Clotting assay procedures (see Table 3) continue to be the most frequently performed assays in the coagulation laboratory. If it has been determined by the screening tests 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.
| 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 of 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, the measurement of PT 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 was insufficient, repeat thrombosis or embolism could occur, but an excess of anticoagulation could produce a fatal hemorrhage.
| 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, presence of an inhibitor such as lupus anticoagulant, or to unfractionated heparin and (less frequently) to low molecular weight heparin.
| 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 to the presence of inhibitors of fibrin formation. The assay is independent of endogenous thrombin and any of the other clotting factors. It is particularly sensitive to heparin.
A modification of the TT assay is the quantitative fibrinogen assay. This assay requires the addition of exogenous thrombin with diluted plasma.