Responsiveness to aspirin in patients with unstable angina pectoris by whole blood aggregometry

To evaluate aspirin responsiveness in patients with unstable angina pectoris (UAP) by whole blood aggregometry. Another goal was to differentiate aspirin-resistant patients into pharmacokinetic or pharmacodynamic type.

We measured platelet aggregation by determining impedance values in 70 normal volunteers and 104 UAP patients on aspirin (100 mg/day > or = 7 days) in four inducing conditions [1 microg/ml collagen, 2 microg/ml collagen, 5 micromol/l adenosine diphosphate (ADP) and 10 micromol/l ADP]. We calculated a cut-off value based on data from normal volunteers to define aspirin responsiveness in cases. Then, the correlation and agreement between the results in the four conditions was analysed to choose a preferred inducing condition for identification of aspirin resistance. Aliquots from all samples were incubated with 0.1 mmol/l aspirin and measured again for aspirin-resistant classification.

Aspirin resistance was observed in 38 patients (36.5%), 51 patients (49.0%), 67 patients (64.4%) and 67 patients (64.4%), respectively, for 1 microg/ml collagen, 2 microg/ml collagen, 5 micromol/l ADP and 10 micromol/l ADP among 104 patients. Collagen at low concentration was suggested as a preferred agent for detecting aspirin inhibitory effect according to the coefficient of sensitivity. After incubation, only three among 38 aspirin-resistant patients showed normal platelet aggregation and were classified into pharmacodynamic type.

In the presence of collagen at low concentration (1 microg/ml), the prevalence of aspirin resistance is about 36.5% in UAP patients, and according to a classification specific for resistant patients, most of the aspirin 'resistance' is just because of pharmacokinetic issues.