Clotting assay procedures (see Table 3) remain the most frequently performed assays in the coagulation laboratory. If the screening tests have determined that a patient has a coagulation disorder, the exact coagulation factor deficiency or abnormality can be identified.
Table 3. Front Line Common Coagulation Testing Methods.| Assay | Description |
| PT | A screening assay for the function of the extrinsic and final common pathway. PT tests the generation of thrombin (stage 2) and conversion of fibrinogen to fibrin in the clotting mechanism. The PT screens for deficiencies of factors I (Fibrinogen), II (Prothrombin), V (Proaccelerin), VII (Proconvertin), and X (Stuart factor). A normal PT assay shows that the factors of the coagulation mechanism are probably not disturbed. Traditionally, PT measurement was the method of choice for monitoring anticoagulant therapy with vitamin K antagonists (warfarin-type oral anticoagulant drugs), especially for the prevention of postoperative thrombosis and pulmonary embolism. If the degree of anticoagulation is insufficient, repeat thrombosis or embolism could occur, but an excess of anticoagulation could produce a fatal hemorrhage. |
| aPTT | The aPTT measures the integrity of the intrinsic and common pathways of the coagulation cascade. This coagulation procedure is commonly used to investigate bleeding patients, monitor standard heparin or unfractionated heparin anticoagulant therapy, and as a preoperative screening test. The aPTT is not sensitive to minor abnormalities in some common-pathway coagulation factors but is useful to screen mild to moderate deficiencies of factors VIII and IX and the contact group coagulation factors (Factors XI, XII, Prekallikrein, and HMWK). This assay can detect deficiencies of all factors except VII and XIII. A prolonged aPTT can be caused by a coagulation factor deficiency, the presence of an inhibitor such as lupus anticoagulant, or unfractionated heparin and (less frequently) low molecular weight heparin. |
| TT | The TT is an older clotting screening test for fibrinogen polymerization and is performed by adding a low concentration of thrombin to plasma. The TT is a global test responsive to fibrinogen (both low and dysfunctional fibrinogen) and the presence of fibrin formation inhibitors. The assay is independent of endogenous thrombin and other clotting factors. It is susceptible to heparin. A modification of the TT assay is the quantitative fibrinogen assay. This assay requires the addition of exogenous thrombin with diluted plasma. |
