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/mm3 (>12.0 × 109/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.
Level of Evidence: Diagnostic study, Level I-1 (testing
of previously developed diagnostic criteria in series of consecutive patients
[with universally applied reference "gold" standard]). See
Instructions to Authors for a complete description of levels of evidence.