Printer Friendly

Monitoring versus testing technologies: present and future.

Testing and monitoring are distinct acts with approaches that have important clinical implications. They differ in technologies used, time required for performing tests, and in some cases, sites in which the assessment takes place.

Testing determines the presence of an abnormality, substance, or disease by measuring a parameter at a fixed point in time. The limitation of testing is that it cannot be done often enough to provide continual diagnostic guidance. Monitoring, on the other hand, examines a parameter continuously or at frequent intervals. The results can warn care givers when undesirable limits are being approached.

A parameter that changes continuously, such as blood gases in a patient with compromised cardiopulmonary function, is best assessed via monitoring. For a parameter that changes slowly, such as serum bilirubin in a patient with advanced cirrhosis, testing is more appropriate.

Monitors have traditionally been thought of as instruments connected directly to patients. The term also applies, however, to discrete measuring devices used at the point of patient care, providing clinical data in real time. The clinical team uses this information in formulating immediate therapeutic decisions.

* Advantages of monitoring. Because a monitor measures a variable in real time, results reflect the patient's actual status. The outcome of a life-threatening event may depend on the speed and accuracy of a laboratory result or on observing the trend of a variable that signals an impending event.

A clinically useful monitor measures output after data input in less time than it takes for the parameter to change. For heart rate and blood pressure, this time consists of seconds; for blood glucose, a few minutes. A blood glucose determination in a diabetic patient obtained one week before admission to the hospital, for example, may be of little value for management after admission. Therefore, monitoring would be more suitable than testing for assessing serum glucose in such a patient. Obtaining the theophylline level of an asthmatic patient who is following a pharmacologic regimen one week before admission for surgery, however, might be very useful. For the asthmatic patient, testing would be as appropriate as monitoring.

* Turnaround time of results. Laboratory turnaround times may be considered from three perspectives. Laboratory time refers to the time required to perform a test after the specimen has arrived in the clinical laboratory. Testing time involves the interval elapsed in obtaining the specimen, transporting it to the laboratory, performing the analysis, and deriving results. The more comprehensive therapeutic time begins with the test order and extends until a therapeutic action based on the test result has been made.

Included in therapeutic time are not only the testing time itself but also the time required for the physician to receive, evaluate, and act upon the results. Prolonged therapeutic time, inevitable when the primary physician is busy or leaves the patient's area, delays therapy, thereby potentially increasing hospital stay and cost.

My colleagues and I recently measured the turnaround times of Stat sodium and potassium levels in the ICU of a teaching hospital.|1~ Laboratory time averaged 7 minutes; testing time, 90 minutes; and therapeutic time, 150 minutes. Bedside electrolyte analysis was associated with much shorter therapeutic times: only a few minutes.

Another group of researchers reported Star lab testing times in two major hospitals.|2~ Blood gas averaged 33 |+ or -~ 15 minutes and 27 |+ or -~ 19 minutes in the two institutions. Testing times for sodium and potassium were 37 |+ or -~ 15 minutes and 75 |+ or -~ 49 minutes; for glucose, 44 |+ or -~ 14 minutes and 75 |+ or -~ 49 minutes.

The rapid therapeutic decisions permitted by monitoring with point-of-care testing, evident in the studies just mentioned, are crucial for treatment of patients in the operating room, emergency department, and critical care unit as well as immediately after cardiac arrest in any setting. Each institution should consider which tests should be available at the point of patient care and under what circumstances.

Monitoring may decrease unneeded tests and therapies by providing specific results on demand. When hypoglycemia is suspected, for example, many clinicians order a blood glucose level, treating the patient with dextrose while awaiting lab confirmation. Unfortunately, hypertonic glucose is painful and can damage the veins. When the test is performed at the bedside, the blood glucose level can be obtained within minutes, making it possible to postpone therapy safely in the meantime. If blood glucose is found to be at a normal level, the search for an alternative diagnosis can begin immediately. We have found that bedside glucose monitoring allows for a more predictable decrease in blood glucose in diabetics receiving treatment for diabetic ketoacidosis than does laboratory glucose testing.

* Desirable characteristics. Since the analyzers will be moved to various locations within the institution, they must be portable and small enough to be accommodated at the crowded bedside and other hospital locations. Similarly, adequate space must be provided for the equipment at the bedside or otherwise near the patient. The ideal is compact equipment that contains all reagents and calibration fluids as well as an enclosed section for waste disposal, to avoid contamination.

If the equipment is to be run by health care workers (especially those with little laboratory training)as part of their regular activities, it should be sturdy, maintenance-free, and simple to operate. Parts and electrodes should be disposable and easily changed. Automatic calibration and external and internal quality control methods must be straightforward.

