Background: The differentiation between septic arthritis and transient synovitis of the hip in children can be difficult. The purpose of the present study was to validate a previously published clinical prediction rule for this differentiation in a new patient population.
Methods: We prospectively studied children who presented to a major children's hospital between 1997 and 2002 with an acutely irritable hip. As in the previous study, diagnoses of septic arthritis (fifty-one patients) and transient synovitis (103 patients) were operationally defined on the basis of the white blood-cell count in the joint fluid, the results of cultures of joint fluid and blood, and the clinical course. Univariate analysis and multiple logistic regression were used to compare the two groups. The predicted probability of septic arthritis of the hip from the prediction rule was compared with actual distributions in the current patient population. The area under the receiver operating characteristic curve was determined.
Results: The same four independent predictors of septic arthritis of the hip (a history of fever, non-weight-bearing, an erythrocyte sedimentation rate of 40 mm/hr, and a serum white blood-cell count of >12,000 cells/mm(3) (>12.0 x 10(9)/L)) were identified in the current patient population. The predicted probability of septic arthritis of the hip from the prediction rule was similar to the actual distributions in the current patient population. The area under the receiver operating characteristic curve for the current patient population was 0.86, compared with 0.96 in the original population.
Conclusions: Clinical prediction rules typically demonstrate diminished performance in a new patient population because they are optimally modeled to the original data set. The previously published clinical prediction rule for the differentiation between septic arthritis and transient synovitis of the hip in children demonstrated diminished, but nevertheless very good, diagnostic performance in a new patient population.