Life-threatening hemorrhage from acquired factor V deficiency with non-inhibitory, hyperclearance-type antibodies successfully treated with rituximab then belimumab.

A 66-year-old woman with a history of Sjögren's disease presented with sudden-onset low back pain that rapidly worsened, necessitating emergency admission. Laboratory testing revealed severe coagulopathy (hemoglobin, 6.7g/dL; prothrombin time, 101.9 s; prothrombin activity, 5.7%; activated partial thromboplastin time, 193.7 s). Imaging demonstrated bilateral pulmonary infiltrates compatible with alveolar hemorrhage and active bleeding within the right iliopsoas muscle. After transcatheter arterial embolization, the patient required mechanical ventilation for progressive respiratory failure. Coagulation studies showed that factor V (FV) activity was below detectable limits, while other coagulation factors were preserved. Despite no prior bleeding tendency, she developed life-threatening hemorrhage due to profound acquired FV deficiency. Mixing studies indicated a deficiency pattern, but plasma exchange resulted in only transient improvement. Although the Bethesda assay yielded negative results, plasma anti-FV immunoglobulin G was strongly positive according to enzyme-linked immunosorbent assay, suggesting the presence of non-inhibitory antibodies, possibly associated with accelerated FV clearance. High-dose glucocorticoids and rituximab induced gradual recovery, allowing extubation. Ten months later, FV activity again declined during glucocorticoid tapering, and tacrolimus, mizoribine, and mycophenolate mofetil proved ineffective. Additional rituximab followed by belimumab successfully restored FV activity without glucocorticoid escalation. The patient has remained stable without further bleeding for three years. Clinicians should consider not only inhibitory antibodies, but also non-inhibitory antibodies associated with accelerated clearance in cases unresponsive to hemostatic therapy. Rituximab combined with belimumab may represent a promising therapeutic approach for autoimmune acquired FV deficiency patients refractory to rituximab.