Monitoring heparin therapy (unfractionated heparin)
Screening for certain coagulation factor deficiencies
Detection of coagulation inhibitors such as lupus anticoagulant, specific factor inhibitors, and nonspecific inhibitors
The activated partial thromboplastin time (APTT) assay is used as a screening test to evaluate the overall integrity of the intrinsic/common coagulation pathway and to monitor patients on heparin therapy.
This test reflects the activities of most of the coagulation factors in the intrinsic and common procoagulant pathway, but not the extrinsic procoagulant pathway, which includes factor VII and tissue factor, nor the activity of factor XIII (fibrin stabilizing factor).
Effective November 2016, APTT will no longer be used as the primary method for therapeutic heparin monitoring, for that purpose, order the heparin anti-Xa assay HEPTP / Heparin Anti-Xa Assay, Plasma.
Prolongation of the activated partial thromboplastin time (APTT) can occur as a result of deficiency of one or more coagulation factors (acquired or congenital in origin), or the presence of an inhibitor of coagulation such as heparin, a lupus anticoagulant, a nonspecific inhibitor such as a monoclonal immunoglobulin, or a specific coagulation factor inhibitor. Prolonged clotting times may also be observed in cases of fibrinogen deficiency, liver disease, and vitamin K deficiency.
Shortening of the APTT usually reflects either elevation of factor VIII activity in vivo that most often occurs in association with acute or chronic illness or inflammation, or spurious results associated with either difficult venipuncture and specimen collection or suboptimal specimen processing.
Activated partial thromboplastin time (APTT) results may be affected by many commonly administered drugs and should be considered as a potential source of unexpected abnormal results.
APTT testing will not detect all lupus anticoagulants, antiphospholipid antibodies, or coagulation inhibitors. SynthASil reagent is reportedly sensitive to decreased concentration of intrinsic factors resulting in an abnormal APTT value when factors VIII, IX, XI, and XII levels were in the 35% to 60% range.
Mixing studies may be indicated to further evaluate specimens with an unexplained prolonged APTT.
1. Clinical and Laboratory Standards Institute (CLSI). One-stage PT and APTT test; Approved Guideline Second Edition. H47-A2, 2008
2. Greaves M, Preston FE: Approach to the bleeding patient. In Hemostasis and Thrombosis: Basic Principles and Clinical Practice. Fourth edition. Edited by RW Colman, J Hirsh, VJ Marder, et al. Philadelphia, JB Lippincott Co, 2001, pp 1197-1234
