Methods and devices for determining factor Xa inhibitors, in particular heparins and fractionated or low-molecular-weight heparins, as well as direct factor Xa inhibitors in blood samples. The methods include contacting a blood sample with a detection reagent that contains at least one thrombin substrate having a peptide residue that can be cleaved by thrombin and is amidically bound via the carboxyl end to an electrogenic substance, and with a known amount of factor X reagent and an activator reagent which induces the conversion of factor X into factor Xa. Subsequently, in a second step, the amount or activity of the electrogenic substance that is cleaved from the thrombin substrate by the factor Xa-mediated thrombin activation and/or the time course thereof is determined as the measurement signal using electrochemical methods. In a third step, the amount of the factor Xa inhibitor in the sample of the blood to be analyzed or a measured quantity that correlates therewith, in particular a clotting time that correlates therewith, is determined on the basis of this measurement signal.