A Case Report of Non-Neutralizing Acquired Factor V Inhibitor Mimicking Deficiency: Diagnostic Challenges and Therapeutic Implications.

Acquired Factor V (FV) deficiency due to inhibitors is a rare coagulopathy that presents significant diagnostic and therapeutic challenges. We report the case of an 81-year-old male with persistent gross hematuria and severe coagulopathy, marked by prolonged prothrombin time (PT), activated partial thromboplastin time (aPTT), and critically low FV activity (< 1%). Initial mixing studies corrected the coagulation abnormalities, suggesting a deficiency rather than an inhibitor; however, standard therapies failed. Fresh frozen plasma (FFP) did not elevate FV levels, and recombinant activated Factor VII (rFVIIa) did not resolve his symptoms, raising suspicion for a non-neutralizing inhibitor that depletes FV by increasing clearance. Clinical improvement was achieved with platelet transfusions, and his FV level normalized after treatment with rituximab and intravenous immunoglobulin (IVIG). PT and aPTT improved from 60 and > 200 to 12 and 32, respectively. It has remained normal with subsequent maintenance immunosuppression with rituximab. This case illustrates the diagnostic complexity created by non-neutralizing FV inhibitors, which accelerate factor clearance without directly impairing activity. It highlights the critical need for integrating clinical and laboratory findings to guide tailored treatment in managing rare coagulopathies.