The presence of leukocyte esterase in the synovial fluid has recently been proposed as a marker for periprosthetic joint infection. However, the sensitivity and specificity of leukocyte esterase has not been determined when matched for the current, most inclusive Musculoskeletal Infection Society (MSIS) criteria for periprosthetic joint infection.The presence of leukocyte esterase was prospectively evaluated in synovial joint aspirates from hips and knees from May 2009 to May 2013. The cohort consisted of 189 hip and knee aspirations (fifty-two positive and 137 negative for infection). If the aspirate was bloody, a centrifuge was used to precipitate red blood cells and obtain clear synovial fluid. A standard chemical test strip (graded as negative, trace, +, or ++) was used to detect the presence of leukocyte esterase. The sensitivity, specificity, positive predictive value, and negative predictive value of the leukocyte esterase strip test were calculated using ++ and ++/+ as two positive strip result scenarios.Synovial fluid was obtained from 221 joints that underwent revision total hip or total knee arthroplasty for either mechanical failure or periprosthetic infection. Due to the lack of adequate criteria for MSIS criteria classification, thirty-two joints were excluded. The leukocyte esterase test with a threshold of +/++ had a sensitivity, specificity, positive predictive value, and negative predictive value of 79.2% (95% confidence interval [CI], 65.9% to 89.2%), 80.8% (95% CI, 73.3% to 87.1%), 61.8% (95% CI, 49.2% to 73.3%), and 90.1% (95% CI, 84.3% to 95.4%), respectively. Using the ++ as a positive leukocyte esterase result, the sensitivity, specificity, positive predictive value, and negative predictive value were 66.0% (95% CI, 51.7% to 78.5%), 97.1% (95% CI, 92.6% to 99.2%), 89.7% (95% CI, 75.8% to 97.1%), and 88.0% (95% CI, 81.7% to 92.7%), respectively.When matched to the current MSIS criteria, the leukocyte esterase strip test yielded a high specificity, positive predictive value, negative predictive value, and moderate sensitivity. These results demonstrate that leukocyte esterase is an accurate, effective marker of periprosthetic joint infection as defined by the MSIS criteria. The leukocyte esterase strip test is a valuable tool that can be used in conjunction with the current battery of diagnostic tests available.