Gram stains have limited application in the diagnosis of infected total knee arthroplasty

SummaryBackgroundThe diagnosis of periprosthetic knee infections can present a challenge to surgeons, especially in the case of chronic presentation. Gram stains are regularly performed as part of the microbiological evaluation of suspected infected total knee arthroplasties. Recently, the utility of this test in diagnosing infections has been questioned. The purpose of this study was to assess the effectiveness of Gram stains performed from surgical site samples by comparing their results to the final diagnosis of infection.MethodsThe results of 347 Gram stains performed at a single center at the time of revision total knee arthroplasty for both septic and aseptic reasons were compared to the final diagnosis based on intra-operative findings and histological evaluation.ResultsGram staining demonstrated a low sensitivity of 7% (95% confidence interval 4–12%), a specificity of 99% (95% confidence interval 97–100%), and positive and negative predictive values of 92% and 57%, respectively.ConclusionsThis study confirmed previous findings of the poor utility of this test for the diagnosis of periprosthetic knee infections. The authors recommend that Gram staining no longer be performed at the time of suspected periprosthetic knee arthroplasty infection.