In vitro and ex vivo effects of aspirin in patients on a low-dose aspirin therapy.

In 19 patients on a low-dose aspirin therapy with 100 mg/d, an insufficient effect of aspirin was observed in five patients (aggregations induced by arachidonic acid and collagen, thromboxane B2-formation in serum and after collagen). Aspirin added in vitro increased the inhibition to a degree comparable to that seen in the other 14 patients, i.e. the insufficient effect could be due to a lack of compliance or to a reduced availability of the drug. In another 20 patients there was a good inhibitory effect of aspirin; additional aspirin did not increase the inhibition of arachidonic acid-induced aggregation and serum-thromboxane B2, but slightly increased collagen-induced aggregation and thromboxane B2 formation. The effect was the same, whether the aspirin was given in vivo or added in vitro.