumed and patients may be placed at increased risk. Global tests are also highly influenced by fibrin formation. This could cause a false positive if coagulation is impaired, which could prompt platelet transfusion unnecessarily. Similar considerations, although to a lesser extent, apply to other point-of-care techniques. As with all tests, routine calibration, quality control and compliance with accreditation standards must be considered.

While this paper set out to review all recent studies and review articles related to the accuracy of point-of-care assessment of antiplatelet agents, it is possible that a proportion were missed. Nevertheless, given the large number of articles reviewed, it is hoped that any inadvertently overlooked articles would not significantly alter the main conclusions.

In summary, there is no simple test for the assessment of antiplatelet agents in the perioperative period. This is due not only to the complexity of platelet function and the multiple pathways of activation, but also to the large variability between patients, and the interdependence between platelet function and other aspects of coagulation. Most traditional point-of-care techniques cannot assess the effect of COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists. In contrast, most devices are able to detect the effect of [G.sub.p][II.sub.b]/[III.sub.a] antagonists. There are several newer techniques that are more promising for the detection of COX-1 inhibitors and [P.sub.2][Y.sub.12] antagonists, but they are still not as sensitive as laboratory platelet aggregometry. All require further study before they can be recommended for routine clinical use.

Accepted for publication on January 17, 2009.

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N. M. GIBBS *

Department of Anaesthesia, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia

* M.B., B.S., M.D., F.A.N.Z.C.A., Head of Department.

Address for reprints: Dr N. M. Gibbs, Department of Anaesthesia, Sir Charles Gairdner Hospital, Nedlands, WA 6009.
TABLE 1
Limitations and potential uses of current point-of-care platelet
function devices for the assessment of antiplatelet agents

 Uses
 time-dependent

 [G.sub.p]
 COX-1 [P.sub.2] [II.sub.b]/
 inhi- [y.sub.12] [III.sub.a]
 Limitations bitors inhibitors inhibitors

Global assessments of coagulation

Standard TEG[R] Not specific for No No Yes *
Sonoclot[R] platelet function
Clot Signature Insensitive to
Analyser[R] COX-1 and
Hemodyne[TM] [P.sub.2][Y.sub.12]
 inhibitors

Global assessments of platelet function

PFA-100[R] Affected by No No Yes *
 platelet count
 and haematocrit
 Wide normal
 range

PlateletWorks[R] Few clinical Yes ** Yes Yes *
 studies
 Mixed results
 Not as sensitive
 as laboratory PA

Specific assessments of antiplatelet agents

Impact[TM] Cone Not widely Yes Yes Yes
and Platelet available
Analyser Few clinical
 studies
 Not as sensitive
 as laboratory PA

Platelet Requires Yes Yes Yes
Mapping[TM] multiple
 channels
 Expensive
 reagents
 Few clinical
 studies
 Not as sensitive
 as laboratory PA

VerifyNow[R] Few clinical Yes Yes Yes
 studies with
 COX-1 and
 [P.sub.2][Y.sub.12]
 inhibitors
 Not as sensitive
 as laboratory PA

COX-1=cyclooxygenase-1, PA=platelet aggregometry. * Device
will detect [G.sub.p][II.sub.b]/[III.sub.a] antagonists but is not
recommended for dose adjustment. ** Applies to arachidonic acid but
not collagen cartridge.

TABLE 2
Inherent problems in the assessment of platelet function

Problem Cause

No standard definition of Complexity of
platelet function function: e.g. adhesion,
 activation, amplification,
 aggregation, platelet
 procoagulant activity

No laboratory standard for the Inter laboratory variation
assessment of platelet function in techniques: Assessments
 affected by technique and
 choice and concentration
 of agonists and responses
 measured

Multiple pathways of platelet Agonists (and antagonists)
activation act via different receptors
 affecting different enzyme
 pathways

Interaction of platelets with Platelet function cannot be
other aspects of coagulation assessed wholly independently
 of other aspects of coagulation

Wide inter-patient variability The wide inter-patient
in platelet function variability makes it difficult to
 define normal vs. abnormal

Arbitrary normal values The normal ranges are based
 on population responses, not
 on a biological standard
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Author:Gibbs, N.M.
Publication:Anaesthesia and Intensive Care
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Geographic Code:8AUST
Date:May 1, 2009
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