aPTT (partial thromboplastin time or activated cephalin time) is used to measure the time required to form a blood clot. It is usually prescribed at the same time as PT (prothrombin time) to investigate episodes of bleeding or clot formation (thromboses). aPTT is also used before surgery or to monitor anticoagulant treatment with standard heparin. aPTT is expressed in seconds.
A normal aPTT generally indicates adequate coagulation function even in the presence of a lupus anticoagulant or a lowered rate of a specific coagulation factor. A prolonged (more than 30 seconds) aPTT may be associated with a risk of ecchymosis (bruising) or bleeding. The causes of a prolonged aPTT are numerous and include a deficiency in the production or presence of a coagulation factor inhibitor (von Willebrand disease, hemophilia, vitamin K deficiency, liver disease, presence of a lupus coagulant, heparin or warfarin [Coumadin®]). An extended aPTT can also accompany leukemia.
A shortened aPTT most often occurs during an acute phase reaction accompanied by inflammation, during the early phase of disseminated intravascular coagulation, or in certain invasive cancers (ovarian, pancreatic or colon).