PB0320 - Estimation of prevalence and factors affecting Aspirin / Clopidogrel resistance in Indian patients evaluated by light transmission aggregometry and VerifyNow

Background: Antiplatelet resistance may be a major cause of recurrent arterial thrombosis in patients on Aspirin or Clopidogrel secondary prophylaxis. Hence identification of antiplatelet responsiveness by measuring platelet function is important.

Aims: To determine the prevalence of Aspirin and Clopidogrel resistance in Indian patients with previous arterial thrombosis, compare the performance characteristics of VerifyNow to the Gold Standard Light Transmission Aggregometry(LTA) to identify antiplatelet resistance and determine the association between antiplatelet resistance and clinical characteristics.

Methods: Platelet function testing was performed by LTA and VerifyNow in patients with arterial thrombosis on Aspirin (75mg OD, n=54) and/or Clopidogrel (75mg OD, n=27) secondary prophylaxis. Aspirin resistance on LTA was defined as >20% platelet aggregation response to 1mM Arachidonic Acid. Clopidogrel resistance on LTA was defined as >30% platelet aggregation response to 2µM Adenosine Diphosphate. VerifyNow results for Aspirin and Clopidogrel response were expressed as Aspirin Reaction Units(ARU) and P2Y12 Reaction Units(PRU) respectively. The cut-off for aspirin and clopidogrel resistance was >550 ARU and >208 PRU respectively. Fisher's Exact Test was used to estimate association between antiplatelet resistance and various clinical characteristics.

Results: The prevalence of Aspirin resistance was 20.3% using LTA and 19.6% using VerifyNow. The prevalence of Clopidogrel Resistance was 25.9% using LTA and 29.6% using VerifyNow. Considering LTA as the gold standard, VerifyNow had 79.5% sensitivity, 18.2% specificity for aspirin resistance (using >550 ARU cut-off) and 80% sensitivity, 57.1% specificity for clopidogrel resistance (using >208 PRU cut-off). There was no significant association between Aspirin / Clopidogrel resistance and history of ischemic stroke, coronary artery disease, recurrent thrombosis, smoking, alcohol, hypertension, diabetes, dyslipidemia or homocysteinemia (Table 1).

Conclusion(s): Although VerifyNow had 80% sensitivity, the specificity for identifying aspirin and clopidogrel resistance using the recommended cut-offs >550 ARU and >208 PRU respectively was low. Further studies may be required to identify relevant cut-offs suitable for each population.