Abstract: A method and kit are provided for monitoring anticoagulant therapy based on measuring the simultaneous influence of factors II and X only, in a test sample (i.e., test plasma or whole blood) of an anticoagulated patient, by creating a corrected sample that circumvents measuring also the influence of fibrinogen, factor V, and factor VII on the clotting process in the corrected sample. One or more coagulation reagents that activate factor VII or factor X, but not factor IX, are added to the corrected sample, and the blood clotting ability of the corrected plasma sample is subsequently determined by measuring blood clotting time. Alternatively, a reagent plasma with added factor VII is mixed into the reagent activator, such as thromboplastin, that is subsequently added to a sample of test plasma or whole blood to correct the test sample and measure the simultaneous influence of only factors II and X.