Effects of low-dose aspirin on in vitro platelet aggregation in the early minutes after ingestion in normal subjects.

Aspirin interferes with platelet aggregation by inhibiting the metabolism of arachidonic acid to thromboxane A2. Although both high- and low-dose aspirin therapies are effective for secondary prophylaxis in patients with atherosclerotic vascular disease, the acute response to low-dose aspirin therapy is controversial. Eighteen volunteer subjects ingested 81, 162, or 324 mg of aspirin in a longitudinal crossover study design. Initial doses were randomly assigned and dosing intervals were separated by 2 weeks. Platelet aggregation in response to 0.9 mM arachidonic acid was measured at baseline, 15, 30, 60, and 90 minutes after ingestion. Thromboxane B2 production was assayed on simultaneously obtained samples after stimulation with arachidonic acid. The median inhibition of aggregation was 97%, 97%, and 97% 15 minutes after ingestion of 81, 162, and 324 mg, respectively. Four subjects had < 20% inhibition 15 minutes after ingesting 81 mg, but all 4 had > 90% inhibition after 30 minutes. Thromboxane B2 production declined by > 93% in all subjects at each dose. There was no difference between doses in inhibition of thromboxane B2 production.