Seropositive Rheumatoid Arthritis Emerging During Acute Phase of Lung Abscess: A Case Report.

Rheumatoid arthritis (RA) is an autoimmune disease where immunological activation often precedes clinical onset. Although respiratory infections are recognized risk factors for RA, the onset of RA shortly after acute infection remains poorly characterized. Herein, we report a rare case of seropositive RA that became clinically apparent during the acute phase of lung infection. An 87-year-old man who had never smoked was diagnosed with a lung abscess caused by methicillin-susceptible Staphylococcus aureus. Post discharge, he developed polyarthritis, and laboratory tests revealed positive results for rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA). He was diagnosed with seropositive RA, and his symptoms improved with corticosteroid therapy. Retrospective review of positron emission tomography scans performed before admission revealed inflammatory uptake in multiple joints, suggesting a preclinical phase. This case suggests that acute respiratory infection may accelerate the transition from latent autoimmunity to clinically overt RA, highlighting the diagnostic value of RF and ACPA testing in new-onset arthritis after infection.