Is the isolation of S. epidermidis from pre-operative synovial fluid culture a predictor of S. epidermidis prosthetic joint infection?

Pathogen identification is key to the management of periprosthetic joint infection (PJI). Staphylococcus epidermidis is a major cause of PJI. Differentiating between invasive and contaminating S. epidermidis from joint specimens is challenging, and usually S. epidermidis is only considered a true invasive isolate when grown from two or more independent tissue samples. Thus, the detection of S. epidermidis in single synovial fluid (SF) samples from preoperative athrocentesis makes it difficult to draw definitive conclusions about its clinical significance, especially when grown from contamination-prone broth enrichment cultures (BEC). This study evaluates the diagnostic value of S. epidermidis detection in preoperative synovial cultures for the diagnosis of S. epidermidis-related PJI diagnosed by intraoperative tissue culture (TC). A total of 292 patients were included in the study, of whom 271 had prosthetic joint infection (PJI) according to EBJIS criteria. Preoperative synovial fluid (SF) cultures detected Staphylococcus epidermidis in 32.5% cases (SF-epi), other pathogens in 43.2% cases (SF-other), and were negative in 24.3%. Intraoperative tissue cultures TC identified clinically significant S. epidermidis in 30.1% of cases. The overall agreement between SF and tissue cultures was 66.1%, and the presence of S. epidermidis in SF was confirmed by TC in 70.5%. The diagnostic accuracy of SF cultures for S. epidermidis PJI was 83.2%, with sensitivity of 76.1% and specificity of 86.3% to detect S. epidermidis PJI. The positive likelihood-ration (LR+) was 5.5. When S. epidermidis from SF-BEC were excluded from analysis, specificity increased to 94.2%, and LR + was 7, but diagnostic sensitivity dropped to 40.5%. This study highlights the important value of using preoperative SF fluid cultures for the diagnosis of S. epidermidis PJI. The integration of BEC improves diagnostic accuracy and sensitivity in S. epidermidis PJI, and thus providing valuable information to guide clinical practice. For definitive antibiotic treatment decisions intraoperative tissue cultures remain mandatory.