Physiological basis
APTT evaluates the intrinsic and common coagulation pathways and adequacy of all coagulation factors except XIII and VII. APTT is commonly used to monitor unfractionated heparin therapy.
Interpretation
Increased in: Deficiency of any individual coagulation factor except Factors XIII and VII, presence of nonspecific inhibitor (eg, lupus anticoagulant), specific factor inhibitor, von Willebrand disease (PTT may also be normal), hemophilia A and B, DIC. Drugs: heparin, direct thrombin inhibitor (eg, hirudin, argatroban), warfarin.
Decreased in: Hypercoagulable states (eg, increased factor VIII levels).
Comments
APTT cannot be used to monitor very high doses of heparin (eg, cardiac bypass surgery) because the clotting time is beyond the analytical measurement range of APTT. Heparin contamination is a very common cause of an unexplained prolonged APTT. Heparin neutralization with heparinase may be needed to rule out this possibility.