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PFA-100 platelet function analyzer.

Q What is the Dade Behring PFA-100 platelet function analyzer? What does it measure? What if the patient is on anti-platelet drugs? What is its application in head trauma injury?

A The Dade Behring PFA-100 platelet function analyzer is used to evaluate primary hemostasis in citrated whole blood. It measures ex vivo platelet binding to collagen/epinephrine (CEPI) or collagen/adenosine diphosphate (CADP). It contains a high-shear flow system to allow blood to pass through a membrane with an aperture. The membrane is coated with collagen fibrils and one of activators epinephrine or ADP (adenosine diphosphate). The platelet activation occurs as blood flows through the aperture. When citrated whole blood mixes with activators, platelets start to form aggregates. The time taken for blood to form a platelet plug that occludes the aperture is an indication of platelet function and is referred to as the closure time (CT). PFA-100 is only a screening test to determine whether abnormal platelet function is present in the patient. There is no critical value at which you can predict whether the patient will bleed in the surgery. If the patient is on an anti-platelet drug, you can only say that the patient's platelet function is compromised due to drugs, using PFA-100, but you cannot predict the bleeding tendency.

Von Willebrand Factor, or vWF, is the key adhesive protein that mediates platelet adhesion and aggregation in the PFA-100 test cartridge. CT is highly sensitive to von Willebrand disease, or vWD. CT is also highly sensitive to qualitative and quantitative defects to platelet receptors that mediate adhesion and aggregation. Thus, CT is also sensitive to inherited or acquired defects in platelet function. If aspirin is the cause of platelet dysfunction, however, only the CT of collagen/epinephrine will be prolonged but not collagen/adenosine diphosphate.

Other factors affecting the CT include low hematocrit (<35%) or low platelet counts (<150,000/mL). In these situations, the prolonged CT may not reflect abnormal function of platelets. Bleeding time (BT) was designed to evaluate platelet function. BT, however, is very imprecise with a high degree of variation. Currently, PFA-100 has almost replaced BT in evaluating platelet function for pre-surgical screening. Abnormal hemostasis is associated with trauma-related morbidity and mortality. Platelet function is one of the key factors in the maintenance of hemostasis. Severe injury usually results in increased platelet activation and function. One study found that the combination of increased platelet activation with decreased function was associated with increased mortality.

--Min Xu, MD, PhD

Clinical Pathologist

Department of Laboratories

Children's Hospital and Regional Medical Center

Seattle, WA

References

1. Kundu SK, et al. Description of an in vitro platelet function analyzer--PFA-100. Semin Thromb Hemost. 1995;21(suppl 2):106-112.

2. Jacoby RC, et al. Platelet activation and function after trauma. J Trauma. 2001;51(4): 639-647.
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Title Annotation:Answering your questions
Author:Baer, Daniel M.
Publication:Medical Laboratory Observer
Geographic Code:1USA
Date:Nov 1, 2006
Words:466
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