The analyzer should use whole blood, eliminating the need to prepare specimens for testing. The output rate should match workload requirements. The unit should be capable of producing a printout of results and of storing data for subsequent analysis and billing. The cost per specimen must be reasonable.

* Accuracy. One source of the special accuracy of point-of-care instrumentation is its elimination of specimen transport and resulting reduction in preanalytical error. It has been well documented that delays in transport of blood gas specimens can adversely affect pH and p|O.sub.2~ results, thereby giving the physician a false impression of patient status.

The quality of results obtained by residents and interns in an acute care setting varies according to the complexity of instruments used.|3~ Instruments requiring the least number of steps to operate produce the most accurate results. The value of simplicity suggests the wisdom of choosing an instrument that uses whole blood, requires no pipetting or dilution, and demands little technical skill to operate.

Researchers have assessed the accuracy of point-of-care techniques for measuring glucose, potassium, sodium, ionized calcium, hematocrit, and blood gases. In some studies, nurses and other non-laboratory-trained direct care providers performed blood glucose analyses at the bedside using dry chemistry reagent sticks. Test results were compared with those obtained in the clinical laboratory with the glucose oxidase method.|4-8~ Values were comparable in most studies. Inaccuracies due to poor technique in some studies were later overcome by training.|9~

Attending to instruments is an important factor for success. The studies suggest that when reflectance meters are used, they must be maintained and calibrated properly. Reagent strips must not be allowed to deteriorate.

* Physicians' knowledge. The installation of point-of-care monitoring presupposes that the expertise to interpret test results will be available on site. To make appropriate decisions at the bedside based on these results, physicians must understand the technology, the extent of its accuracy, and its limitations. Misinterpreting a test result could lead to a deleterious decision.

Some point-of-care technology may prove to be less accurate than instrumentation in the clinical laboratory. Nevertheless, related problems can be averted if the physician understands the difference between results and determines it not to be clinically significant. Training of all staff, including the medical staff, is a continual process when point-of-care instruments are used.

* Quality assurance. Endless demands for quality assurance are frequently used when the value of point-of-care testing and monitoring is in dispute. Yet good QA programs can certainly be created for such testing with the cooperation of laboratory professionals on the team.|9~ Many new instruments generate printouts or can store information that can be used afterward to document instrument performance, calibration, and quality control efforts.

A good QA program tests both instrument and operator. Blind and unblind quality control samples are available from various organizations. The individual institution's policies should define who will perform point-of-care testing, what type of training will be required, how the competence of users will be determined, how the reliability of the test result will be documented, how results are to be used, and who will supervise testing.

* Documentation. Point-of-care analysis is easily documented, as required not only for hospital records but also for reimbursement by third-party providers. Test results may be recorded on flow sheets, on laboratory slips kept at each point-of-care site, or (in some of the new instruments) on computer disks. Billing can be derived from any of these sources. An alternative to billing tests individually is to set a flat daily fee for either a specific procedure or the duration of patient stay.

Point-of-care monitoring is intended as an extension of the central laboratory, not as a replacement for it. To become involved with point-of-care testing, the central laboratory may have the benefit of being able to redistribute some of its labor. Laboratorians' expertise will be needed to educate operators, clinicians, and others; to monitor the QA program; and to help assure that the instruments function properly.

Whether point-of-care testing can improve patient care is central to clinical pathology and most of medicine. At my institution, we performed a study to determine whether point-of-care testing of blood potassium in cardiac bypass surgery patients could reduce the incidence of arrhythmias and the need for antiarrhythmic agents. One group of patients received standard care consisting of blood potassium determinations in the Stat laboratory of the operating room. Blood potassium levels were taken for another group of patients every few minutes with a portable blood gas/electrolyte analyzer. Measuring potassium with the portable instrument was associated with a 50% reduction in arrhythmias and in use of antiarrhythmic agents. The second group demonstrated a significantly decreased need for inotropic therapy with dopamine or dobutamine as well.
Table I

 Laboratory Oximeter

No. of patients 15 15
Average no. ABGs/day 8 3(*)
Average time to optimize ventilator 6 1.5(*)
(hours)
Average time to wean from ventilator 8 2(*)
(hours)
Average ICU stay (days) 2.1 1.2(*)
Average hospital stay (days) 7 6(*)
Average hospital cost $3,150 $2,400

* p|is less than~0.05

Bedside oxyhemoglobin saturation monitoring with pulse oximeter
versus central laboratory blood gases in postoperative patients


In a prospective randomized study, investigators evaluated the efficacy and accuracy of having patients monitor their own prothrombin times at home with a portable monitor. Their results were compared with prothrombin times obtained in a laboratory.|10~ Accuracy was found to be comparable in both settings. In fact, home monitoring achieved better anticoagulation control than did monitoring in the clinic, possibly because of the more frequent testing possible at home. Monitoring anticoagulation therapy also cost less with the home monitor if such factors as the cost to the patient of each test, charges lot clinic visits, and transportation expenses were considered.

