The current diagnostic cornerstone for septic arthritis contains gram stains, bacterial culture, and cell count with a differential of aspirated synovial fluid. Recently, a synovial leukocyte esterase (LE) test has been used for diagnosing septic arthritis. Since this test measures the esterase activity of leukocytes, there is always a dilemma for using this test in patients with inflammatory arthritis.
We collected the synovial fluid specimens as part of the general diagnostic protocol for patients suspected of Juvenile Idiopathic Arthritis (JIA) or Septic Arthritis (SA). Each group included 34 patients. We compared the result of the synovial LE test with the result of the culture of each patient.
The mean ages of patients were 64.14 ± 31.27 and 50.88 ± 23.19 months in the JIA group and septic arthritis group, respectively. The LE test results were positive in 30 specimens, trace in 3 and negative in one in the first-time test and were positive in 31 specimens and trace in 3 in the second-time test, while it was negative in all patients with JIA. Hence, the sensitivity of the synovial LE test was 80.8%, the specificity, PPV, and NPV were 78.6, 70.0, 86.8% respectively based on a positive culture.
The leukocyte esterase strip test can be used as a rapid, bedside method for diagnosing or excluding bacterial infections in different body fluids. The synovial LE test can be used as an accurate test to rapidly rule in or out an acute articular bacterial infection, even in patients with concurrent inflammatory arthritis.

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OBJECTIVE: The aim of this study was to evaluate the characteristics of synovial fluid (SF) white cell count (SWCC) and neutrophil/lymphocyte percentage in the diagnosis of prosthetic joint infection (PJI) for particular threshold values.
RESULTS: This was a prospective study of 391 patients in whom SF specimens were collected before total joint replacement revisions. SF was aspirated before joint capsule incision. The PJI diagnosis was based only on non-SF data. Receiver operating characteristic plots were constructed for the SWCC and differential counts of leukocytes in aspirated fluid. Logistic binomic regression was used to distinguish infected and non-infected cases in the combined data. PJI was diagnosed in 78 patients, and aseptic revision in 313 patients. The areas (AUC) under the curve for the SWCC, the neutrophil and lymphocyte percentages were 0.974, 0.962, and 0.951, respectively. The optimal cut-off for PJI was 3,450 cells/μL, 74.6% neutrophils, and 14.6% lymphocytes. Positive likelihood ratios for the SWCC, neutrophil and lymphocyte percentages were 19.0, 10.4, and 9.5, respectively. Negative likelihood ratios for the SWCC, neutrophil and lymphocyte percentages were 0.06, 0.076, and 0.092, respectively.
CONCLUSIONS: Based on AUC, the present study identified cut-off values for the SWCC and differential leukocyte count for the diagnosis of PJI. The likelihood ratio for positive/negative SWCCs can significantly change the pre-test probability of PJI.

Differentiating septic arthritis of the pediatric hip from other causes of hip pain and effusion continues to present a diagnostic challenge for the clinician. Although septic arthritis traditionally has been reported to have a synovial white blood cell count of 75,000 cells/mm3 or greater, lower counts can be seen in this condition. In cases where a synovial sample has been obtained and the cell count falls in the intermediate range between 25,000 and 75,000 cells/mm(3), it is unclear what proportion of these cases may be truly septic hips. In this evidence-based review, we examine Heyworth et al\'s study focusing on the predictive value of this intermediate white cell count range in a Lyme-endemic region.