Comparing r-Tissue Factor and Mammalian Tissue Reagents for Prothrombin Time

We compared two recombinant human tissue factor (r-TF) prothrombin time (PT) reagents with two commercially available mammalian PT reagents. The two r-TF reagents were more sensitive to the lower concentrations of factors II, VII, IX, andX. The r-TF preparations were found to give longer clotting times than the mammalian thromboplastins when plasma samples from oral anticoagulant–treated patients were tested. When international normalized ratios (INRs) were calculated, however, no significant differences were noted among reagents when INR values were less than or equal to 3.0; however, at INR values greater than 3.0, differences were noted. The r-TF was more sensitive to the thrombin inhibitors than were heparins. Because of different origins and composition, the currently available thromboplastin reagents are not equivalent, and the results obtained from these should be interpreted carefully.