Rapid and specific diagnosis may decrease morbidity and mortality. We have cared for a number of patients in the last few years in whom cardiac arrest occurred as a result of unsuspected hyperkalemia or ionized hypocalcemia. Prompt and specific treatment was possible thanks to point-of-care monitoring of electrolytes. More studies are needed to address the impact of portable testing equipment on the quality of patient care.

We studied two groups of patients recovering from surgery without complications. In the first group, blood gas levels were measured by the central laboratory. The other group was monitored with pulse oximeters. We compared the average time required to wean patients from the ventilator, the average length of stay in the intensive care unit and in the hospital overall, and the average hospital cost.

We found that on average, those monitored for oxygenation at the bedside had shorter ventilator weaning time, ICU stay, and total hospital stay as well as lower hospitalization costs than those tested through the laboratory. The most significant factor in cost saving was the more rapid weaning from the ventilator, permitting earlier transfer out of the ICU.

Blood glucose monitoring with dry chemistry reagent test strips improved the care of a group of diabetics admitted to the ICU with ketoacidosis. Treated more quickly, patients stayed a shorter time in the ICU and in the hospital, thus generating lower hospitalization costs. Other investigators have reported reduced costs for monitoring blood glucose levels and hospital stay in diabetics whose blood glucose was monitored at the bedside after initially being tested through the central laboratory.|11~

In a recent study, we reported that the testing of blood gases, electrolytes, and hematocrit with a point-of-care analyzer was less expensive than when done through the central laboratory.|12~ The greatest sources of saving were lower operating costs and heightened productivity.

Another study demonstrated a saving to the hospital when near-patient testing was performed in an operating room as a supplement to testing in the central laboratory. Faster test turnaround time permitted cases to be finished earlier. The amount of overtime pay needed for operating room personnel dropped, as did waiting time for patients entering the hospital's emergency room. One cost analysis demonstrates that instituting point-of-care monitoring can lower total annual operating costs for measuring certain blood parameters.|13~
Table II

 Laboratory Bedside

Average ICU stay (days) 2.5 1.4(*)
Average hospital stay (days) 8.0 5.0(*)
Average glucose tests performed 36 36
Average cost of glucose test $3.50 $0.45
Total average cost of glucose testing $126.00 $16.20
Total hospital cost $3,925.00 $2,280.00

* p|is less than~0.05

Bedside versus clinical laboratory blood glucose monitoring for
treatment of diabetic katoacidosis


* Point-of-care testing and critical care. Critical care units were devised to provide more intensive nursing care and closer monitoring of patients. At first, monitoring consisted of continuous ECG recording for arrhythmias, closely followed by continuous invasive measurement of blood pressure and evaluation of urine output. Dry chemistry reagent test strips were developed for urinalysis and assessment of blood glucose levels. Accuracy of the latter was improved with the introduction of reflectance meters.

The pulse oximeter for monitoring hemoglobin oxygen saturation represented the next advance in bedside testing. Tissue oxygenation had previously been monitored with transcutaneous and transconjunctival oximeters. Technological problems limited the use of these techniques. The pulse oximeter is accurate and easy to use and can improve patient care by alerting health care providers to hypoxemia. Pulse oximeters, now the standard of care in operating rooms, are fast becoming the standard in critical care units as well, although they have not supplanted traditional blood gas testing.

Technology is available for continuous assessment of end-tidal C|O.sub.2~, an approximation of arterial pC|O.sub.2~, and mixed venous oxyhemoglobin saturation. Instruments are currently under development for continuous intra-arterial measurement of p|O.sub.2~, pC|O.sub.2~, and pH. It is nevertheless vital to consider the incremental cost/benefit relationship before adopting these new techniques.

Recent technologic advances have led to the development of small portable analyzers capable of performing complicated biochemical tests at the point of patient care. The first such instruments used an individual test card for each analysis and included only electrolytes and pH. Cartridge-based technology has enabled expansion of these tests, which now include blood gases, electrolytes, hemoglobin or hematocrit, glucose, BUN, and creatinine. Some such instruments are maintenance-free and require very little training to operate. Bedside monitoring is well accepted by the critical care staff. Requests for additional bedside testing continue to grow.

