Figure 1.

Laboratory testing algorithm for suspected acquired hemophilia A. After exclusion of interfering substances as a cause of prolonged aPTT, a two-hour mixing study should be done with normal pooled plasma. For samples where the aPTT corrects, a factor deficiency should be suspected, and specific factor assays conducted. For samples where the aPTT does not correct, testing for lupus anticoagulant should be done and, if negative, specific testing for a FVIII inhibitor should be conducted. Inhibitors can be measured via Bethesda assay ideally with heat inactivation.