MASKING OF HEPARIN ACTIVITY IN THE ACTIVATED COAGULATION TIME (ACT) BY PLATELET PROCOAGULANT ACTIVITY

The effect of platelet procoagulant activity in the Activated Coagulat ion Time (ACT) was measured in whole blood anticoagulated with various levels of heparin before or after reversal with protamine. Similar st udies were carried out on blood anticoagulated with hirudin to disting uish procoagulant activity from heparin neutralization in platelet pre parations. At 0.5 - 1.0 units/mL antithrombin activity with heparin or hirudin, the ACT was lowered progressively by the addition of increas ing concentrations of lysed platelets to as much as 20 seconds below t he baseline clotting time obtained with unanticoagulated blood samples . Neutralization of higher concentrations of heparin with protamine pr oduced an ACT below baseline in the presence of lysed platelets. Aprot inin (400 KIU/mL) prolonged the ACT slightly in heparinized whole bloo d, but did not prevent the lowering of the ACT by lysed platelets to b aseline or below. Recirculation of heparinized whole blood in a simula ted cardiopulmonary bypass circuit generated platelet microparticles d etected 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 X(a) assay of heparin did not show a change with increasing microparticle concen tration during recirculation. These findings indicate a masking of hep arin activity by the procoagulant activity of platelet membrane microp articles that could affect reversal of heparin based on the ACT.