Bottom line: Gram stain and culture of synovial fluid are part of the gold standard for diagnosing septic arthritis, but alone are not very sensitive. They continue to be part of the diagnostic strategy for suspected septic arthritis and to guide treatment decisions.
Summary: Margaretten ME, et al. Does this adult patient have septic arthritis? JAMA. 2007 Apr 4;297(13):1478-88.
Systematic review of English-language studies located in Pubmed and EMBASE. Included studies reporting data on accuracy of history, physical examination, serum, or synovial fluid laboratory data for diagnosing septic arthritis. RESULTS: 14 studies (N=6,242) of varying quality met the criteria. Reports that synovial fluid culture is considered the gold standard for assessing presence of septic arthritis, but previous studies indicate a sensitivity of synovial fluid Gram stain to be 29%-50%. Sensitivity of culture is around 82%. This review recommends clinical follow-up to catch patients missed by Gram stain and culture.