Avoiding errors in the laboratory evaluation of potent lupus anticoagulants

A 56-year-old female was referred to us for evaluation of a prolonged partial thromboplastin time (PTT) without correction in a 1:1 mix (evidence of an inhibitor), markedly low factor IX activity (1% or less on multiple occasions), and factor IX inhibitor (ranging from 19 to 40 Bethesda units on multiple occasions). Her prothrombin time (PT) was normal. No other laboratory results were provided at the time of her referral. Her physician had treated her with cyclophosphamide and prednisone for a presumed acquired factor IX antibody in preparation for orthopedic surgery.