The validity of the one-stage prothrombin estimation* as a guide to the control of coumarin-type anticoagulant therapy has been questioned periodically. The method has been criticized by some laboratory workers for poor reproducibility, and by some clinicians as being responsible for episodes of hemorrhage or active thromboembolism while the patient's prothrombin time has been reported in the "therapeutic range." There are experts who plead for more specific tests1-4 that measure proconvertin, "pure prothrombin," or Stuart factor. Others have described better methods to measure "coagulability" in general,5,6 using tests such as the "standard clotting time" or "heparin tolerance" test.
The clinician has been less concerned with the complex theories of coagulation than with the empirical clinical results of using a given test as a guide to the administration of anticoagulants. For example, if the presence of hematuria could be used by him as a quantitative guide for coumarin administration,