"Accessory After the Factors": A Rare Case of an Acquired Factor VIII Inhibitor in a 75-Year-Old Man on Rivaroxaban.

Direct oral anticoagulants (DOACs) are used to treat several conditions such as non-valvular atrial fibrillation, deep vein thrombosis, and pulmonary embolism. DOACs and other anticoagulants block crucial steps in the coagulation cascade and ultimately prevent clot formation. Generally, individuals initiated on an anticoagulant are predisposed to or have a propensity to form clots. Patients with hemophilia are given anticoagulants only in very rare cases. In this report, we discuss the case of a 75-year-old man with a history of atrial fibrillation managed on rivaroxaban; he presented to the emergency department with fatigue, easy bleeding, symptomatic anemia, and significantly elevated partial thromboplastin time (PTT) with an undiagnosed acquired factor VIII inhibitor. Reports of DOAC use and concomitant factor inhibitor autoimmunization, as seen in this case, are scarcely explored in the existing literature. While DOACs are popular anticoagulants, their variable effects on both prothrombin time (PT) and PTT make it difficult to detect superimposed bleeding disorders. In patients with severe anemia or significant elevations in PT or PTT, an expedited workup, including factor assays, may be a reasonable option as evidenced by this case.