Plasma salicylate level and aspirin resistance in survivors of myocardial infarction.

To investigate the effect of aspirin on the platelets of survivors of myocardial infarction we correlated plasma salicylate level with platelet reactivity in ten patients and ten normal controls. The patients and controls were tested at the end of 2 week periods on 75, 150 and 300 mg aspirin daily by mouth. Platelet reactivity was measured, under high shear stress conditions, using cartridges containing adrenaline and adenosine diphosphate in a PFA-100 platelet function analyser. The time taken by the developing platelet aggregate to close an aperture in the collagen membrane of the cartridge, the closure time, was taken as an index of platelet reactivity. There was no difference in baseline haematocrit, platelet count or plasma vWF antigen level between the groups. There was a dose-dependent increase in closure time of the adrenaline containing cartridge in the controls (P < 0.001), but not in the patients (P = 0.08), compatible with a reduced anti-platelet effect of aspirin in the patients. Furthermore, plasma salicylate level was higher in the patient group (P < 0.05).