Rheumatoid Arthritis Mimickers Presenting With Polyarticular Arthritis: A Case Series Highlighting Diagnostic Pitfalls.

Rheumatoid arthritis (RA) is the most common chronic inflammatory arthritis and a leading cause of disability worldwide. However, several diseases may mimic RA, particularly in patients presenting with polyarticular pain or seronegative inflammatory arthritis. Misdiagnosis may result in inappropriate immunosuppressive therapy and delayed treatment of the underlying disease. Recognizing these mimickers remains a critical challenge in rheumatology practice. We conducted a retrospective case series of patients evaluated at a tertiary rheumatology center between January 2021 and December 2025. Patients presenting with clinical features suggestive of RA but subsequently diagnosed with alternative conditions were included. Clinical presentation, laboratory findings, imaging studies, diagnostic reasoning, and treatment outcomes were reviewed. Five patients initially suspected to have RA were ultimately diagnosed with alternative conditions. These included ferric carboxymaltose-induced hypophosphatemic osteomalacia, calcific tendinitis, polyarticular gout, remitting seronegative symmetrical synovitis with pitting edema (RS3PE) associated with malignancy, and carcinomatous polyarthritis secondary to colon adenocarcinoma. Several diagnostic clues including atypical joint involvement, abnormal laboratory findings, imaging abnormalities, and systemic symptoms helped establish the correct diagnoses. RA mimickers represent an important diagnostic challenge in rheumatology practice. Crystal arthropathies, metabolic bone diseases, calcific tendinitis, and paraneoplastic syndromes may closely resemble RA. Awareness of atypical clinical features and recognition of diagnostic red flags are essential for accurate diagnosis. Early identification of these conditions allows appropriate treatment and prevents unnecessary exposure to immunosuppressive therapies.