Platelet involvement in the activated coagulation time of heparinized blood.

The activated coagulation time (ACT), commonly used during cardiopulmonary bypass (CPB), is assumed to measure only differences in heparin levels. Studies were undertaken to determine whether platelet activation/inhibition might also influence the test. When either the platelet inhibitor prostacyclin or the platelet activator adenosine diphosphate (ADP) was added to healthy donor blood containing 2 IU of heparin per ml, there was significant prolongation of the ACT (prostacyclin: mean prolongation, 60.6%; ADP: mean prolongation, 52.3%). In blood taken from the extracorporeal circuit of ten CPB cases, the prolongation with the platelet inhibitor carbacyclin, a prostacyclin analogue, was infinite. It is concluded that the ACT as used during CPB must be interpreted as a measure of both platelet procoagulant activity and heparin activity. Furthermore, this dual sensitivity, while fortuitous, is probably advantageous; patients with hyporeactive platelets will automatically receive less heparin.