Abstract 3284: Limited Accuracy of a New Point-Of-Service Assay for Assessing Inhibition of Platelet Aggregation in Response to Clopidogrel and Aspirin
Suboptimal response (“resistance”) to the platelet inhibitory effects of clopidogrel has been implicated in increased risk of coronary thrombotic events in various clinical settings. Traditional measurement of platelet inhibition involves cumbersome procedures and thus a new point-of-service (POS) assay specific to P2Y12 receptor antagonists was recently made available (Accumetrics VerifyNow). The purpose of this study was to compare this assay to traditional platelet inhibition measures.
Methods 50 healthy subjects (20 M, 30 F) aged 40 – 65 taking no medications and with no history of CV or other disease provided blood samples at drug-free baseline and at 2, 6, 24 and 30 hours after dosing with clopidogrel (loading dose 300 mg; 75 mg at 24 hrs) and ASA 325 mg. Platelet inhibition was assessed with 5 and 20 uM ADP as the agonist using platelet rich plasma (PRP) and using whole blood with Ultegra. Data were analyzed as continuous variables as well as classifying subjects according to degree of inhibition: “resistant” < 10% inhibition; “hyporesponder” 10–29% inhibition; normal response = 30–50% inhibition; “hyper-responder” >50% inhibition. Percent inhibition was calculated as baseline minus post-drug inhibition.
Results Though the population mean inhibition assessment was very similar between POS and PRP methods were very similar (e.g. Hour-30 POS = 50.8 ± 27.7% vs 5 uM ADP in PRP = 47.8 ± 13.3%), at all points in time, the correlations, while statistically significant, were rather modest (Pearson and Spearman correlation coefficients ranged from 0.33 to 0.69). Examined categorically 37 of 49 (75.6%) were misclassified by VerfiyNow including 7 of 49 (14.2%) who were misclassified by ≥ 2 categories.
Conclusions Though the new assay yields population means comparable to the conventional method of measuring platelet aggregation with PRP, the results in individual subjects with the POS device differ substantially from PRP. The results of the POS measurements may therefore not be applicable to data derived from prior clinical studies demonstrating variable response to clopidogrel as predictive of clinical events. The utility of the POS device in predicting clinical outcomes needs to be verified independently and measurements using PRP remain the gold standard.