Factors distinguishing septic arthritis from transient synovitis of the hip in children. A prospective study.

BACKGROUND:
Distinguishing septic arthritis from transient synovitis of the hip in children can be challenging. Authors of recent retrospective studies have used presenting factors to establish algorithms for predicting septic arthritis of the hip in children. This study differs from previous work in three ways: data were collected prospectively, C-reactive protein levels were recorded, and the focus was on children in whom the findings were so suspicious for septic arthritis that hip aspiration was performed.

METHODS:
Over four years, we prospectively collected data on every child (a total of fifty-three) who underwent hip aspiration because of a suspicion of septic arthritis at our institution. Diagnoses of confirmed septic arthritis, presumed septic arthritis, and transient synovitis were determined on the basis of the results of Gram staining, culture, and a cell count of the hip aspirate. Presenting factors and laboratory values were recorded. To evaluate the strength of predictors, we performed univariate and multivariate analysis on data from forty-eight patients who met the inclusion criteria.

RESULTS:
Univariate analysis showed that fever, the C-reactive protein level, and the erythrocyte sedimentation rate were strongly associated with the final diagnosis (p < 0.05). On multivariate analysis, the C-reactive protein level and erythrocyte sedimentation rate were found to be significant predictors. However, the erythrocyte sedimentation rate was not independent of the C-reactive protein level on backward elimination, and the C-reactive protein level was the only risk factor that was strongly associated with the outcome at a 5% significance level. Patients with five predictive factors had a 98% chance of having septic arthritis, those with four factors had a 93% chance, and those with three factors had an 83% chance.

CONCLUSIONS:
This prospective study of children who presented with findings that were highly suspicious for septic arthritis of the hip builds on the work of previous authors. We found fever (an oral temperature >38.5 degrees C) was the best predictor of septic arthritis followed by an elevated C-reactive protein level, an elevated erythrocyte sedimentation rate, refusal to bear weight, and an elevated serum white blood-cell count. In our study group, a C-reactive protein level of >2.0 mg/dL (>20 mg/L) was a strong independent risk factor and a valuable tool for assessing and diagnosing children suspected of having septic arthritis of the hip.