Differential Diagnosis for Distinguishing Infection from Flare of Rheumatoid Arthritis

When attempting to differentiate between an infection and a flare of rheumatoid arthritis (RA), it's crucial to consider various diagnoses due to the overlapping symptoms and the potential severity of both conditions. Here's a structured approach to the differential diagnosis:

• Single Most Likely Diagnosis

  • Flare of Rheumatoid Arthritis: This is often the most likely diagnosis in a patient with a known history of RA presenting with increased joint pain, swelling, and stiffness, especially if these symptoms are symmetric and involve typical joints such as the hands, wrists, and knees. A flare can be triggered by various factors, including non-adherence to medication, stress, or other medical conditions.

• Other Likely Diagnoses

  • Septic Arthritis: A significant concern in patients with RA, especially if they are on immunosuppressive therapy. Symptoms can include sudden onset of severe pain, swelling, and decreased range of motion in a single joint, often accompanied by systemic signs of infection like fever.
  • Crystal Arthropathy (Gout or Pseudogout): These conditions can mimic a septic joint or an RA flare and are more common in patients with RA. They typically present with acute onset of intense pain, redness, and swelling in a single joint.
  • Osteoarthritis Flare: While osteoarthritis (OA) is generally a degenerative condition with a more gradual onset, a flare can occur, especially in patients with pre-existing OA, and may be confused with an RA flare or infection.

• Do Not Miss Diagnoses

  • Endocarditis: Although less common, endocarditis can present with joint pain and fever, mimicking an infection or RA flare, especially in patients with a history of valvular heart disease or intravenous drug use.
  • Lyme Arthritis: Caused by Borrelia burgdorferi, Lyme disease can cause arthritis that may resemble RA or septic arthritis, particularly in endemic areas.
  • Gonococcal Arthritis: A form of septic arthritis caused by Neisseria gonorrhoeae, which can present with skin lesions, tenosynovitis, and arthritis, often in younger, sexually active individuals.

• Rare Diagnoses

  • Relapsing Polychondritis: A rare autoimmune disorder that can cause cartilage inflammation, leading to joint pain and swelling, among other symptoms.
  • Sarcoid Arthritis: Part of a systemic disease that can cause joint inflammation and mimic RA or an infectious process.
  • Hemochromatosis-related Arthropathy: A condition caused by iron overload, leading to joint pain and arthritis, which can be mistaken for OA or RA.

Each of these diagnoses requires careful consideration of the patient's history, physical examination, laboratory tests (including inflammatory markers, blood cultures, and joint aspirate analysis), and sometimes imaging studies to accurately differentiate between infection and an RA flare.