Activated Partial Thromboplastin Time

Also known as aPTT or PTT, is laboratory test that measures the integrity of the intrinsic pathway of coagulation and commonly used for monitoring heparin therapy. It is sensitive to detecting deficiencies in factors involved in the intrinsic pathway such as factor VIII, IX, XI, and XII common pathway such as factor X or phospholipid-dependent inhibitors such as lupus anticoagulants. As noted above, it is also prolonged if certain anticoagulants such as heparin are present. The test is performed by taking citrated plasma to clot at 37 degrees C after the addition of calcium (a set amount of phospholipid is also added but no Tissue Factor is added so it will not evaluate the extrinsic pathway). PTT could also be used comparatively in mixing studies when one suspects factor deficiency or presence of inhibitors. In a patient with prolonged PTT (& normal PT), first the presence of heparin must be excluded. Once heparin is excluded, the patient's plasma can be mixed with a known normal plasma sample to see if the PTT corrects. Correction of the PTT in such setting is suggestive of a factor deficiency while lack of correction may imply presence of an inhibitor (antibody), such as lupus anticoagulant.