To check how well blood-thinning medications (anti-coagulants) are working to prevent blood clots; to help detect and diagnose a bleeding disorder. As part of an investigation of a bleeding or thrombotic episode; to monitor unfractionated (standard) heparin anticoagulant therapy. Who should be tested? If you have unexplained bleeding or blood clotting; if you are on unfractionated (standard) heparin anticoagulant therapy. Sometimes it is needed as part of a pre-surgical screening.