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Platelet Function Analyzer
The platelet function analyzer (PFA)-100 is the most widely used global test of primary hemostasis. It performs an in vitro test of platelet plug formation, referred to as the PFA, by measuring the time to occlusion of a window in a coated membrane through which blood is forced at high shear rate. It was designed to replace the bleeding time (BT) test, which is no longer recommended.
Methods
In the PFA-100 instrument (Siemens, Tarrytown, NY), 0.8 mL of citrated whole blood is forced at high shear rate (5000–6000/second) through a window in a membrane coated with collagen and epinephrine or collagen and adenosine diphosphate. The time for platelets to adhere and aggregate to produce closure is referred to as closure time (CT). The PFA-200 retains the same measurement system.
Test Interpretation
CT appears to be inversely correlated with functional von Willebrand factor, in particular with ristocetin cofactor, and is equivalent to or better than BT as a screening tool for von Willebrand disease (VWD). Its sensitivity for VWD Types 2A, 2B, and 3 is greater than 98%, but it is less sensitive to Type 1 VWD. The PFA is sensitive to severe platelet function defects but lacks sensitivity for more common platelet function disorders. Like the BT, the CT lacks sufficient sensitivity and specificity to be used alone for screening of individuals for platelet disorders and should be used along with clinical history to determine which patients should undergo diagnostic studies. Few studies have as yet been undertaken to establish a clear role for the PFA in predicting clinical outcomes and monitoring therapy. The use of the PFA in assessing response to antiplatelet medications is discussed in Chapter 143 .
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