Concurrence of prothrombin nonneutralizing antibody and factor XI neutralizing inhibitor in lupus anticoagulant positive patient

Isolated prothrombin antibody or isolated factor XI inhibitor have been reported separately in lupus-anticoagulant positive patients. We report the first case of a lupus-anticoagulant positive patient that both simultaneously occurred. A 30-year-old man with a history of systemic lupus erythematosus was positive for lupus-anticoagulant. He exhibited significantly high bleeding score compared to previous reports of lupus-anticoagulant positive patients with isolated prothrombin or factor XI deficiency. Examination via one-stage clotting assays revealed decreased levels of both prothrombin and factor XI. Factor parallelism was proven for prothrombin but not for factor XI. The factor XI inhibitor was quantified at 2.1 Bethesda units in the Bethesda assay, and antiprothrombin nonneutralizing antibody tested positive in ELISA. This study suggests that the concurrence of prothrombin nonneutralizing antibody and factor XI neutralizing inhibitor can aggravate bleeding tendency synergistically in lupus-anticoagulant positive patient. Bethesda assay or ELISA may be considered depending on the factor-parallelism in one-stage clotting assay.