The value of pre-operative aspiration in the diagnosis of an infected prosthetic knee: a retrospective study and review of literature.

The purpose of this study was to compare the results of cultures of preoperative aspiration samples with those of swabs taken intra-operatively. The records of 70 revision arthroplasties of the knee from 69 patients, 49 females and 20 males, were reviewed. The mean age of the patients at time of revision arthroplasty was 67 years (range: 34 to 89). Sixty-eight knees from 67 patients were included. There were 32 true positives, 17 true negatives, 6 false positives and 13 false negatives. This led to a specificity, sensitivity and accuracy of 57%, 84% and 72%, respectively. Based on these findings, pre-operative aspiration has a positive predictive value of 71% and a negative predictive value of 74%. When the aspiration sample yields a positive culture, the chances are high that the prosthetic knee is infected. When aspiration is negative, infection cannot be ruled out. Our study suggests that, in such cases, a coagulase negative Staphylococcus (CNS) infection has to be considered.