Analysis of synovial fluid in culture-negative samples of suspicious periprosthetic infections.

Synovial fluid analysis can help to rule out a periprosthetic infection. The goal of this study was to evaluate the prognostic significance of synovial fluid suspicious for a periprosthetic infection given the synovial leukocyte count and percent of neutrophils. From August 2006 to November 2008, patients who had synovial fluid aspirated for painful knees and elevated C-reactive protein levels after total knee arthroplasty but revealed no growth of any microorganism were retrospectively evaluated by medical record review. Mean follow-up period was 827.7±250.6 days from the date of joint aspiration results. The optimal cut-off values for synovial leukocyte counts and percent neutrophils were determined using receiver operating characteristic curves. From a total of 56 patients, 14 were classified as having poor results. The remaining 42 patients had an uneventful clinical course with a stable prosthesis on radiographs without specific therapy during the follow-up period. The receiver operating characteristic curve showed that a synovial leukocyte count of 3600/uL had an 86% sensitivity and 79% specificity; with 89% synovial neutrophils, the sensitivity was 72% and specificity 62%. The synovial fluid leukocyte count and percent neutrophils were significant predictors of outcome in patients with cultures that were negative but suspicious for periprosthetic infections.