The ACT test is used to monitor the effect of high dose heparin before, during, and shortly after surgeries that require intense anticoagulation measures, such as cardiac bypass surgery, cardiac angioplasty, and dialysis. It is ordered in situations where the PTT test is not clinically useful or takes too long. The ACT is sometimes used, along with the prothrombin time (PT) to help determine whether a bleeding episode is due to excessive anticoagulation or to depletion of coagulation factors.