* Which tests? Only tests that require rapid turnaround time and whose results lead to enhancement of patient care should be performed at the point of care. Quality control and accuracy are usually better in the central laboratory, where trained medical technologists perform the measurements.

Tests that are particularly valuable to perform at the point of patient care include those to determine blood pressure, heart rate, cardiac output, pulmonary artery occlusion pressure, ECG monitoring, urine output, and temperature. Oxygenation monitoring by pulse oximeter and pC|O.sub.2~ monitoring by end-tidal C|O.sub.2~ were recently added to the list. My opinion concerning which tests are appropriate to be done at the bedside includes blood glucose, potassium, ionized calcium, blood pH, pC|O.sub.2~, p|O.sub.2~, clotting times (PT/PTT, ACT), and hemoglobin or hematocrit.

Tests that may be done at the point of care in the future include blood lactate, mixed venous oxygen saturation, BUN/creatinine, tissue p|O.sub.2~, and drug levels. Monitoring the function of the liver, brain, kidney, and other organs may be helpful in identifying dysfunction at an early stage, before organ failure can take hold. It will be important to document the benefits of these tests before they are widely used.

* Categories of point-of-care analyzers. Portable analyzers can be classified in four major categories: discrete specimen noninvasive, continuous noninvasive, discrete invasive, and continuous invasive.|14~ Each has distinct advantages and disadvantages.

Most blood p|O.sub.2~ measurements are made with Clark-type polarographic oxygen sensors.|14~ Because the sensors drift, they require frequent calibration. Among optical sensing methods recently developed for measuring p|O.sub.2~, the most popular uses fluorescent dye immobilized in a thin layer adjacent to optical fibers.|14~ In the presence of oxygen, the intensity of fluorescence emission from the dye is quenched in proportion to the concentration of p|O.sub.2~. This technique depends greatly on the intensity of the source, the resolution of the detector, and the stability of the dye. Because these devices too can drift, accurate results are a must. This |O.sub.2~ sensing method, now available for extracorporeal monitoring, is being developed for in vivo arterial p|O.sub.2~ monitoring as well.

Tissue oxygenation has been measured with transcutaneous oxygen tension, conjunctival oxygen tension, pulse oximetry, and mixed venous oxygen saturation (Sv|O.sub.2~). Transcutaneous analyzers use Clark-type polarographic oxygen sensors. Sensors, commonly used for infants and neonates, are heated to assure arterialization of blood. Transcutaneous oxygen tension monitoring of adults, however, is frequently inaccurate and associated with technical problems.|15~ Measurements are affected by skin thickness, peripheral blood perfusion, shunting, and circulatory status (cardiac output).

In one method for measuring oxygen tension, a device is placed in the conjunctiva of the eye. This sensor, suitable for use only for short periods, produces inaccurate results if the patient is hypotensive. Only pulse oximeters and Sv|O.sub.2~ monitoring have proved reliable in the clinical setting.

Noninvasive pulse oximetry can provide a simple, rapid, continuous, and accurate assessment of capillary oxyhemoglobin saturation.|15-17~ The pulse oximeter estimates hemoglobin oxygen saturation by measuring the light absorbance of perfused tissue at 660 and 940 nm. The device computes the difference between the maximum and minimum absorbance at each wavelength to generate a "pulse-added" absorbance signal. The ratio of the pulse-added absorbance at the two wavelengths is used to estimate oxygen saturation by means of an empirical algorithm built into the oximeter software. This algorithm was created by measuring pulse-added absorbances in healthy, awake volunteers breathing various gas mixtures.|18,19~ Data for calibration were obtained by inducing hypoxia in normal subjects with saturations above 70%.

Performance characteristics of oximeters are not uniform.|20~ The accuracy of saturation measurements depends on proper application of the sensor; perfusion status, since the results are inaccurate at low perfusion; and the presence of any interfering compounds in the blood, such as carboxyhemoglobin, methemoglobin, markedly elevated bilirubin, methylene blue, or indocyanine green. Pulse oximeters are less precise in the presence of low oxygen saturation (|is less than~ 70%)|16,19~ and are insensitive to p|O.sub.2~ changes when hemoglobin is saturated (|is less than~ 100 mm Hg). Another common problem is interference with measurements by external light sources.

