ABSTRACT
The activated partial thromboplastin time (PTT) is the grandchild of the Lee-White whole blood clot time (WBCT). Both tests were developed to assist the diagnostic process for patients who exhibited features consistent with hemophilia, i.e., the pretest probability was extremely high. Probably <0.1%-1.0% of PTTs ordered in current medical practice fit that indication with the most common indication now being routine admission order; i.e., a pretest probability that is extremely low. The question asked of the PTT has evolved from 'why does this patient bleed?' to 'will this patient bleed?' As the PTT was never intended to answer that question, one must be careful regarding interpretation of results of that test. As many situations not related to hemorrhage are associated with perturbations of the PTT, a prolonged PTT is not strongly predictive of hemorrhage nor does a normal PTT provide shelter against hemorrhagic risk.
