Urological Malformations Identify the High-Burden Phenotype Among Children Hospitalized for Presumed Urinary Tract Infection: A Retrospective Cohort

Background/Objectives: The clinical utility of renal ultrasound after pediatric urinary tract infection (UTI) remains controversial, particularly because not all ultrasonographic abnormalities have the same prognostic significance. This study aimed to determine whether nephrourological malformations identify the phenotype associated with greater subsequent clinical burden among children hospitalized with presumed UTI and interpretable renal ultrasound findings, and to differentiate this phenotype from non-malformative ultrasound abnormalities. Methods: A retrospective single-center hospital-based cohort study was conducted in children aged 2 months to 17 years hospitalized with a clinical diagnosis of UTI at a general hospital of the Mexican Social Security Institute between 2020 and 2025. The cohort included both microbiologically confirmed UTI cases and probable clinical/microbiologically unconfirmed UTI cases. Of 182 registered patients, 130 with interpretable renal ultrasound were included. The primary exposure was the presence of adjudicated nephrourological malformation. As a secondary exposure, within the subgroup without malformation, abnormal non-malformative ultrasound findings were compared with normal ultrasound findings. Outcomes included outpatient follow-up, subspecialty referral, and hospital readmission. Crude associations were expressed as relative risks (RR), and adjusted analyses were estimated using modified Poisson regression with HC3 robust errors. Results: Twenty-nine of 130 patients (22.31%) were classified as having nephrourological malformations, and 31 (23.85%) had abnormal non-malformative ultrasound findings. Malformations were associated with higher outpatient follow-up (79.31% vs. 44.55%; RR 1.78, 95% CI 1.34–2.37), greater subspecialty referral (79.31% vs. 49.50%; RR 1.60, 95% CI 1.22–2.10), and increased readmission (44.83% vs. 13.86%; RR 3.23, 95% CI 1.72–6.08). In adjusted models, malformations remained associated with follow-up (aRR 1.72, 95% CI 1.25–2.37), referral (aRR 1.59, 95% CI 1.17–2.16), and readmission (aRR 3.38, 95% CI 1.58–7.23). In contrast, abnormal non-malformative ultrasound findings showed no significant adjusted associations. Microbiologically confirmed UTI was present in 47/130 patients (36.15%), and malformations were more frequent in this subgroup than in probable/non-confirmed clinical UTI (34.04% vs. 15.66%; p = 0.027). Conclusions: In this single-center hospital-based cohort, subsequent clinical burden was concentrated in the nephrourological malformation phenotype rather than in the broader category of “abnormal ultrasound”. These findings suggest that renal ultrasound may serve as a useful prognostic stratification tool beyond its role as a nonspecific detector of abnormalities following pediatric UTI. Given the observational design, these associations should be confirmed in larger prospective studies.