Some aspirin-treated patients have a blunted pharmacological response predisposing to clinical failure. The investigators hypothesize that the blunted response often results from increased rate of platelet production and some failures will be prevented by administering aspirin twice daily. The overall objective is to develop a valid method to quantify platelet production (without the use of radioactive isotopes) in order to examine the hypothesis that enhanced platelet production is a common cause of poor aspirin responsiveness in patients with atherothrombosis.

The specific objective of this study is to validate the Cayman Chemical serum thromboxane immunoassay as a measure of platelet production in healthy subjects who are treated with aspirin by demonstrating that the recovery of their serum thromboxane B2 levels occurs at a rate of about 10% per day after aspirin cessation.

Inclusion Criteria:

• Healthy non-smoking volunteers;
• Age ≥ 18 years;

Exclusion Criteria:

• Allergy or intolerance to aspirin;
• Current pregnancy;
• Use of aspirin or drugs interfering with platelet function (NSAIDs, anticoagulants) within one week of study enrolment.