Case Report: A hidden cause of severe hemorrhage: a diagnostic and therapeutic challenge.

Acquired factor XIII (FXIII) deficiency is a rare but life-threatening cause of hemorrhagic diseases. We report a case of a patient presenting with life-threatening bleeding with normal routine coagulation tests. The diagnosis was confirmed by a specific FXIII activity assay. The subsequent absence of ANA delayed the definitive diagnosis, which was ultimately established as ANA-negative systemic lupus erythematosus (SLE) based on comprehensive clinical criteria and multisystem involvement. This case presented a profound diagnostic dilemma, requiring not only the identification of a rare acquired FXIII deficiency as the cause of unexplained bleeding, but also the confirmation of SLE despite seronegativity, relying on its multisystem manifestations.