A Comparison of the Incidence of PFA-100^® Aspirin Non-Responders with 3.2% and 3.8% Citrated Blood (with and without Addition of in Vitro Aspirin)

Abstract number: P1566

Harrison¹ P, Segal¹ H, Silver² L, Rothwell² PM

¹¹Oxford Haemophilia Centre & Thrombosis Unit, Churchill Hospital, Oxford, UK ¹¹Oxford Haemophilia Centre & Thrombosis Unit, Churchill Hospital, Oxford, UK ²²Stroke Prevention Research Unit, Department of Clinical Neurology, University of Oxford, UK

The PFA-100^® identifies some patients as aspirin (ASA) non-responders, but the sensitivity is higher than with aggregometry. This may be partly caused by the differences in test principles and/or the impact of variables (e.g. VWF) on the Closure Time (CT). Also there are known CT differences between 3.2% and 3.8% citrated blood and patients may also be ASA non-compliant or under-dosed resulting in potential overestimation of ASA non-responders. We have studied patients taking daily ASA (75–300 mg) after TIA, ischaemic stroke, and myocardial infarction (MI) in an ongoing population-based incidence study that started in 2002 (Oxford Vascular Study). Citrated (3.2%) blood was tested within both Collagen/ADP (CADP) and Collagen Epinephrine (CEPI) cartridges in the PFA-100, a high shear test of platelet activity. An ASA non-response was defined as the absence of a prolonged CT (<164 secs) within the CEPI cartridge. From May 2003, 133/546 (24.3%) consecutive samples have been identified as ASA non-responders, and we have since performed additional immediate CEPI cartridge analysis using 3.2% citrated blood (after in vitro incubation with 100 µM aspirin, n= 114/133) and 3.8% citrated blood (n= 91/133) from samples (if available) taken simultaneously. 39/114 (34.2%) only remained ASA non-responsive (CT < 164 secs) after incubation with in vitro ASA. 28/91 (30.8%) only remained ASA non-responsive using 3.8% citrate (CT < 202 secs). 10/76 (13.2%) remained ASA non-responsive by both additional tests. ASA non-responsiveness in the PFA-100 is therefore significantly influenced by citrate concentration and calcium levels. Incubation of samples with in vitro ASA potentially eliminates samples from patients who may be ASA under-dosed or non-compliant. As we are in the process of collecting outcome data from ASA responders and non-responders, both variables (anticoagulant concentration and ASA under-dosage/non-compliance) are probably important in determining the true prognostic value of the PFA-100 test.