Septic arthritis vs Reactive arthritis
How to tell Septic arthritis and Reactive arthritis apart when a board question makes them look alike.
Septic arthritis is hot infected monoarthritis needing drainage; reactive arthritis is sterile post-infectious arthritis with eye/GU/GI clues.
Side by side
Septic arthritis is acute infected joint with severe monoarticular pain, swelling, warmth, fever, painful limited passive range of motion, and purulent synovial fluid.
Reactive arthritis is sterile inflammatory arthritis after GI or GU infection with asymmetric oligoarthritis, enthesitis, conjunctivitis/uveitis, urethritis, and HLA-B27 association.
Hematogenous seeding or direct inoculation infects the synovium, rapidly destroying cartilage if not drained and treated.
Immune-mediated arthritis follows infection elsewhere; the joint itself is not infected on culture.
Synovial WBC often greater than 50,000/mm3 with neutrophil predominance, positive Gram stain/culture, low glucose, fever, and need for urgent drainage support septic arthritis.
Sterile synovial fluid, recent Chlamydia, Salmonella, Shigella, Campylobacter, or Yersinia infection, HLA-B27, and arthritis-urethritis-conjunctivitis pattern support reactive arthritis.
The trap
A preceding infection can cause either bacteremia-seeded septic arthritis or sterile reactive arthritis, so synovial Gram stain/culture and severity decide.
The repair move
For acute arthritis, tap the joint and decide infected synovium versus sterile post-infectious inflammation.
Try a free question
CoreA patient has fever and an acutely swollen knee with severe pain on passive motion; arthrocentesis shows purulent fluid with many neutrophils and gram-positive cocci. Which diagnosis is most likely?
Show answer and explanation
Answer: A. Septic arthritis. Fever, acute monoarthritis, painful passive motion, purulent synovial fluid, and bacteria support septic arthritis.
Why the other choices are tempting
- B. Reactive arthritis has sterile synovial fluid after GI/GU infection.
- C. Osteoarthritis is chronic degenerative joint pain.
- D. Rheumatoid arthritis flare is inflammatory but not culture-positive purulent infection.
- E. Gout can be acute but crystals, not bacteria, define it.
Flashcards
Tap a card to flip it.
What motion finding supports septic arthritis?
Severe pain with passive range of motion supports septic arthritis.
What synovial fluid result supports septic arthritis?
Purulent fluid, positive Gram stain/culture, or WBC above 50,000/mm3 with neutrophil predominance supports septic arthritis.
Quick answers
What is the fastest way to tell Septic arthritis from Reactive arthritis?
Septic arthritis is hot infected monoarthritis needing drainage; reactive arthritis is sterile post-infectious arthritis with eye/GU/GI clues.
What is the key clue for Septic arthritis?
Septic arthritis is acute infected joint with severe monoarticular pain, swelling, warmth, fever, painful limited passive range of motion, and purulent synovial fluid.
What is the key clue for Reactive arthritis?
Reactive arthritis is sterile inflammatory arthritis after GI or GU infection with asymmetric oligoarthritis, enthesitis, conjunctivitis/uveitis, urethritis, and HLA-B27 association.
What is the common trap with Septic arthritis vs Reactive arthritis?
A preceding infection can cause either bacteremia-seeded septic arthritis or sterile reactive arthritis, so synovial Gram stain/culture and severity decide.