Partial thromboplastin time

In hematology, the partial thromboplastin time (PTT) is "the time required for the appearance of fibrin strands following the mixing of plasma with phospholipid platelet substitute (e.g., crude cephalins, soybean phosphatides). It is a test of the intrinsic pathway (factors VIII, IX, XI, and XII) and the common pathway (fibrinogen, prothrombin, factors V and X) of blood coagulation. " It is used as a screening test and to monitor anticoagulant therapy.

Both the PTT and the prothrombin time (PT) are subject to variation due both to the specific batches of reagents, and to laboratory methods. A calculation involving both the PT and PTT, called the International Normalized Ratio (INR), corrects for the variability and is the more useful value for monitoring. When the PT or PTT sugggests an error in coagulation, more detailed studies become appropriate.