Acute bacterial arthritis: how long should you wait for culture results?

BACKGROUND: The optimal incubation period for synovial fluid cultures is unknown. OBJECTIVES: Our study was designed to determine the positivity rate and time to positivity of synovial fluid cultures from adults with suspected acute bacterial arthritis. METHODS: We reviewed the charts of 94 adults who had acute monoarthritis. Patients were classified as low risk or high risk for acute bacterial arthritis. The positivity rate and time to positivity of synovial fluid in combined agar plate and broth culture were calculated. RESULTS: The overall positivity rate was 22.3% (21 of 94). None of the 21 low-risk patients had a positive culture. Twenty-one (28.7%) of 73 high-risk subjects showed growth, with a mean time to positivity of 36.7 ± 27.1 hours. While half of these turned positive within a day of incubation, growth was detected at up to 90 hours. CONCLUSIONS: In patients with acute monoarthritis, especially those at high risk for infection, it is reasonable to incubate cultures for 4 days before considering them to be negative.