Synovial fluid culture: agar plates vs. blood culture bottles for microbiological identification

Bacteriological diagnosis of septic arthritis (SA) is complicated. Agar plates are the main culture method and yields 40-60% of positive bacterial detection. Addition of bottled culture broth (Bactec®) as a method for detecting synovial microorganisms is common. The advantages of this method and the combination of both have not been thoroughly investigated. This study evaluates an added value of the Bactec culture broth as a single method or as combined with the agar-plate culture.

All culture aspirates of SA-suspected patients were analyzed. All cases with a positive result by either method were reviewed for background data and clinical diagnosis.

Out of 5000 synovial fluid samples, a clinical diagnosis of SA was suspected in 1024 cases. Samples processed by both culture methods were extracted during the same event. Bactec® vials were positive for significant bacterial detection in 113/148 cases (76.4%) while agar-plate cultures were positive in only 96/154 (62.3%) representing higher sensitivity of 0.5 vs. 0.42 and a positive predictive value (PPV) of 0.76 vs. 0.62. Bacterial detection by both methods combined was positive in 137/221 (62%) and did not achieve a significant increment.

The Bactec® method has many advantages in bacteriological identification of synovial infection, including a broader identification spectrum, faster response time, and superior qualities of identification although being more expensive. This method has a better yield in detecting septic arthritis and might be considered a single method for synovial fluid culture in cases suspected for SA.Key Points• The Bactec method had improved detection rates.• Culturing by agar plates and Bactec revealed higher sensitivity and lower specificity.• The use of the blood culture bottles (Bactec system) alone will raise the detection rate of septic arthritis with lower false positive rates and at lower costs.