A comparison of the effects of aspirin and dipyridamole on platelet aggregation in vivo and ex vivo

The effects of aspirin and dipyridamole, two drugs which inhibit platelet function, are different when tested in vivo and ex vivo. We have developed an in vivo test of platelet aggregation and have used this to compare the effect of aspirin and dipyridamole on collagen-induced platelet aggregation both in vivo and ex vivo. Aspirin was more effective when tested ex vivo than when tested in vivo. On the other hand, dipyridamole inhibited collagen-induced platelet aggregation in vivo and in heparinized plasma ex vivo, but had no effect when tested in citrated plasma ex vivo.