Evaluation for suspected acquired hemophilia A in adults

AHA: acquired hemophilia A; aPTT: activated partial thromboplastin time; PT: prothrombin time.

* A mixing study is usually faster than a factor VIII activity level in most institutions. Both should be obtained, and if one is suggestive, treatment can be started for bleeding if needed while awaiting results of the confirmatory testing. The initial mixing study can be normal (aPTT corrects) in acquired hemophilia A, but the aPTT prolongs after the 2-hour incubation at 37°C because the inhibitors are time- and temperature-dependent.

¶ Confirmatory testing includes factor VIII activity, mixing study with 2-hour incubation at 37°C, and inhibitor titer by Bethesda assay. Tests that are already done do not need to be repeated; tests that were not yet done should be obtained. Acquired hemophilia A is characterized by:

• Factor VIII activity: <50%; most have activity of 1 to 5%.
• Mixing study: Fails to correct, especially at the 2-hour time point at 37°C.
• Bethesda assay: Identifies inhibitor and titer. A titer of <5 BU is considered low titer; ≥5 BU is considered high titer.