The PTT is used primarily to investigate unexplained bleeding or clotting. It may be ordered along with a prothrombin time (PT) test to evaluate hemostasis, the process that the body uses to form blood clots to help stop bleeding. These tests are usually the starting points for investigating excessive bleeding or clotting disorders. Several proteins called coagulation factors are involved in hemostasis and the formation of blood clots. When an injury occurs and bleeding begins, some coagulation factors are activated in a sequence of steps (coagulation cascade) that eventually help to form a clot. The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK). A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen). By evaluating the results of the two tests together, one can gain clues as to what bleeding or clotting disorder may be present. The PTT and PT are not diagnostic but usually provide information on whether further tests may be needed. PTT results are typically reported in seconds. A PTT result that falls within a laboratory's reference interval usually indicates normal clotting function. However, mild to moderate deficiencies of a single coagulation factor may be present. The PTT may not be prolonged until the factor levels have decreased to 30% to 40% of normal. Also lupus anticoagulant may be present but may not prolong the PTT result. If the lupus anticoagulant (LA) is suspected, a more sensitive LA-sensitive PTT or a dilute Russell viper venom time (DRVVT) can be used to test for it.