Abstract
<title>Abstract</title> <p>Purpose To validate the performance of the NUM-score for stratifying the risk of nephrourological malformations (NUM) in children with febrile urinary tract infection (fUTI), and to assess its ability to predict clinically relevant malformations requiring treatment (NUMRT) and guide imaging decisions. Methods We conducted a retrospective observational study including 235 pediatric patients under 16 years old with fUTI evaluated in a tertiary care emergency department. Patients were stratified into low-, moderate-, and high-risk groups according to the NUM-score. Diagnostic performance for predicting NUMRT and guiding imaging decisions was assessed at two predefined thresholds: indication for renal-bladder ultrasound (RBUS) and indication for voiding cystourethrography (VCUG). Results The prevalence of NUMRT was 5%, 21%, and 54% in the low-, moderate-, and high-risk groups, respectively. For RBUS indication (moderate/high vs. low risk), the NUM-score showed high sensitivity (90%) and negative predictive value (95%). For VCUG indication (high vs. moderate risk), specificity was 95%. Overall, 35% of RBUS examinations could have been avoided, with only four missed NUMRT cases, all diagnosed after a subsequent UTI episode. Notably, 33% of patients with normal RBUS had vesicoureteral reflux on VCUG. Conclusion The NUM-score accurately stratifies the risk of nephrourological malformations and identifies patients with clinically relevant disease. It may support a more selective and efficient use of imaging after fUTI, reducing unnecessary investigations while ensuring appropriate identification of patients requiring further evaluation.</p>