A Case of Adult-Onset Still's Disease With Positive Antinuclear Antibodies and Positive Antineutrophil Cytoplasmic Antibodies.

Adult-onset Still's disease (AOSD) is a rare autoinflammatory disease characterized by nonspecific symptoms such as fever, maculopapular rash, and arthralgias. The exact etiology and pathogenesis remain unclear despite advancements in medical science. Diagnosis is typically established using the Yamaguchi criteria, which include a negative antinuclear antibody (ANA) test as one of the minor criteria. However, some patients with AOSD exhibit positive ANA and even positive antineutrophil cytoplasmic antibodies (ANCA), complicating the diagnostic process. We present the case of a 19-year-old Asian woman of Yakut ethnicity who initially presented with symptoms resembling an upper respiratory tract infection. Laboratory tests revealed the presence of both ANA and ANCA. The diagnosis of AOSD was confirmed based on clinical presentation and the Yamaguchi criteria. Subsequent pulse therapy with prednisolone resulted in significant clinical improvement and a one-year remission. A review of the literature revealed that simultaneous ANCA and ANA positivity in AOSD has not been previously reported. Follow-up over 12 months showed no evidence of other autoimmune or autoinflammatory diseases, suggesting that the positive ANA and ANCA results may be either false positives or atypical laboratory manifestations in AOSD, which should be considered in the diagnosis.