Activated Partial Thromboplastin Time (APTT), Plasma

CPT CODE:

USEFUL FOR:

Monitoring heparin therapy (unfractionated heparin: UFH)
Screening for certain coagulation factor deficiency(ies)
Detection of coagulation inhibitors such as lupus anticoagulant, specific factor inhibitors, and nonspecific inhibitors

SPECIMEN REQUIRED:

Whole Blood - Available to Local accounts onlyDraw 4.5 mL Sodium Citrated whole blood and send ambient.APT must be completed within 4 hours of draw time.
Plasma - Available to Clinical Trial accounts onlyDraw blood in a light blue-top 3.2% Sodium Citrate tube(s) and send0.5 mL citrated platelet-poor plasma frozen in plastic vial.

TRANSPORT TEMPERATURE:

Frozen\Refrig NO\Ambient NO

CLINICAL INFORMATION:

The activated partial thromboplastin time (APTT) test reflects the activities of most of the coagulation factors, including Factor XII andother "contact factors" (prekallikrein [PK] and high molecular weightkininogen [HMWK] and Factors XI, IX, and VIII in the intrinsic procoagulant pathway, as well as coagulation factors in the commonprocoagulant pathway that include Factors X, V, II and fibrinogen(Factor I). The APTT also depends on phospholipid (a partialthromboplastin) and ionic calcium, as well as an activator of the contact factors (e.g., silica), but reflects neither the extrinsic procoagulant pathway that includes Factor VII and tissue factor, northe 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 antibodiesdirected 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 inhibitorantibodies, 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.

CLINICAL INTERPRETATION:

Since APTT reagents can vary greatly in their sensitivity to UFH, itis important for laboratories to establish a relationship between APTT response and heparin concentration. The therapeutic APTTrange in seconds should correspond with an UFH concentration of 0.3-0.7 U/mL as assessed by heparin assay (inhibition of Factor Xaactivity with detection by a chromogenic substrate). In our laboratory, we have found the therapeutic APTT range to be approximately 60-90 seconds.
Prolongation of the APTT can occur as a result of deficiency of 1or more coagulation factors (acquired or congenital in origin), or thepresence of an inhibitor of coagulation such as heparin, a lupusanticoagulant, a "nonspecific" inhibitor such as a monoclonalimmunoglobulin, or a specific coagulation factor inhibitor.
Shortening of the APTT usually reflects either elevation of Factor VIIIactivity in vivo that most often occurs in association with acute orchronic illness or inflammation, or spurious results associated witheither difficult venipuncture and specimen collection or suboptimalspecimen processing.

REFERENCE VALUES:

21-33 seconds