We recommend correlating the pulse oximeter reading with an oxyhemoglobin saturation level obtained from a blood gas result or co-oximeter measurement. Pulse oximeters are best used to monitor patients for periods of hypoxemia,|19~ which may go unrecognized by clinical parameters. These states include endotracheal tube placement, monitoring of ventilator function, airway obstruction (secretions, bronchospasm), central line placement, pulmonary edema, right heart catheterization, suctioning, bronchoscopy, and endoscopy. When used appropriately by trained clinicians, pulse oximetry may decrease the need for arterial blood gas analyses in stable patients, but it is not a substitute for blood gas analysis in all cases.

Many conditions commonly found in critically ill patients impair oxygen delivery to the tissues by altering the ratio of supply and demand. Continuous monitoring of mixed venous oxygen saturation (Sv|O.sub.2~), which provides an estimate of that ratio, can be accomplished with a fiberoptic pulmonary artery catheter.|21~ When the oxygen needs of tissue exceed its delivery, tissue p|O.sub.2~ falls, increasing p|O.sub.2~ diffusion from capillaries into the tissue. A falling capillary p|O.sub.2~ leads to an off-loading of oxygen from hemoglobin and to a drop in Sv|O.sub.2~. Decreased Sv|O.sub.2~ thus suggests that oxygen demand exceeds supply.

Decreased oxygen supply may result from decreased cardiac output or decreased arterial oxygen content, evidenced by respiratory failure, increased ventilation-perfusion mismatching, and low hemoglobin. Increased oxygen demand may result from hyperthermia, shivering, agitation, pain, increased work of breathing, and many other causes of a quickened metabolic rate.

Thus continuous monitoring of Sv|O.sub.2~ may serve as an early warning for alterations in cardiorespiratory status. Decreased Sv|O.sub.2~, for example, may be an early indicator of deteriorating hemodynamic status from pneumothorax. Knowing this may assist the care giver in titrating vasoactive agents, volume loading, adjusting the controls of the ventilator, and performing tasks directly related to patient care, such as suctioning and positioning.

Some Sv|O.sub.2~ catheters use three-reference wavelengths; others use two-reference wavelengths.|22~ The two-reference system is reported to drift over time, resulting in significant deviations from true measured Sv|O.sub.2~.

Measurement of pC|O.sub.2~ is most often done with the Severinghaus-type potentiometric gas sensor. C|O.sub.2~ diffuses across a gas-permeable membrane into a thin bicarbonate layer that is in contact with a pH electrode. The pH change, proportional to the pC|O.sub.2~, is measured.

Newer polymeric membranes have been developed for discrete specimen testing. Optical sensors for detecting pC|O.sub.2~ in blood are based on the change in pH of bicarbonate electrolytes held behind a gaspermeable membrane. Change in pH is detected by assessing the absorbance or fluorescence of an acid-base indicator dye. Optical sensing of C|O.sub.2~ is being developed for in vivo use.

End-tidal C|O.sub.2~ tension (|ETCO.sub.2~) is a continuous noninvasive method used for monitoring arterial pC|O.sub.2~.|23-24~ We have found the technique useful in monitoring patients' ventilation after endotracheal intubation, during transport, in the operating room, and as they are weaned from the ventilator. |ETCO.sub.2~ supplements blood gas analysis but does not replace it.

New bedside techniques currently used only experimentally include real-time dual oximetry|25~ and cerebral oxygenation and cytochrome monitoring.|26~ Dual oximetry, which estimates venous admixture and body oxygen utilization, combines pulse oximetry and pulmonary artery oximetry. The method has been useful for titrating positive airway pressure in patients experiencing respiratory failure.|25~ Cerebral oxygenation can be determined with an instrument that monitors oxidized cytochrome aa3 and oxyhemoglobin levels and may be useful for early detection of cerebral hypoxia.|26~

Measurement of pH is usually done by potentiometric ion-selective electrodes.|14~ While many laboratory instruments use pH-selective glass membranes, newer instruments use polymeric membrane systems. Polymeric membranes function as ion-selective transducers when an appropriate ionophore (Na+, K+, iCa+ +, H+) is incorporated into the polymeric film.

Membrane electrodes measure thermodynamic ion activities of whole blood, eliminating the need for centrifugation of specimens. Because these electrodes measure activity rather than concentration, they are not affected by proteins or lipids in the blood. Optical pH sensors use acid-base indicator dyes and measure changes in pH using optical absorption or fluorescence. These sensors are currently being developed for in vivo use.

Electrolytes are usually measured with potentiometric ion-selective membrane electrodes. Newer instruments use polymeric membranes with specific ionophores (Na+, K+, iCa + +) similar to pH. We are not aware of any current optical sensors for electrolyte analysis.

