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Background: Platelet activity may be measured remotely as surface P-selectin expression and varies between antiplatelet agents and patient responses to them.Methods: In TARDIS, patients with acute non-cardioembolic ischaemic stroke or transient ischaemic attack were randomised to intensive (combined aspirin, clopidogrel and dipyridamole, ACD) or guideline antiplatelets (combined aspirin and dipyridamole, AD; or clopidogrel alone, C). On-treatment platelet function, assessed as surface P-selectin expression, was measured remotely at 7 days post randomisation. The tests detected the effects of aspirin and of clopidogrel (Platelet Solutions Ltd). Data are mean difference (MD) with 95% confidence intervals (CI).Results: 626 patients (intensive 312, guideline AD 142, C 172) had P-selectin taken at day 7: mean age 68.3 (10.0), 66.3% male. In a comparison of on-treatment P-selectin (Table 1), the aspirin test showed that platelet reactivity was lower with ACD than AD (p=0.005) and C (p<0.001). ACD failed to suppress P-selectin levels below 500 units in 13 (4.4%) patients. Conversely, the clopidogrel test demonstrated that platelet reactivity was higher in patients receiving ACD than C (p=0.020). Treatment with C did not suppress P-selectin levels below 860 units in 22 (12.9%) patients.Conclusion: The aspirin test indicated that ACD was more effective in supressing stimulated platelet surface P-selectin than AD. However, the clopidogrel test showed treatment with C to be more effective than ACD, although 12.9% of patients had high residual platelet reactivity.