Pseudo-prolongation of the partial thromboplastin time

A patient with a negative personal and family history for bleeding was being prepared for bronchoscopy when an extremely long activated partial thromboplastin time was detected. Analyses for the contact phase factors were normal. It was determined subsequently that the prolongation was artifactual. The prolongation was due to a peculiar set of circumstances requiring an intrinsically short partial thromboplastin time, a rapid-acting commercial accelerating agent, and a clot-sensing device having a built-in lag phase longer than the resulting partial thromboplastin time. The authors wish to bring to the attention of laboratory personnel this curiosity.