The conventional method for measuring hematocrit is microcentrifugation (spun hematocrit). Although reliable, this technique is inconvenient to perform outside the central laboratory. Newer analyzers TABULAR DATA OMITTED measure Hct with conductivity, under the concept that blood conductivity is inversely proportional to blood cell volume, and adjust for sodium concentration. Hemoglobin is measured by lysing red blood cells and spectrophotometrically measuring the change in absorption of the specimen.

Table III

Evolution of point-of-care monitors

I. Simple observation

Urine color, plasma color

II. Observation plus simple bedside test

Urine protein, specific gravity, osmolality

III. Observation plus specialized test (satellite laboratory)

Urine microscopic analysis

Blood sedimentation rate

Sputum Gram stain

Blood hematocrit

IV. Complex biochemical tests requiring expensive equipment, technical expertise (centralized laboratory)

Serum Na+, K+, glucose, Cl-, C|O.sub.2~, iCa++

BUN, creatinine

p|O.sub.2~, pC|O.sub.2~, pH

AST, ALT, bilirubin

Drug levels

Many others

V. Bedside monitoring (critical care units)

Blood pressure, heart rate, ECG

Urine output

Dry chemistry reagent strips (urine, blood glucose, urine leukocyte esterase)

Dry chemistry reagent strips plus reflectance meter (blood glucose)

VI. Point-of-care testing

TABULAR DATA OMITTED
Analyzers Parameters measured

Gem-6 Plus, Gem-Stat, Gem ABG, Na+, K+, iCa++, Hct
Premier (Mallinckrodt Sensor
Systems, Ann Arbor, Mich.)(1,2)

200 Series (Ciba-Corning, ABG, Na+, K+, iCa++, Hb
Medfield, Mass.)(2)

Stat Profile (Nova ABG, Na+, K+, iCa++, Hb
Biomedical, Waltham, Mass.)(2) glucose

IL BGE (Instrumentation ABG, Na+, K+, iCa++, Hct
Laboratory, Lexington,
Mass.)(2)

ABL500/510(Radiometer ABG, Na+, K+, iCa++
America, Westlake, Ohio)(2)

Lytening (Baxter Lytening Na+, K+, iCa++, Cl
Systems, Danvers, Mass.)(2)

AVL 980 Series, AVL 995 ABG, Na+, K+, iCa++, Hb
(AVL Scientific, Roswell, Ga.)(2)

1. Suitable for point-of-care use

2. Suitable for near-patient laboratory
Table IV

Portable analyzers: A sampler

Test Analyzer

Urine: glucose, Dry chemistry reagent strips;
ketones, protein, osmometer
pH, blood,
bilirubin, specific
gravity, leukocyte
esterase, osmolality

Blood glucose Dry chemistry reagent strips
 (reflectance meter)

Hemoglobin/ Spun Hct: Gem-6 Plus, Gem-Star, Gem
hematocrit Premier (Mallinckrodt Sensor Systems,
 Ann Arbor, Mich.); other(*)

Prothrombin time Coumatrack (Du Pont, Wilmington, Del.)

Activated clotting Hemochron (Future Tech, Birmingham,
time Ala.)

Blood gases and pH Gem-6 Plus, Gem-Stat, Gem Premier;
 other(*)

Blood Na+, K+ Gem-6 Plus, Gem-Stat, Gem Premier;
 other(*)

Blood iCa++ Gem-6 Plus, Gem-Stat, Gem Premier;
 other(*)

Hemoglobin oxygen Pulse oximeters: Nellcor, Hayward,
saturation Calif.; Ohmeda, Madison, Wis.;
 Criticare Systems, Milwaukee, Wis.;
 Physio-Control, Redmond, Wash.;
 Novametrix Medical Systems,
 Wallingford, Conn.

 Mixed venous pulmonary catheters:
 Oximetrix, Mountain View, Calif.;
 Edwards Division, Baxter Healthcare,
 Santa Ana, Calif.

End-tidal C|O.sub.2~ Criticare Systems, Milwaukee, Wis.;
 Fenem, New York, N.Y.

* These analyzers are not portable but could be adapted for
near-patient testing: Stat Profile (Nova Biomedical, Waltham,
Mass.); 200 Series (Ciba-Corning, Medfield, Mass.); ABL 500/510
(Radiometer America, Westlake, Ohio); IL BGE (Instrumentation
Laboratory, Lexington, Mass.); AVL 980 Series, AVL 995 (AVL
Scientific, Roswell, Ga.); Lytening (Baxter Lytening Systems,
Danvers, Mass.).


