Clinically unsuspected pyelonephritis in children younger than 7 years.

Purpose: Diagnosis of pyelonephritis can be challenging in young children. Our purpose is to evaluate the incidence and characteristics of CT-diagnosed pyelonephritis that was not clinically suspected in children under 7 years of age.

Methods: We retrospectively identified children < 7 years with CT diagnosis of pyelonephritis between 2011 and 2024. Demographic, clinical, and laboratory data were extracted from the medical record. One pediatric radiologist reviewed all CT scans and recorded the findings. Wilcoxon rank sum tests were used to compare age with clinically unsuspected pyelonephritis and negative urinalysis; Chi-square tests compared extent of pyelonephritis with renal atrophy and dilated (grades 3-5) VUR.

Results: 104 children (mean age 4.8 years; 79 females) met inclusion. 92/104 (88.5%) had no UTI history; 34/104 (32.7%) had urinary symptoms. Pyelonephritis was clinically unsuspected in 53/104 (51.0%), with no age group difference (p = 0.579). Urinalysis was negative in 17/104 (16.3%). 26 children received antibiotics prior to sampling. CT showed pyelonephritis in 126 kidneys (48 right, 34 left, 22 bilateral); 7 children had renal abscesses. Renal scarring developed in 11/47 with follow up renal imaging (23.4%). VUR was found in 41/51 with voiding cystourethrogram (80.4%), including 26 with grade 3-5 VUR. No association was found between extent of renal involvement and atrophy/scarring (p = 0.978) or VUR (p = 0.751).

Conclusion: CT-diagnosed pyelonephritis in young children is often clinically unsuspected and may present with negative urine tests. Follow-up US and voiding cystourethrogram are warranted to assess for scarring and VUR, even in the absence of prior UTI.