[Acquired factor V inhibitor]

Since acquired factor V inhibitor (FV-INH) has been first reported in Germany in 1955, about 200 cases have been recorded globally. The incidence of FV-INH is extremely low, with a rate of 0.023-0.09 per million persons per year. FV-INH formation is caused by infection, use of antibiotics and other drugs, surgery, and diseases, including malignancy and autoimmune disorder. Some patients with FV-INH present with abnormal clinical laboratory test results but have no hemorrhagic symptoms. Others experience life-threatening bleeding. Moreover, thrombosis can sometimes occur. The diagnosis is based on prolonged prothrombin time (PT) and activated partial thromboplastin time (APTT), an inhibitor pattern shown by a cross-mixing test of PT and APTT, decreased factor V activity, and detection of FV-INH. Treatment includes hemostatic and immunosuppressive therapy. However, in some cases, the monitoring of progression alone is appropriate. In terms of hemostatic therapies, infusion of platelet concentrates and administration of recombinant factor VIIa are highly useful. However, no definitive treatment strategy has been established. In about 50% of cases, FV-INH is eliminated spontaneously. Therefore, immunosuppressive therapy is recommended only for hemorrhagic patients or those at high risk of hemorrhage. Prednisolone is generally used for the management of immunosuppression. However, some reports have shown that the administration of rituximab, cyclophosphamide, and intravenous immunoglobulin and plasma-exchange can be utilized as treatments.