AbstractThe activated partial thromboplastin time (aPTT) is frequently used to assess overall competency of the intrinsic pathway of coagulation. An abnormal value may be caused by any of several abnormalities along this pathway or by many other variables including the presence of inhibitors, poor collection of the sample, or variables in the laboratory. When the cause for the prolongation is unknown to the requesting physician, the hematologist may be consulted. In this prospective study, the cause and perceived hemostatic risk to 100 consecutive patients referred to us for consultation regarding a prolonged aPTT of previous unknown cause were evaluated. We found that these abnormal aPTTs may be either indicative of a hemostatic defect, in 50% of the cases, or of no particular risk, in 36% of the cases. In 14%, the aPTT was artifactually prolonged. Most (81%) patients with a prolonged aPTT due to a hemostatic defect had an abnormal hemostatic history but some (19%) did not. Even among true abnormal tests, the degree of abnormality indicated little or nothing about hemostatic competency. We conclude that the cause of an abnormal aPTT is more important than the result itself. These data may be of use to those who consult on such matters.