Clinical Information

The activated partial thromboplastin time (APTT) test reflects the activities of most of the coagulation factors, including factor XII and other "contact factors" (prekallikrein [PK] and high-molecular-weight kininogen [HMWK]) and factors XI, IX, and VIII in the intrinsic procoagulant pathway, as well as coagulation factors in the common procoagulant pathway that include factors X, V, II and fibrinogen (factor I). The APTT also depends on phospholipid (a partial thromboplastin) and ionic calcium, as well as an activator of the contact factors (eg, silica), but reflects neither the extrinsic procoagulant pathway that includes factor VII and tissue factor, nor the activity of factor XIII (fibrin stabilizing factor).

The APTT is variably sensitive to the presence of specific and nonspecific inhibitors of the intrinsic and common coagulation pathways, including lupus anticoagulants or antiphospholipid antibodies. Lupus anticoagulants may interfere with in vitro phospholipid-dependent coagulation tests, such as the APTT, and prolong the clotting time. Lupus anticoagulants are antibodies directed towards neoepitopes presented by complexes of phospholipid and proteins, such as prothrombin (factor II) or beta 2 glycoprotein I, but these antibodies do not specifically inhibit any of the coagulation factors. Clinically, lupus anticoagulant represents an important marker of thrombotic tendency. In contrast, patients with specific coagulation inhibitors, such as factor VIII inhibitor antibodies, have a significant risk of hemorrhage and often require specific treatment for effective management. Both types of disorders may have similar prolongation of the APTT.