Close collaboration between clinicians, laboratory staff and a haemophilia centre experienced in the management of inhibitors is necessary to diagnose and monitor patients with acquired haemophilia A (AHA). Initial investigation in any patient with unexplained bleeding symptoms should include a full blood count to assess platelet number and a coagulation screen, including activated partial thromboplastin time (aPTT) and prothrombin time (PT).1-3
Any acute or recent bleeding symptoms in a patient with no previous history of a bleeding disorder and an unexplained, isolated prolonged aPTT and a normal PT are suggestive of AHA and should prompt further investigation to achieve a differential diagnosis. Irrespective of the presence or absence of bleeding symptoms, an isolated prolonged aPTT outside the normal range should be investigated further. If a patient exhibits bleeding symptoms suggestive of AHA, further investigation is prudent even if the aPTT appears to be normal.
Algorithm for the laboratory diagnosis of suspected acquired haemophilia A.1,3
Mixing tests can be used to distinguish between factor deficiencies and the presence of an inhibitor. A prolonged aPTT after 1–2 h using a mixture of normal and patient plasma is indicative of autoantibodies, however factor activity assays (FVIII, IX, XI, XII) should be performed in parallel.1-3
Factor activity assays
An isolated low FVIII activity level is suggestive of AHA. A decreased level of all intrinsic pathway coagulation factors may represent an artefact caused by depletion of FVIII in the substrate plasma by the inhibitor or the presence of a lupus anticoagulant due to phospholipid inhibition in the assay, therefore specific lupus anticoagulant tests should also be performed.
Inhibitor titre quantification (Bethesda) assay
The Nijmegen-modified Bethesda assay can be used to quantify the titre of alloantibodies, which exhibit type 1 kinetics, however use of the Bethesda assay to quantify the complex, non-linear type 2 kinetics associated with autoantibodies may inaccurately represent the potency.4,5 Autoantibody titre and residual FVIII activity levels correlate poorly with clinical severity and bleeding risk, therefore treatment decisions should not be solely based on these laboratory values.1,2,6
Lupus anticoagulant tests
Lupus anticoagulant may also be associated with a prolonged aPTT, a positive Bethesda assay and low intrinsic factor levels, therefore specific tests may need to be undertaken to distinguish between lupus anticoagulant and acquired FVIII inhibitors.5 Autoantibodies to FVIII and lupus anticoagulant may be present in the same patient sample, therefore a FVIII-antibody-specific enzyme-linked immunosorbent assay (ELISA) or an aPTT reagent insensitive to lupus anticoagulant may be useful to distinguish between the two.2
1. Collins P, Baudo F, Huth-Kuhne A, et al. Consensus recommendations for the diagnosis and treatment of acquired hemophilia A. BMC Res Notes 2010;3:161.
2. Huth-Kuhne A, Baudo F, Collins P, et al. International recommendations on the diagnosis and treatment of patients with acquired hemophilia A. Haematologica 2009;94:566-75.
3. Franchini M, Castaman G, Coppola A, et al. Acquired inhibitors of clotting factors: AICE recommendations for diagnosis and management. Blood Transfus 2015;13:498-513.
4. Kasper CK, Aledort L, Aronson D, et al. Proceedings: A more uniform measurement of factor VIII inhibitors. Thromb Diath Haemorrh 1975;34:612.
5. Verbruggen B, van Heerde WL, Laros-van Gorkom BA. Improvements in factor VIII inhibitor detection: From Bethesda to Nijmegen. Semin Thromb Hemost 2009;35:752-9.
6. Baudo F, Collins P, Huth-Kühne A, et al. Management of bleeding in acquired hemophilia A: Results from the European Acquired Haemophilia (EACH2) registry. Blood 2012;120:39-46.