Abstract: The present invention relates to a method to determine the concentration of anticoagulants as a function of the inhibition of thrombin formation in an assay mixture. The assay mixture is composed of two reagents and the diluted plasma sample. The diluted plasma sample containing an anticoagulatory substance is mixed with a clotting factor reagent, which is a combination of an excess amount of purified coagulation factors of the endogenous system, phospholipids and a weak thrombin inhibitor, which can be potentiated by an anticoagulatory substance. Then a coagulation cascade activator and calcium chloride, an activator reagent, is added to the mixture and formed thrombin is measured after a chosen incubation time. A proportionality between the inhibition of the thrombin formation and the anticoagulatory active components allows for a determination of the concentration of anticoagulatory active components in the sample.