BACKGROUND Tissue factor is the active ingredient in thromboplastin reagents used to perform prothrombin time (PT) clotting tests to monitor oral anticoagulant therapy and to screen for clotting factor deficiencies. Thromboplastins are complex mixtures prepared from extracts of brain or placenta, although newer thromboplastins contain recombinant tissue factor incorporated into phospholipid vesicles. Thromboplastins can vary widely in their sensitivity to reductions in the levels of vitamin K-dependent clotting factors. A system to compensate for this, the International Sensitivity Index (ISI) and International Normalized Ratio (INR), has revolutionized the monitoring of oral anticoagulant therapy. The INR system is also sometimes used to monitor coagulopathies in patients with sepsis or liver failure, applications for which it was not originally designed and for which it has not been rigorously validated. OBJECTIVE To better understand thromboplastin performance, we systematically investigated which properties of recombinant thromboplastins influence their sensitivities to changes in the levels of specific clotting factors. RESULTS We now report that relative sensitivities to changes in the plasma levels of factors V, VII, X (FV, FVII, FX) and prothrombin are differentially influenced by a recombinant thromboplastin's content of phospholipid and sodium chloride. Furthermore, thromboplastins of similar ISI values may exhibit quite different sensitivities to each of these clotting factors. CONCLUSIONS Differing sensitivities of thromboplastin reagents to individual clotting factor levels have implications for monitoring of oral anticoagulant therapy and interpreting results of the PT assay.