Septic arthritis vs Gout and rheumatoid arthritis
How to tell Septic arthritis and Gout and rheumatoid arthritis apart when a board question makes them look alike.
Septic arthritis is infected purulent joint until proven otherwise; gout is crystals; RA is chronic symmetric autoimmune synovitis.
Side by side
Septic arthritis causes acute hot swollen painful joint with fever, very high synovial WBC count, positive Gram stain or culture, and urgent joint destruction risk.
Gout causes crystal-proven episodic monoarthritis; RA causes chronic symmetric inflammatory polyarthritis with anti-CCP and erosions.
Bacterial infection in the joint space rapidly recruits neutrophils and damages cartilage.
Gout is crystal-driven neutrophilic inflammation; RA is autoimmune pannus-mediated chronic synovitis.
Purulent synovial fluid, WBC often above 50,000, low glucose, and positive culture support septic arthritis.
Urate crystals support gout; anti-CCP, MCP/PIP pattern, and erosions support RA.
The trap
All can be painful and inflammatory, but fever, culture, synovial WBC, crystals, and chronic distribution separate them.
The repair move
For an acutely hot joint, rule out infection with aspiration before settling on crystal or autoimmune disease.
Try a free question
CoreA 58-year-old man with diabetes has 24 hours of acute pain, swelling, warmth, and erythema of the right knee with fever 38.8 C. The knee cannot bear weight. Arthrocentesis: turbid yellow fluid, WBC 92,000/uL with 92% neutrophils, glucose 28 mg/dL (serum 142), Gram stain shows gram-positive cocci in clusters. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Septic arthritis. Septic arthritis presents with acute monoarthritis, fever, very high synovial WBC (often >50,000 with neutrophil predominance), low synovial glucose, and positive Gram stain/culture; Staphylococcus aureus is the most common organism in adults; treatment is urgent joint drainage plus IV antibiotics.
Why the other choices are tempting
- B. Acute gout has crystals on polarized microscopy (negatively birefringent needles) and Gram stain is negative.
- C. RA flare has chronic symmetric polyarthritis and elevated anti-CCP/RF rather than monoarticular sepsis.
- D. Pseudogout has rhomboid positively birefringent CPPD crystals and chondrocalcinosis.
- E. Reactive arthritis follows infection but the joint is sterile on Gram stain and culture.
Flashcards
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What test is decisive in septic arthritis?
Synovial fluid Gram stain and culture are decisive for septic arthritis.
What antibody supports RA?
Anti-CCP supports rheumatoid arthritis, especially chronic symmetric MCP/PIP inflammatory arthritis with erosive disease.
Quick answers
What is the fastest way to tell Septic arthritis from Gout and rheumatoid arthritis?
Septic arthritis is infected purulent joint until proven otherwise; gout is crystals; RA is chronic symmetric autoimmune synovitis.
What is the key clue for Septic arthritis?
Septic arthritis causes acute hot swollen painful joint with fever, very high synovial WBC count, positive Gram stain or culture, and urgent joint destruction risk.
What is the key clue for Gout and rheumatoid arthritis?
Gout causes crystal-proven episodic monoarthritis; RA causes chronic symmetric inflammatory polyarthritis with anti-CCP and erosions.
What is the common trap with Septic arthritis vs Gout and rheumatoid arthritis?
All can be painful and inflammatory, but fever, culture, synovial WBC, crystals, and chronic distribution separate them.