Detection of Irpex lacteus in the Lung in a Case with Microscopic Polyangiitis-mimicking Manifestations Positive for Myeloperoxidase-antineutrophil Cytoplasmic Antibody.

Antineutrophil cytoplasmic antibody (ANCA) testing is valuable for the diagnosis of ANCA-associated vasculitis; however, ANCA can be positive in infectious diseases. We herein report a case of myeloperoxidase (MPO)-ANCA-positive microscopic polyangiitis-mimicking manifestations in which the bronchoalveolar lavage was positive for Irpex lacteus. After initiating treatment for I. lacteus with isavuconazole, the patient's symptoms of pain in the lower legs improved, and MPO-ANCA became negative. This case provides speculation on the relationship between fungus and ANCA production and highlights the importance of an exhaustive search for infectious agents when administering immunosuppressive therapies.