Masking of heparin activity in the activated coagulation time (ACT) by platelet procoagulant activity.

The effect of platelet procoagulant activity in the Activated Coagulation Time (ACT) was measured in whole blood anticoagulated with various levels of heparin before or after reversal with protamine. Similar studies were carried out on blood anticoagulated with hirudin to distinguish procoagulant activity from heparin neutralization in platelet preparations. At 0.5-1.0 units/mL antithrombin activity with heparin or hirudin, the ACT was lowered progressively by the addition of increasing concentrations of lysed platelets to as much as 20 seconds below the baseline clotting time obtained with unanticoagulated blood samples. Neutralization of higher concentrations of heparin with protamine produced an ACT below baseline in the presence of lysed platelets. Aprotinin (400 KIU/mL) prolonged the ACT slightly in heparinized whole blood, but did not prevent the lowering of the ACT by lysed platelets to baseline or below. Recirculation of heparinized whole blood in a simulated cardiopulmonary bypass circuit generated platelet microparticles detected by flow cytometry. An increase in platelet microparticles was associated with a decrease in the amount of protamine needed to reach the baseline ACT in blood samples removed from the circuit at various time points during recirculation. A chromogenic anti-Factor Xa assay of heparin did not show a change with increasing microparticle concentration during recirculation. These findings indicate a masking of heparin activity by the procoagulant activity of platelet membrane microparticles that could affect reversal of heparin based on the ACT.