An assay based on free oscillating rheometry to measure the activity of coagulation factors is described. The method can be used in blood and plasma and is particularly suitable for screening and monitoring coagulation disturbances in point-of-care testing (POCT) in environments where quick analysis with minimal preanalytical work is needed. In this study the endpoint as clotting onset time (COT) is determined by a deviation from initial viscoelastic properties of an oscillating sample. The model system entails the clotting of citrated blood or plasma clotting by repletion of Ca2+. COT was shown to give a dose-dependent response to added thrombin and to be resistant to high concentrations of corn trypsin inhibitor, indicating measurement of the tissue-factor-dependent pathway of coagulation activation. COT in recalcified blood and plasma covariated with prothrombin time (PT) according to Owren, and activated partial thromboplastin time (aPTT). The technique and instrument used proved to be quick and easy to handle, and suitable for POCT as well as for examinations in the laboratory.