How sensitive is the synovial fluid white blood cell count in diagnosing septic arthritis?

This study was conducted to determine the sensitivity of the current standard for synovial fluid leukocytosis analysis in diagnosing infectious arthritis or a septic joint. How accurate is the standard synovial fluid white blood cell (WBC) cutoff of 50,000 WBC/mm3 to rule out septic arthritis?

We conducted a retrospective study at an urban tertiary care medical center with 50,000 adult emergency department visits per year. The study population consisted of patients with infectious arthritis confirmed by synovial fluid culture growth of a pathogenic organism. The study period lasted from January 1996 to December 2002. Extracted data included synovial fluid leukocyte count, Gram's stain, culture, past medical history, and discharge diagnosis. Fisher exact test was used to compare proportions. Sensitivity and means were calculated with 95% confidence intervals (CI).

There were 49 culture-positive synovial fluid aspirates in the 6-year study period. Nineteen (39%) of 49 patients (95% CI, 25%-52%) had a synovial WBC of less than 50,000/mm3 and 30 (61%) of 49 patients (95% CI, 48%-75%) had a synovial WBC of more than 50,000/mm3. The sensitivity of the 50,000 synovial WBC/mm3 cutoff was 61% (95% CI, 48%-75%). Twenty-seven (55%) of 49 patients had a negative Gram's stain (95% CI, 41%-69%) and 15 (56%) of 27 patients (95% CI, 37%-74%) with negative Gram's stain had a synovial WBC of less than 50,000/mm3.

A synovial fluid WBC cutoff of 50,000/mm3 lacks the sensitivity required to be clinically useful in ruling out infectious arthritis.