Objective: The routine measurement of IgA anticardiolipin (aCL) and IgA anti-β 2 glycoprotein I (anti-β 2 GPI) antibodies remain controversial despite several studies demonstrating an association with thromboembolic disease in patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). This controversy may be a contributing factor for the current under use of IgA antiphospholipid antibodies. We aimed to investigate the nature of discrepant IgA anti-β 2 GPI reactivity to help define the diagnostic value of IgA antiphospholipid antibodies.Material and Methods: Four sera selected from SLE/APS patients and positive for antiphospholipid antibodies but having discrepant IgA anti-β 2 GPI reactivity on two commercial assays were studied. IgA antibodies were affinity purified to investigate anti-β 2 GPI reactivity. Column wash through and eluent fractions were tested on both IgA anti-β 2 GPI assays. Results were normalized to total protein. Assay conjugates and standards from the discrepant assays were interchanged.