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Platelet Function Testing: Auditing Local Practice and Broader Implications

BACKGROUND: Platelet function testing is a common test procedure used for assessing patients with mucocutaneous bruising and/or bleeding and for monitoring anti-platelet therapy. However, standardization of practice is poorly applied, and experts differ on several aspects of its application. OBJECTIVE: This study reports on a local audit of current practice in consideration of recent reports, expert opinion and current CLSI guidelines. METHODS: We undertook an assessment of our laboratory test practice for light transmission aggregometry (LTA) as a diagnostic screening process for platelet function, as well as performance of PFA-100 closure times used for screening primary haemostasis. For LTA testing, we wished to assess the validity or otherwise of platelet count adjustments using autologous platelet poor plasma (PPP), as used by some experts and as also recommended by the current CLSI guidelines for performance of platelet aggregation. For PFA-100 testing, we assessed the effect of different blood collection tubes. RESULTS: For most test cases undergoing LTA, platelet count adjustment using autologous PPP resulted in considerable diminution of detectable platelet function using several agonists, and in particular collagen, ADP and epinephrine. These effects could result in differing conclusions regarding the likelihood or severity of a platelet function disorder. For the PFA-100, different blood collection tubes resulted in slightly different closure times that could also potentially influence the conclusion of ‘normality’ or otherwise for investigated patients. CONCLUSIONS: This audit of local practice indicates that the process of platelet count adjustment using autologous PPP provides adverse outcomes related to identification of platelet dysfunction. Accordingly, we recommend that all laboratories validate this practice if used at their facility. For PFA-100 testing, local validation of the normal reference range is required according to local conditions and collection practice. Otherwise, laboratories may inappropriately identify platelet function disorders when these may not exist. ABBREVIATIONS USED: ADP - adenosine diphosphate; CLSI – Clinical and Laboratory Standards Institute; CTs - closure times; C/Epi - collagen/ epinephrine; C/ADP - collagen/adenosine diphosphate; Epi – epinephrine; fc -final concentration; LTA - light transmission aggregometry; PPP - platelet poor plasma; PRP - platelet rich plasma; VWD - von Willebrand disease; WBA - whole blood aggregometry.