Objective The inhibitory response to clopidogrel considerably varies among individuals and clopidogrel resistance is a risk factor for thrombotic events in patients with cardiovascular disease. Based on the platelet aggregation evaluated by the VerifyNow P2Y12 Assay, the present study investigated clopidogrel resistance and the effect of cilostazol addition. Methods We measured the ability of 20 μM ADP to aggregate platelets using the VerifyNow P2Y12 Assay. Clopidogrel resistance was defined as % inhibition of <20% in this assay. Patients We examined 77 patients (53 men and 24 women, aged 65.8±9.9 years) with ischemic stroke or carotid artery stenting who received clopidogrel (75 mg) for >7 days at our hospital between October 2009 and March 2010. For 62 patients (42 men and 20 women, aged 65.3±9.9 years) 75 mg clopidogrel alone was administered (clopidogrel only group); the other 15 patients (11 men and 4 women, aged 67.9±9.9 years) received 75 mg of clopidogrel plus 100 or 200 mg of cilostazol (cilostazol combination group). Results Clopidogrel resistance was identified in 18 (29%) of the 62 patients in the clopidogrel only group. The percent inhibition was significantly higher in the cilostazol combination group than in the clopidogrel only group (41.7±28.0% vs. 64.9±22.7%, p=0.005). None of the patients in the cilostazol combination group had % inhibition of <20%. Conclusion Clopidogrel resistance developed in 29% of patients given clopidogrel alone. The addition of cilostazol to clopidogrel may have intensified platelet inhibition.

Objective Clopidogrel has potent antiplatelet effects, but recent interest has focused on clopidogrel resistance, in which platelet function is not inhibited despite taking the drug. This study evaluated clopidogrel resistance in ischemic stroke patients. Methods After taking oral clopidogrel 75 mg/day for "1 week, platelet aggregometry was performed by turbidimetry in all patients, and by a screen filtration pressure method using whole blood in 37 patients. Using turbidimetry, resistance was defined as platelet maximum aggregation rate "34% with aggregationinducing agent ADP 1 μmol/L, or "66% with ADP 4 μmol/L. Using the screen filtration pressure method, resistance was defined as a minimum concentration of !3 μmol/L ADP to induce secondary aggregation of platelets. Patients This study was conducted in 72 patients (52 men, 20 women; mean age, 69 ± 8 years; range, 50-84 years) with non-cardiogenic ischemic cerebrovascular disease. Results Based on turbidimetry, the rate of clopidogrel resistance was 8.3% with ADP 1 μmol/L and 18.1% with 4 μmol/L. Based on the screen filtration pressure, the rate of clopidogrel resistance was 8.1%. The differences between turbidimetry and screen filtration pressure methods, regarding the measurement of the presence of resistance in the same patient, were observed. Conclusion Clopidogrel resistance varies greatly depending on the method of measuring platelet aggregation and the definition of resistance. Rates of 8-18% were obtained using our methods and criteria.

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