Distinguishing Lyme from septic knee monoarthritis in Lyme disease-endemic areas.

OBJECTIVE Because Lyme and septic arthritis may present similarly, we sought to identify children with knee monoarthritis at low risk for septic arthritis who may not require arthrocentesis. METHODS We performed a retrospective study of children with knee monoarthritis presenting to 1 of 2 pediatric centers, both located in Lyme disease-endemic areas. Septic arthritis was defined by a positive result on synovial fluid culture or synovial fluid pleocytosis with a positive blood culture result. Lyme arthritis was defined as a positive Lyme serologic result or physician-documented erythema migrans rash. All other children were considered to have other inflammatory arthritis. A clinical prediction model was derived by using recursive partitioning to identify children at low risk for septic arthritis, and the model was then externally validated. RESULTS We identified 673 patients with knee monoarthritis; 19 (3%) had septic arthritis, 341 (51%) had Lyme arthritis, and 313 (46%) had other inflammatory arthritis. The following predictors of knee septic arthritis were identified: peripheral blood absolute neutrophil count ≥10 × 10(3) cells per mm(3) and an erythrocyte sedimentation rate ≥40 mm/hour. In the validation population, no child with a absolute neutrophil count <10 × 10(3) cells per mm(3) and an erythrocyte sedimentation rate <40 mm/hour had septic arthritis (sensitivity: 6 of 6 [100%], 95% confidence interval [CI]: 54-100; specificity: 87 of 160 [54%], 95% CI: 46-62). Overall, none of the 19 children with septic arthritis were classified as low risk (10%, 95% CI: 0-17). CONCLUSIONS Laboratory criteria can be used to identify children with knee monoarthritis at low risk for septic arthritis who may not require diagnostic arthrocentesis.