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)