• Julia K Deanehan, Peter A Nigrovic, Matthew D Milewski, Sharman P Tan Tanny, Amir A Kimia, Brian G Smith, and Lise E Nigrovic.
• From the *Division of Emergency Medicine, Boston Children's Hospital; †Harvard Medical School , Boston, MA; ‡Johns Hopkins Medical School and Johns Hopkins Children's Center Baltimore, MD; ∥Division of Immunology, Boston Children's Hospital; §Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, Boston, MA; ¶Department of Orthopedics and Rehabilitation, Yale University School of Medicine, New Haven; and #Elite Sports Medicine, Connecticut Children's Medical Center, Farmington, CT; and **University of Melbourne School of Medicine, Melbourne, Australia.
• Pediatr Emerg Care. 2014 Jan 1;30(1):16-9.

BackgroundAlthough Lyme and septic arthritis of the knee may have similar clinical presentations, septic arthritis requires prompt identification and treatment to avoid joint destruction. We sought to determine whether synovial fluid cell counts alone can discriminate between Lyme, septic, and other inflammatory arthritis.MethodsWe conducted a retrospective cohort study of children aged 1 to 18 years with knee monoarthritis who presented to 1 of 2 pediatric emergency departments located in Lyme endemic areas. We included children who had both a synovial fluid culture and an evaluation for Lyme disease. Septic arthritis was defined as a positive synovial fluid culture or synovial fluid pleocytosis (white blood cell [WBC] ≥40,000 cells/μL) with a positive blood culture. Lyme arthritis was defined as positive Lyme serology without a positive bacterial culture. All other children were considered to have other inflammatory arthritis. We compared the synovial fluid counts by arthritis type.ResultsWe identified 384 children with knee monoarthritis, of whom 19 (5%) had septic arthritis, 257 (67%) had Lyme arthritis and 108 (28%) had other inflammatory arthritis. Children with other inflammatory arthritis had lower synovial WBC and absolute neutrophil count, as well as percent neutrophils, than those with either Lyme or septic arthritis. There were no significant differences in the synovial fluid WBC, absolute neutrophil count, and percent neutrophils for children with Lyme and septic arthritis.ConclusionsIn Lyme endemic areas, synovial fluid results alone do not differentiate septic from Lyme arthritis. Therefore, other clinical or laboratory indicators are needed to direct the care of patients with knee monoarthritis.

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