False-positive results in ELISA-based anti FVIII antibody assay may occur with lupus anticoagulant and phospholipid antibodies

Summary. The evaluation of a prolonged aPTT often includes Lupus Anticoagulant, Antiphospholipid Antibodies, and Factor VIII (FVIII) inhibitors. We have noticed that patient samples positive for lupus antibody (LA) are frequently also positive for FVIII IgG antibodies in an enzyme‐linked immunosorbent assay (ELISA), indicating the need for follow‐up testing with a more labour‐intensive functional assay for FVIII inhibition. This study evaluates the potential for a FVIII IgG ELISA to yield false‐positive results in patient samples positive for LA or other antiphospholipid antibodies. A total of 289 residual de‐identified patient samples positive for LA (n = 143), anti‐cardiolipin IgG (n = 84), or beta2‐glycoprotein antibody (n = 62) were tested for FVIII IgG using a commercial ELISA. Samples with positive FVIII IgG ELISA results were further tested for FVIII activity using a clot‐based FVIII inhibitor assay. The FVIII IgG ELISA yielded positive results in 39 (13%) of the samples tested, including 13/143 (13%) LA‐positive, 15/85 (18%) aCL IgG‐positive and 6/62 (10%) β2‐glycoprotein IgG‐positive samples. The clot‐based FVIII inhibitor assay yielded negative results in all 39 FVIII IgG‐positive specimens tested, indicating discrepancy with the FVIII IgG ELISA results. Patient specimens positive for LA, aCL IgG, or β2‐glycoprotein IgG may yield false‐positive results for FVIII antibodies. Caution is warranted in interpreting FVIII antibody results in these cases.