A case of coagulation factor V inhibitor induced by immune checkpoint inhibitor therapy.

A 73-year-old woman with lung adenocarcinoma (cT4N3M1a: Stage IVA) was treated with atezolizumab as the eighth line of therapy. Four weeks after the fourth dose of atezolizumab, the prothrombin time (PT) and activated thromboplastin time (APTT) were prolonged. Coagulation factor V (FV) activity was decreased, and FV inhibitors were observed. There was no history of PT or APTT prolongation or bleeding before the use of atezolizumab. Atezolizumab-induced coagulation FV inhibitor was diagnosed. After 2 weeks, the PT and APTT spontaneously normalized. FV activity improved and the FV inhibitors disappeared after 6 and 9 weeks, respectively.