BACKGROUND Ecarin Clotting Time (ECT) assay specifically determines the inhibition of meizothrombin by direct thrombin inhibitors (DTI). Blood coagulation factor levels lowered by vitamin K antagonists (VKA) may prolong ECT. Concomitant treatment of VKA with DTI may influence differently the two published ECT methods. METHODS Lepirudin (100-3,000 ng/ml), argatroban (300--3,000 ng/ml) and melagatran (30--1000 ng/ml) were added to normal plasma (NP; n=12) samples and to plasma of patients on stable vitamin K antagonist therapy with warfarin (VKAP; n=12). ECT assays were performed according to [5] (method 1) and according to [6] (method 2). Data were subjected to multifactorial variance analysis. RESULTS Normal ranges were 35.5+/-2.8 s in NP versus 31.8+/-1.2 s in VKAP with method 1 (p< 0.001) and 44.3+/-3.9 s in NP vs. 51.4+/-8.3 s in VKAP with method 2 (p< 0.004). Besides the inhibitors (p<0.0001), the method used (p<0.0001) and the group (NP vs. VKAP, p=0.003) had an influence on the ECT. Inhibitors (p< 0.02) or method used (p< 0.03) and the group (NP vs. VKAP, p=0.0001) influenced also the ECT ratio. CONCLUSIONS Both ECT methods are suitable for monitoring different DTIs over a large linear range with both methods during concomitant treatments with vitamin K antagonists. The ECT ratio improves but not abolishes the differences between the methods. Additive effects of vitamin K antagonists on ECT methods have to be taken into consideration in clinical routine.