Gram staining of the preoperative joint aspiration for the diagnosis of infection after total knee arthroplasty

Gram staining of joint fluid for the diagnosis of postoperative joint infection after total knee arthroplasty is considered to have limited efficacy because of the low sensitivity. However, the specificity of the gram staining is reported to be high in most reports. This study aimed to evaluate the sensitivity and specificity of the gram staining when used on the aspirated joint fluid in patients with suspected postoperative knee joint infection after total knee arthroplasty. We retrospectively reviewed the reports of synovial fluid samples retrieved from suspected infected joints at eight hospitals between 2012 and 2019. A total of 179 samples of aspirated joint fluid from knee joints (80 culture-positive samples and 99 culture-negative samples) were evaluated in this study. Of the 80 gram stains performed on samples from infected patients, there were 60 true positives and 20 false negatives. In contrast, of the 99 stains performed on samples from aseptic knees, there were 99 true negatives and no false positives. The sensitivity and specificity for detecting periprosthetic knee infections were 75.0% and 100.0%, respectively. Further, we divided infected samples into the early aspiration group (within 14 days) and the late aspiration group (15 days or more) based on the duration between the onset of symptoms and aspiration. The sensitivity of the gram staining was 84.2% and 41.2% in the first and second groups, respectively. In this study, gram staining of preoperatively aspirated joint fluid for the infected periprosthetic knee joint with short-lived symptoms showed high sensitivity. •We reviewed joint fluid data retrieved from suspected infected joints after TKA.•The sensitivity and specificity of the gram staining were 75.0% and 100.0%.•The sensitivity of the gram staining was 84.2% in the early aspiration group.•Sample centrifugation and short-lived symptoms may relate to high sensitivity.