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Am Fam Physician. 2004;70(12):2357-2358

Clinical Question: How can septic arthritis be differentiated from transient synovitis of the hip in children?

Setting: Outpatient (specialty)

Study Design: Decision rule (validation)

Synopsis: A total of 154 consecutive children with intact immune systems and no recent surgery presented to a tertiary care center with an acutely irritable hip. The children were evaluated in a systematic manner that included a history, physical examination, laboratory assessment, radiography, and joint fluid analysis. Although C-reactive protein tests have proved better than erythrocyte sedimentation rate in the diagnosis of septic arthritis in children, C-reactive protein tests were not readily available early in this study and, therefore, were not included in data analysis to avoid potential bias.

The researchers used explicit diagnostic standards. A truly septic joint was defined as a positive joint fluid culture or a white blood cell count in the joint fluid of higher than 50,000 cells per mm3 (50 × 109 per L) plus a positive blood culture. Septic arthritis was suspected when patients had a white blood cell count in the joint fluid of greater than 50,000 cells per mm3 with negative cultures of the joint aspirate and blood. The researchers combined these two groups to constitute septic arthritis. They defined transient synovitis as having a white blood cell count in the joint fluid of less than 50,000 cells per mm3 with negative culture, resolution of symptoms without antibiotics, and no subsequent evolution of symptoms during a mean follow-up of 11.8 months.

In a previous study, four factors were found to differentiate septic joints from those with transient synovitis: a history of fever, an inability to bear weight, an erythrocyte sedimentation rate higher than or equal to 40 mm per hour, and a peripheral white blood cell count of more than 12,000 cells per mm3 (12 × 109 per L). These same variables performed well here. The sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio are, respectively, as follows: one factor (100 percent, 26 percent, 1.4, 52); two factors (90 percent, 68 percent, 2.8, 0.15); three factors (59 percent, 89 percent, 5.4, 0.5); and all four factors (16 percent, 99 percent, 16, 0.8). The area under the receiver operating characteristic curve, a measure of overall diagnostic accuracy, was 0.86, which is very good.

Bottom Line: To distinguish septic arthritis from transient synovitis of the hip in children, the following factors are helpful: a history of fever, inability to bear weight, erythrocyte sedimentation rate greater than or equal to 40 mm per hour, and a peripheral white blood cell count greater than 12,000 per mm3. The presence of none of these factors rules it out, while the presence of all four virtually rules it in. (Level of Evidence: 1a)

POEMs (patient-oriented evidence that matters) are provided by Essential Evidence Plus, a point-of-care clinical decision support system published by Wiley-Blackwell. For more information, see Copyright Wiley-Blackwell. Used with permission.

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Copyright © 2004 by the American Academy of Family Physicians.

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