A quantitative method for determining the plasma levels of thrombin-specific inhibitors is based on the quantitative thrombin time using plasma dilutions, excess fibrinogen and thrombin. The plasma dilutions and excess fibrinogen act in concert to eliminate the effects that coagulopathies have on standard coagulation tests. The method is relatively simple and provides superior results to standard conventional tests. The method is suitable for performance in clinical hematology laboratories on a routine basis using commercially available instrumentation.