The authors of What is the concordance rate of preoperative synovial fluid aspiration and intraoperative biopsy in detecting periprosthetic joint infection of the shoulder ? sought to evaluate the degree of agreement between preoperative synovial fluid culture results and intraoperative tissue culture results in 50 patients meeting the 2014 Musculoskeletal Infection Society criteria for shoulder PJI.
Concordance between preoperative aspiration and intraoperative tissue culture was identified in only 28 patients out of 50 (56%).
Preoperative cultures positive for Cutibacterium acnes demonstrated sensitivity, specificity, positive predictive value (PPV) and NPV lower than 0.8. More favorable concordance was observed for monomicrobial preoperative cultures, particularly for Gram-negative organisms and methicillin sensitive Staphylococcus aureus. Gram-negative pathogens demonstrated the highest sensitivity and specificity, while polymicrobial infections exhibited the lowest sensitivity and positive predictive value (PPV).
Comment: In this study there was frequent disagreement between the results of cultures of a preoperative shoulder joint fluid aspirate and the results of cultures of 5-8 intraoperative tissue samples in shoulders meeting the 2014 Musculoskeletal Infection Society criteria for shoulder PJI. The accuracy of preoperative joint fluid cultures was greater for virulent organisms; the accuracy was less for the most common organisms causing shoulder PJI: Cutibacterium and coagulase negative Staph. The reason for the discordance, especially for Cutibacterium, is due to the facts that (1) this bacterium is most commonly found in biofilms adherent to soft tissue and implants rather than freely floating in the joint fluid and (2) a single sample of joint fluid is statistically less likely to detect bacteria than multiple samples of tissue.
In our practice, joint aspiration is most helpful in the presence of an obvious periprosthetic infection where the diagnosis of PJI is not in doubt but where identifying the causative bacterium before surgery would help inform the choice of surgery and immediate postoperative antibiotic therapy (see Periprosthetic shoulder infections, single stage and two-stage revision).
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