Septic Arthritis Masquerading as a Flare of Rheumatoid Arthritis: A Not So Straightforward Diagnosis.

A typical presentation of septic arthritis (SA) includes pain, swelling, and erythema in the affected joint. Often, patients complain of inability to bear weight on the affected limb. However, some patients may present with subtle symptoms of pain and no fever or obvious swelling of the affected limb thus making the initial suspicion of SA low. Especially, patients with rheumatoid arthritis (RA) may present with polyarticular joint pain and initial synovial fluid analysis from an infected joint not consistent with overt septic arthritis. In such situations, the diagnosis of septic arthritis could be missed on delayed. In this case report, we present a 79-year-old female with a history of RA who presents with polyarticular pain, most notably in her right knee. SA was not initially suspected because of her history of RA and her current presentation with polyarticular pain. The initial synovial analysis did not suggest SA as well. However, cultures of synovial fluid from her right knee confirmed SA. Thus, we have highlighted that physicians should have a high suspicion for SA when addressing joint pain in RA patients.