[Development of asymptomatic acquired factor V inhibitor after the administration of antibiotics].

Acquired factor V (FV) inhibitor is a rare coagulation disorder, the causes and clinical symptoms of which are known to vary widely. Acquired FV inhibitor mostly occurs with exposure to fibrin glues during surgical procedures. We experienced a case with asymptomatic acquired FV inhibitor caused by antibiotic therapy for aspiration pneumonia.A man in his eighties had been treated for chronic atrial fibrillation, cerebral infarction and spinal canal stenosis. He was admitted after antibiotic therapy for aspiration pneumonia in a previous hospital. On admission, the screening test for blood coagulation revealed extreme prolongation of both prothrombin time (PT) and activated partial thromboplastin time (APTT). Vitamin K was administered intravenously after cessation of warfarin, but neither PT nor APTT showed any improvement. Subsequently, a cross mixing test was performed and showed an inhibitor pattern. Furthermore, a high titer of FV inhibitor was detected by the Bethesda method and an acquired FV inhibitor was thus diagnosed. Despite the presence of FV inhibitor, the patient showed no bleeding symptoms. Eight months later, both PT and APTT had diminished to normal clotting time values without immunosuppressive therapies.