Use of a novel platelet function analyzer (PFA-100) with high sensitivity to disturbances in von Willebrand factor to screen for von Willebrand's disease and other disorders.

Abstract

The PFA-100 is a new platelet function analyzer which uses whole blood and high shear stress blood flow to simulate primary hemostasis and assess platelet function. A small volume of blood is introduced into a disposable cartridge, and forced through a capillary tube. Platelet adhesion and aggregation is then initiated following exposure to either collagen/ADP [C/ADP] or collagen/epinephrine [C/Epi] coated membranes. Movement of blood through the capillary, and its subsequent occlusion is monitored and yields the measured endpoint (closure time [CT] in seconds). Using two approaches, we assessed the sensitivity of this system to disturbances in the function of von Willebrand Factor (VWF). Firstly, we assessed the ability of the PFA-100 to detect the presence of von Willebrands Disease (VWD). Using normal individuals (N = 18), CTs (in seconds; mean [range = mean +/- 2SD]) were (i) C/ADP, 95 [66-124], (ii) C/Epi, 128 [98-158]. A panel of 47 patients undergoing evaluation for clinical hemostatic defects inclusive of VWD were also evaluated. All samples from patients confirmed to have VWD following specific VWF studies [N = 9; 3 x Type 1, 1 x Type 3, 1 x Type 2A, 4 x Type 2B] gave prolonged CTs (>/= 200 s) for both C/ADP and C/Epi membranes; in contrast, all patients yielding normal CT values were found to yield normal VWF results (i.e., were found not to suffer from VWD). Patients with hemophilia (1 x hemophilia A, 1 x hemophilia C) gave normal PFA-100 CT, while those with clinical thrombocytopaenia (N = 3) gave prolonged PFA-100 CT. A number of other patient samples also gave abnormal CT values which in some cases could be linked to recent aspirin consumption. In the second evaluation process, and using normal blood, we have assessed the ability of various antibodies to influence the CT. Of the monoclonal antibody panel tested [N = 20], only a proportion of those against VWF [6/10] or gp1b/IX [CD42; 2/5] were found to be inhibitory (i.e., prolonged the CT). Data using polyclonal antibodies (against platelets, VWF, fibrinogen and fibronectin) is more complex but largely confirms the sensitivity of the system to VWF. On the basis of these results, we conclude that the PFA-100 is highly sensitive to disturbances in VWF and to the presence of VWD and may thus provide a valuable screening test for VWD in certain specific circumstances (i.e., acute need conditions or remote testing sites; normal CT result generally effective as negative predictor, i.e. not severe VWD). However, since abnormal CT values were obtained in clinical situations other than evident VWD, the PFA-100 cannot be used as a specific diagnostic tool to establish the presence of VWD. Thus, any abnormal PFA-100 CT result should be thoroughly evaluated by follow-up specific testing to establish the true clinical disorder affecting the individual under investigation, inclusive of appropriate VWF assays if VWD is clinically suspected.