Direct thrombin inhibitors (DTIs) represent a new class of promising anticoagulation agents. The DTIs frequently are used to provide initial anticoagulation, with long-term therapy requiring eventual transition to coumarins. Unfortunately, DTIs not only prolong the activated partial thromboplastin time but also can affect international normalized ratio (INR) values. We approximated the DTI effect on INRs by each drug to pooled plasma at concentrations between 0.1 and 1.2 microg/mL. We then concurrently tested these samples using 14 prothrombin time (PT) reagents. By using repeated measures analysis of variance, we found significant differences (P < .05) between the median INRs for lepirudin and argatroban for all PT reagents, between lepirudin and bivalirudin for all reagents except PT-Fibrinogen HS Plus (P = .07), and between bivalirudin and argatroban for all reagents except Thromborel S (P = .05). The DTI effect on INRs was dependent on drug, drug concentration, and reagent. Argatroban had the most effect on INRs, while lepirudin had the least effect. Reagents with a lower international sensitivity index were less affected by DTI; ThromboMax HS was the least sensitive PT reagent to any DTI.