Real-time blood gases can be measured with in-line extracorporeal analyzers.|27~ Blood is recirculated through tubing from artery to vein. The analyzer pulls blood from the circuit for periodic analysis. This approach, which requires heparinization of the blood, has been used only during cardiac bypass and hemodialysis. Other problems that can occur from this procedure include the risk of infection, the formation of emboli, and potential damage to circulating blood cells.

In vivo continuous sensors are being developed for measuring blood gases and electrolytes. This technology has been limited by an inability to develop small, durable, stable sensors. It is also impossible to recheck calibration with the sensors. Thrombus formation is another concern. Expense will be another consideration, since these single-use systems will probably cost $200 to $250 per patient for every two to three days of use. These devices are being designed for use through arterial and venous catheters.

Only one manufacturer now produces a portable instrument capable of analyzing blood gases. The analyzer also measures Hct, Na+, K+, and iCa++. When the instrument was evaluated during cardiac surgery, results for all analytes except Hct and iCa++ were comparable to those obtained in the clinical laboratory.|28~ Differences between results from the portable analyzer and those from the laboratory were 4.7 |+ or -~ 2.7% for Hct and 0.195 |+ or -~ 0.11 mmol for iCa++. Another study of cardiac surgery patients reported comparable results for pH, p|O.sub.2~, pC|O.sub.2~, K+, iCa++, and Hct.|27~

Yet another group of investigators reported an excellent correlation between results from the two types of instruments for all analytes except p|O.sub.2~ values of less than 60 mm Hg.|29~ The mean difference between the two kinds of measurements for p|O.sub.2~ |is less than~ 60 mm Hg was -2.3 |+ or -~ 5.5 mm Hg. Measurement of pH, p|O.sub.2~, pC|O.sub.2~, Na+, K+, and Hct was found comparable in laboratory and point-of-care tests in the study at our institution cited earlier.|12~

Recent refinements in technology have improved the accuracy for low p|O.sub.2~, iCa++, and Hct measurements. In a recent study of critically iII patients, we found results for all analytes to be comparable when tests done by non-laboratory-trained health care providers with the point-of-care analyzer were contrasted with those performed in our ICU Stat laboratory.

Selective use of point-of-care analyzers and monitoring equipment improves patient care by providing real-time diagnostic information. The resulting reduction in turnaround time provides timely data about the critical care patient to the clinical staff, improving their therapeutic decisions. Higher productivity, fewer repeat tests, and shorter hospital stays with consequently lower costs.

The trend toward increased use of point-of-care testing will continue. Although these tests will not replace those performed in the central laboratory, advances in technology will permit results that are of laboratory quality to be obtained at the point of patient care.

1. Zaloga, G.P. Evaluation of bedside testing options for the critical care unit. Chest 97(suppl.): 185S-190S, 1990.

2. Salem, M.; Chernow B.; Burke R.; et al. Bedside diagnostic testing: Its precision, rapidity, and utility in blood preservation. Crit. Care Med. 19(suppl.): 84, 1991

3. Nanji, A.A.; Poon, R.; and Hinberg, I. Decentralized clinical chemistry testing: Quality of results obtained by residents and interns in an acute care setting. J. Intensive Care Med. 3: 272-277, 1988.

4. Zaloga, G.P. Bedside reagent testing: Blood, CSF, and bacterial cultures. J. Crit. Illness 3: 85-94, 1988.

5. Chernow, B.; Diaz, M.; Cruess, D.; et al. Bedside blood glucose determinations in critical care medicine: A comparison analysis of two techniques. Crit. Care Med. 10: 463-465, 1982.

6. Maisels, M.J., and Lee, C.A. Chemstrip glucose test strips: Correlation with true glucose values less than 80 mg/dl. Crit. Care Med. 11: 293-295, 1983.

7. Shapiro, B. Savage, P.J.; Lomatch, D.; et al. A comparison of accuracy and estimated cost of methods for home blood glucose monitoring. Diabetes Care 4: 396-402, 1981.

8. Godine, J.E.; Hurxthal, K.; and Nathan, D.M. Bedside capillary glucose measurement by staff nurses in a general hospital. Am. J. Med. 80: 803-806, 1986.

9. Belsey, R.; Morrison, J.I.; Whitlow, K.J.; et al. Managing bedside glucose testing in the hospital. JAMA 258: 1634-1638, 1987.

10. White, R.H.; McCurdy, S.A.; Von Marensdorff, H.; et al. Home prothrombin time monitoring after the initiation of warfarin therapy. Ann. Intern. Med. 111: 730-737, 1989.

