Synovial Fluid Culture: Agar Plates versus Blood Culture Bottles for Better Microbiological Identification

Background: Bacteriological diagnosis of septic arthritis (SA) is complicated. Agar plates are the main culture method and yields 40-60% of positive bacterial detection as compared to clinical diagnosis. Addition of bottled culture broth (Bactec) as a method for detecting synovial microorganisms is also 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.

Methods: All culture aspirates of adults suspected to sustain SA were analyzed. Samples that had been submitted in both methods i.e. agar and Bactec were included. All cases with a positive result by either method were reviewed for clinical diagnosis and background data.

Results: During the study period, 5000 synovial fluid samples were detected. In 1024 cases where the diagnosis of SA was made, both culture methods were collected 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 of 0.76 vs. 0.62. Bacterial detection in both methods was positive in 137/221 and didn`t achieve a significant increment.

Discussion: The blood culture broth method (Bactec) has many advantages in bacteriological identification of synovial joint infection. These include 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.