Activated partial thromboplastin time

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The Activated Partial Thromboplastin Time (APPT) evaluates all the classic clotting factors except factor VII and factor XIII. It is therefore a haematological test that measures mainly the intrinsic clotting pathway. It is used to help monitor therapy with heparin and measures the time a fibrin clot forms after addition of the contact activator which is negatively-charged phospholipids and calcium ions to the patient's citrated plasma. Factor XII, prekallikrein, and high-molecular weight kininogen (HMWK) bind to kaolin, glass, or another artificial surface. Once bound, reciprocal activation of factor XII and prekallikrein occurs. Factor XIIa triggers clotting via the sequential activation of factors XI, IX, X, and II (prothrombin). Therapeutic values with heparin are twice to three times the lab reference time of the standard test. The test on its on can be used to evaluate dysfunctional clotting and is normally complete in between 2.5 to 4 seconds, depending upon reagents/ standardization and normal variability.

It is sometimes expressed as a index ratio (APTTR) in a similar way that INR expresses the prothrombin time.