Which synovial fluid finding is most characteristic of septic arthritis?

• A. Synovial WBC often >50,000 with neutrophils and low glucose.
• B. Crystals present indicate crystal arthritis.
• C. Cultures are always negative.
• D. Glucose is normal.

Answer: (A)

Septic arthritis tends to produce a highly inflammatory joint fluid. The most characteristic finding is a synovial fluid white blood cell count that is markedly elevated, with neutrophil predominance, along with a low glucose level. When the WBC count in the joint fluid is around or above 50,000 cells per microliter and most cells are neutrophils, this pattern strongly suggests a bacterial infection within the joint. The low glucose occurs because bacteria and inflammatory cells consume glucose, reflecting the intense inflammatory milieu in septic arthritis.

Crystals can be seen in some joints, but their presence does not define septic arthritis, and crystals may coexist without excluding infection. Cultures can be positive, so they are not always negative. Glucose is not typically normal in septic arthritis; it is usually reduced due to bacterial activity and inflammation.