• Matthew D Milewski, Aristides I Cruz, Christopher P Miller, Ashley T Peterson, and Brian G Smith.
• Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, CT 06520-8071, USA. mdmilewski@gmail.com
• J Bone Joint Surg Am. 2011 Feb 2;93(3):252-60.

BackgroundThe present study was designed to evaluate the prevalence of Lyme arthritis in children who had a joint aspiration at a tertiary care children's hospital in an endemic area and to identify clinical factors useful to differentiate Lyme arthritis from septic arthritis at the time of the initial presentation.MethodsThe records of all children with an age of eighteen years or less who were managed with aspiration for joint effusions at our institution from 1992 to 2009 were reviewed. Data collection included a review of aspirates; an analysis of cell count, culture results, and hematological inflammatory markers; and a review of surgical intervention.ResultsA total of 506 joint aspirations were analyzed. One hundred and fifteen aspirations were excluded. In the remaining group of 391 patients, 123 (31%) were subsequently diagnosed with Lyme arthritis. Fifty-one patients had culture-positive septic arthritis. The two cohorts were significantly different in terms of the presence of a fever of >101.5°F (>40.6°C) at the time of presentation, the refusal to bear weight, the peripheral white blood-cell count, and joint fluid cell count. The erythrocyte sedimentation rate and the C-reactive protein level were not significantly different between the two cohorts. Multivariate analysis demonstrated that refusal to bear weight was the strongest predictor of the diagnosis of septic arthritis over Lyme arthritis.ConclusionsFor any child presenting with a joint effusion in a Lyme-endemic area of the Northeastern United States, the likely prevalence of Lyme arthritis is 31% overall and 45% in the presence of knee effusion. Children with joint effusions resulting from Lyme disease are more likely to have knee involvement, a lower peripheral white blood-cell count, and a lower joint fluid cell count, and they are less likely to have fever or complete refusal to bear weight, when compared with children with septic arthritis.

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