Azathioprine hypersensitivity syndrome in anti-myeloperoxidase anti-neutrophil cytoplasmic antibody-associated vasculitis

Two patients with anti-neutrophil cytoplasmic antibody–associated vasculitis (AAV) and rapid onset of high fever, tachycardia and systemic hypotension accompanied by elevated laboratory markers of infection were diagnosed with azathioprine hypersensitivity syndrome only after repeat exposure. Azathioprine hypersensitivity can closely mimic sepsis and/or vasculitis activity and should be considered in AAV, a condition with frequent use of this drug. We discuss the pitfalls in diagnosis and the possible pathophysiologic background.