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

Background-Because optimal use of combinations of antiplatelet and antithro mbotic drugs requires improved methods for assessment of therapeutic effica cy, 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 inhibito r (a specific factor XIIa inhibitor without effect on other coagulation fac tors). Clotting was initiated with relipidated tissue factor and detected w ith a Hemochron ACT instrument. Results were reproducible with samples from 25 healthy volunteers (mean time to clot, 125+/-17 seconds). Blood was als o exposed to pharmacological concentrations of antithrombotic and antiplate let agents in vitro. Heparin (0.25 anti-IIa/Xa U/mL) prolonged the rime to clot by 2.4-fold (172 seconds, P<0.05); hirudin (1.0 anti-IIa U/mL), by 3-f old (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 det ectable with both heparin and hirudin. Thus, addition of 3 <mu>g/mL abcixim ab 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 fo r each). Addition of abciximab to enoxaparin did not further prolong the ti me to clot (increment, 13 seconds; P=NS). Conclusions-The assay developed should facilitate improved dose selection, titration, and monitoring of combination antithrombotic and antiplatelet tr eatment regimens.