Microscopic Polyangiitis Initially Suspected of Being Anti-synthetase Syndrome.

Anti-synthetase syndrome is an autoimmune disorder associated with anti-aminoacyl-tRNA synthetase antibodies, including anti-Jo-1. However, antibody positivity alone is insufficient for the diagnosis because overlapping features with other diseases may lead to a misdiagnosis. We herein report a 69-year-old woman initially suspected of having anti-synthetase syndrome due to anti-Jo-1 antibody positivity and muscle weakness. Additional findings, including mononeuritis multiplex, purpura, crescentic glomerulonephritis, and strongly positive myeloperoxidase-antineutrophil cytoplasmic antibodies, led to a diagnosis of microscopic polyangiitis. Although anti-Jo-1 antibodies are highly specific, false-positive results can occur. This case highlights the need for a comprehensive evaluation beyond antibody testing to avoid diagnostic errors.