Septic Arthritis: The Daily PANCE Blueprint
A 65-year-old man presents with 2 days of a hot, swollen, exquisitely painful left knee and fever. He cannot bear weight, and the knee is warm with a large effusion and pain on any passive movement. He has no history of trauma. Which of the following is the most appropriate next step?
A. Start empiric IV vancomycin before sampling
B. MRI of the knee
C. Arthrocentesis (joint aspiration) with synovial fluid analysis
D. Plain radiograph and outpatient follow-up
E. Blood cultures and observe for 24 hours
Answer and topic summary
The answer is C. Arthrocentesis (joint aspiration) with synovial fluid analysis.
An acutely hot, swollen, exquisitely painful single joint with fever is septic arthritis until proven otherwise — a joint-threatening emergency. The best next step is urgent arthrocentesis: synovial fluid Gram stain, culture, cell count (typically >50,000 WBC/µL with neutrophil predominance), and crystal analysis confirm the diagnosis and guide therapy. The trap is starting empiric IV vancomycin BEFORE sampling — aspirate first (or at the same time) so antibiotics do not sterilize the cultures, then begin empiric IV antibiotics right after. MRI and plain films only delay care, and “blood cultures and observe” is unsafe for a joint that can be destroyed within days.
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