False-Positive Myeloperoxidase-Antineutrophil Cytoplasmic Antibody in a Patient with Rheumatoid Arthritis

Objective: Mistake in diagnosis Background: Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a form of vasculitis predominantly affecting small blood vessels and systemic organs, including the lungs and kidneys. The serum ANCA is an im-portant diagnostic marker for AAV. However, ANCA levels can be nonspecifically elevated in autoimmune dis-eases like rheumatoid arthritis (RA) and some infectious diseases. Furthermore, RA and AAV can occur togeth-er. Therefore, when ANCA is detected in patients with RA, interpretation of the results is often difficult. Case Report: A 71-year-old woman with a 15-year history of RA was admitted to our hospital with a fever and anorexia. She was treated with prednisolone 5 mg/day and iguratimod 50 mg/day for the RA. She presented with bi-lateral frosted glass shadows in the lungs, acute kidney injury, positive myeloperoxidase (MPO)-ANCA results, and elevated b-D-glucan levels, suggesting AAV or pneumocystis pneumonia. A renal biopsy and bronchoal-veolar lavage ruled out AAV. A polymerase chain reaction of the bronchoalveolar lavage fluid was positive for Pneumocystis jirovecii DNA, leading to a diagnosis of pneumocystis pneumonia. After admission, the patient continued to receive intravenous supplemental fluids, and renal function improved. Based on her pathologi-cal test results and clinical course, acute kidney injury was diagnosed as prerenal failure due to dehydration in the background of chronic kidney disease. Conclusions: Even if MPO-ANCA is positive in patients with RA, it is important to consider the possibility of a false-positive result and perform a thorough and aggressive examination.