11. Trundle, D.S., and Weizenecker, R.A. Capillary glucose testing: A cost-saving bedside system. Lab Management 24:59-62, May 1986.

12. Zaloga, G.P.; Hill, T.R.; Strickland, R.A.; et al. Bedside blood gas and electrolyte monitoring in the critically ill patient. Crit. Care Med. 17: 920-925, 1989.

13. Statland, B.E., and Brzys, K. Evaluating Stat testing alternatives by calculating annual laboratory costs. Chest 97(suppl.): 198-203, 1990.

14. Misiano, D.R.; Meyerhoff M.E.; and Collison, M.E. Current and future directions in the technology relating to bedside testing of critically ill patients. Chest 97(suppl.): 204S-214S, 1990.

15. Durand, M., and Ramanathan, R. Pulse oximetry for continuous oxygen monitoring in sick newborn infants. J. Pediatr. 109: 1052-1056, 1986.

16. Boxer, R. A.; Gottesfeld, I.; Singh, S.; et al. Noninvasive pulse oximetry in children with cyanotic congenital heart disease. Crit. Care Med. 15: 1062-1064, 1987.

17. Yeldenman, M., and New, W. Evaluation of pulse oximetry. Anesthesiology 59: 344-352, 1983.

18. Barker, S.J., and Tremper, K.K. The effect of carbon monoxide inhalation on pulse oximetry and transcutaneous p|O.sub.2~. Anesthesiology 66: 677-679, 1987.

19. Schnapp, L.M., and Cohen, N.H. Pulse oximetry--uses and abuses. Chest 98: 1244-1250, 1990.

20. Severinghaus, J.W., and Naifeh, K.H. Accuracy of response of six pulse oximeters to profound hypoxia. Anesthesiology 67: 551-558, 1987.

21. Reinhart, K. Principles and practice of Sv|O.sub.2~ monitoring. Intensive Care World 5: 121-124, 1988.

22. Gettinger, A.; De Traglia, M.C.; and Glass, D. In vivo comparison of two mixed venous saturation catheters. Anesthesiology 66: 373-375, 1987.

23. Whitesell, R.; Asiddao, C.; Gollman, D.; et al. Relationship between arterial and peak expired carbon dioxide pressure during anesthesia and factors influencing the difference. Anesth. Analg. 60: 508, 1981.

24. Healey, C.J.; Fedullo, A.J.; Swinburne, A.J.; et al. Comparison of noninvasive measurements of carbon dioxide tension during withdrawal from mechanical ventilation. Crit. Care Med. 15: 764-768, 1987.

25. Rasanen, J., Downs, J.B.; and DeHaven, B. Titration of continuous positive airway pressure by real-time dual oximetry. Chest 92: 853-856, 1987.

26. Jobis-Vandervliet, F.F.; Fox, E.; and Sugioka, K. Monitoring of cerebral oxygenation and cytochrome aa3 redox state. Int. Anesthesiol. Clin. 25: 209-230, 1987.

27. Strickland, R.A.; Hill, T.R.; and Zaloga, G.P. Bedside analysis of arterial blood gases and electrolytes during and after cardiac surgery. J. Clin. Anesthesiol. 1: 248-252, 1989.

28. Bashein, G.; Greydanus, W.K.; and Kenny, M.A. Evaluation of a blood gas and chemistry monitor for use during surgery. Anesthesiology 70: 123-127, 1989.

29. Nicolson, S.C.; Jobes, D.R.; Steven, J.M.; et al. Evaluation of a user-operated patient-side blood gas and chemistry monitor in children undergoing cardiac surgery. J. Cardiothorac. Anesth. 3: 741-744, 1989.

Gary P. Zaloga, M.D., FCCM is professor of anesthesia and medicine, department of anesthesia, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, N.C.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:MLO Special Issue: Point-of-Care Testing
Author:Zaloga, Gary P.
Publication:Medical Laboratory Observer
Date:Sep 1, 1991
Words:5824
Previous Article:Why testing is being moved to the site of patient care.
Next Article:Technology's answer to labor and resource constraints.
Topics:


Related Articles
Self testing: a big future?
Stat testing in the new CLIA era.
A collaborative approach to managing risk.
Why testing is being moved to the site of patient care.
Laboratory medicine: a 25-year retrospective.
Stats: tolerable for some, a major headache for others.
Managing information from bedside testing.
Point-of-care testing in pediatric hospitals.
How to check costs and quality of point-of-care testing.
Tips for managing your POCT program.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters