Coagulation screening test
These are frequently conduced in cases of unexpected bleeding post-operatively or significant or unexpected blood loss associated with diarrhea or vomiting.
The assays essentially test in vitro the time required for fibrin clot formation. The prothrombin time is initiated by the addition of tissue factor and calcium to the test sample and is a measure of the extrinsic and common pathways. Patients deficient in one or more factors in this pathway demonstrate prolonged clotting time in the prothrombin time.
Clot formation in the activated partial thromboplastin time is initiated by a reagent containing phospholipid and negatively charged contact particles followed by the addition of calcium to the reaction mixture. This test is sensitive to factor deficiencies in the intrinsic and common pathways.
The activated clotting time is a simplified test that is influenced by platelet count, platelet dysfunction, hematocrit and plasma protease activity.
- Factor II deficiency, factor VII deficiency, factor IX deficiency, factor X deficiency - prolonged activated prothrombin and thromboplastin times with a normal thrombin clotting time
- Factor VII deficiency - normal activated prothrombin and thromboplastin time and prolonged thrombin clotting time
- Factor VIII deficiency, factor IX deficiency, factor XI deficiency, prekallikrein deficiency - prolonged activated prothrombin time and activated clotting time
- Factor XII deficiency - prolonged partial thromboplastin and activated clotting times, but normal thrombin clotting time and fibrinogen concentration
- Furie B & Furie BC (1992) Molecular and cellular biology of blood coagulation. N Engl J Med 326:800-806
- Jobes DR et al (1989) Limitations for ACT. Anesth Analg 69:142-144