Activated Partial Thromboplastin Time Test
Activated partial thromboplastin clotting time (aPTT) is one of a handful of diagnostic coagulation tests that provides a measurement for the length of time it takes an individual's blood to form a clot. Blood clotting factors come together in a cascade process when a blood vessel is damaged to form a blood clot and stop bleeding. Clotting factors XII, IX, II, X, XI, VIII, V, fibrinogen, prekallikrein, and high-molecular-weight kininogen are components that make up the intrinsic clotting pathway and the common final pathway. These factors are measured in the aPTT test. The partial thromboplastin time test is performed before the activated partial thromboplastin time.
The difference between these two tests is a special activation solution is added to the sample for the aPTT that increases the clotting time to provide a smaller reference range. If both the PTT and aPTT tests yield abnormal results, it is suggested the patient has a deficiency of factor I, V, X, or II. A normal PTT with an abnormal aPTT result suggests the patient has a deficiency of factor IX, XIII, VIII, or X. The typical presentation for factor IX deficiency or hemophilia B is a normal PTT with a prolonged aPTT.