partial thromboplastin time (PTT), a test for detecting coagulation defects of the intrinsic system by adding activated partial thromboplastin to a sample of test plasma and to a control sample of normal plasma. The time required for the formation of a clot in test plasma is compared with that in the normal plasma. A delayed clotting time suggests an abnormality in one or more factors of the intrinsic system. If indicated, specific factor abnormalities can be identified by exposing the test plasma to a series of plasma samples with known factor deficiencies and observing for coagulation, which occurs only if the test plasma provides the missing clotting factors. PTT is one of the basic tests used to measure specific factor activity and to detect hemophilias. It can be used to monitor liver function and to monitor the activity of the anticoagulant heparin. The normal activated PTT in plasma is 25 to 45 seconds after the addition to the plasma sample of partial thromboplastin reagent and ionized calcium. Also called activated partial thromboplastin time. Compare prothrombin time. See also hemostasis.
