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Background: Acquired hemophilia A (AHA) is a rare autoimmune bleeding disorder caused by autoantibodies against factor VIII. Extremely high-titer inhibitors may cause laboratory interference patterns mimicking com-bined factor deficiencies.
Methods: We report a 28-year-old woman with extensive postpartum hematomas. Investigations included coagulation screening, factor assays, serial dilution studies (undiluted to 1:16), and anti-factor VIII antibody quantification using the Nijmegen method. Factor VIII activity was measured by both one-stage clot-based and chromogenic assays. Immunological workup excluded associated autoimmune conditions. Treatment consisted of first-line immunosuppression with rituximab and prednisone followed by emicizumab (for refractory disease).
Results: Initial testing showed isolated aPTT prolongation (ratio 3.82) with apparent deficiency of factors VIII, IX, XI, XII (all 0%). Serial dilutions revealed differential recovery with factors IX, XI, XII progressively normalized while factor VIII remained at 0%. Anti-factor VIII antibodies exceeded 256 BU/mL. At one-year follow-up, factor VIII activity reached 54% and inhibitor titer 2.1 BU/mL.
Conclusions: Extremely high-titer anti-factor VIII antibodies can cause spurious multi-factor deficiencies through assay interference rather than true multiple factor inhibition. Serial dilution studies are essential for accurate diagnosis. This interference phenomenon must be recognized in order to avoid misdiagnosis and ensure appropriate treatment of severe AHA.
DOI: 10.7754/Clin.Lab.2025.251045
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