The platelet function screen or PFA-100® test is an in vitro system capable of detecting platelet dysfunction in a citrated whole blood sample under high shear flow conditions. The blood sample is made to flow through an aperture in a membrane coated with either collagen and epinephrine (COL/EPI) or collagen and ADP (COL/ADP). The time taken for blood to form a platelet plug that occludes the aperture is called the closure time and is an indication of platelet function. The system is able to detect defects in platelet adhesion, platelet aggregation, and von Willebrand factor, and is thus highly sensitive to inherited and acquired defects in platelet function, as well as von Willebrand disease (vWD). Sensitivity for diagnosis of vWD has varied from 88% to 100% in different studies; the only sub-types of vWD not detected have included occasional cases of Type 1 and the rare Type 2N. The new platelet function screen has been shown to be significantly more sensitive and specific than the bleeding time in detection of platelet function disorders and vWD and is more sensitive than platelet aggregation in detection of vWD and inherited platelet function defects.
Normal ranges for closure times with COL/EPI and COL/ADP are 80 -192 and 60 -112 seconds respectively.
Normal closure times with both COL/EPI and COL/ADP indicate normal platelet function.
A prolonged closure time with COL/EPI and normal result with COL/ADP is characteristic of platelet dysfunction secondary to aspirin or aspirin-like drugs. This pattern may also be found in mild hereditary platelet function disorders such as storage pool disease, and in occasional cases of mild type 1 vWD. Use of platelet-inhibitory drugs should be ruled out in these cases prior to further laboratory testing.
Prolonged closure times with both COL/EPI and COL/ADP are typical of either vWD or significant hereditary platelet function disorders, warranting further laboratory investigation including a vWD panel (ristocetin cofactor, von Willebrand factor antigen, factor VIII and platelet aggregation).
Other factors producing an abnormal result on platelet function screen include significant thrombocytopenia and anemia. Thus the test is not useful in patients with a platelet count less than 100,000 / uL, or hematocrit less than 30%. One 5.0 mL sodium citrate (light blue top) tube is required (3.2% sodium citrate is preferred). The sample must be received by the laboratory within 3 hours of collection.
The Platelet Function Screen should not be used for random pre-operative screening; its use should be restricted to those patients with a history suggestive of hereditary or acquired platelet dysfunction, or vWD. The algorithm below delineates the recommended approach to the laboratory investigation of such patients.
