Novel, bedside, tissue factor-dependent clotting assay permits improved assessment of combination antithrombotic and antiplatelet therapy.

ABSTRACT

BACKGROUND: Because optimal use of combinations of antiplatelet and antithrombotic drugs requires improved methods for assessment of therapeutic efficacy, we developed an assay designed to increase sensitivity that is based on initiation of clotting by tissue factor in minimally altered whole blood. METHODS AND RESULTS: Blood samples were obtained from healthy subjects, and the contact pathway of coagulation was inhibited with corn trypsin inhibitor (a specific factor XIIa inhibitor without effect on other coagulation factors). Clotting was initiated with relipidated tissue factor and detected with a Hemochron ACT instrument. Results were reproducible with samples from 25 healthy volunteers (mean time to clot, 125+/-17 seconds). Blood was also exposed to pharmacological concentrations of antithrombotic and antiplatelet agents in vitro. Heparin (0.25 anti-IIa/Xa U/mL) prolonged the time to clot by 2.4-fold (172 seconds, P<0.05); hirudin (1.0 anti-IIa U/mL), by 3-fold (250 seconds P<0.05); and enoxaparin (0.6 anti-Xa U/mL), by 2 -fold (123 seconds, P<0.05). Additive effects of antiplatelet agents were readily detectable with both heparin and hirudin. Thus, addition of 3 microg/mL abciximab to 1.0 anti-IIa/Xa U/mL heparin and to 1.0 anti-IIa U/mL hirudin further prolonged the times to clot by 140 and 67 seconds, respectively (P<0.05 for each). Addition of abciximab to enoxaparin did not further prolong the time to clot (increment, 13 seconds; P=NS). CONCLUSIONS: The assay developed should facilitate improved dose selection, titration, and monitoring of combination antithrombotic and antiplatelet treatment regimens.