Complicated Urinary Tract Infections in Children

Definition and Criteria

A complicated urinary tract infection (UTI) in children is defined as an infection that occurs with underlying urological abnormalities, atypical presentation, or recurrent episodes that increase the risk of renal scarring and long-term complications, according to the American College of Radiology 1, 2
Atypical UTI is characterized by the patient being seriously ill, poor urine flow, presence of an abdominal or bladder mass, elevated creatinine, septicemia, failure to respond to appropriate antibiotics within 48 hours, or infection with non-Escherichia coli organisms, as recommended by the American College of Radiology 1, 2

Recurrent UTI is defined as 2 or more episodes of UTI with acute pyelonephritis/upper tract UTI, OR 1 episode of UTI with acute pyelonephritis/upper tract UTI plus 1 or more episodes of UTI with cystitis/lower tract UTI, OR 3 or more episodes of UTI with cystitis/lower tract UTI, according to the American College of Radiology 1, 2

Presence of vesicoureteral reflux (VUR) is a consideration in complicated UTI, as noted by the American College of Radiology 1, 3

Upper tract refers to kidneys and ureters, while lower tract is distal to the ureters, as defined by the American College of Radiology 1, 2
Pyelonephritis (upper tract infection) presents with systemic symptoms such as high fever, malaise, vomiting, abdominal or flank pain, and tenderness, according to the American College of Radiology 1, 2

Age less than 2 months is a higher risk factor for bacteremia and urinary anomalies, as noted by the American College of Radiology 1, 2
Male gender in early infancy is also a risk factor, according to the American College of Radiology 1, 2
Concomitant bacteremia occurs in 4-36.4% of cases, as reported by the American College of Radiology 1, 2
Renal scarring occurs in approximately 15% of children after first UTI episode, according to the American College of Radiology 1, 2

Voiding cystourethrography may be indicated for recurrent or atypical UTIs, as recommended by the American College of Radiology 4

Renal scarring can lead to hypertension and chronic renal failure, as noted by the American College of Radiology 1, 2

Children with a history of Vesicoureteral Reflux (VUR) have an increased prevalence of Urinary Tract Infections (UTIs), with frequency increasing from 35% to 74% in those with recurrent UTIs 5
The presence of VUR increases the risk of upper tract infection and damage when a bladder infection occurs 6

Ultrasound of the kidneys and bladder may be considered based on clinical judgment, though routine imaging is not necessary for every UTI episode in children >6 years with known VUR 7, 8

The child's history of VUR increases their risk for upper tract involvement and potential renal scarring 6
Monitor response to treatment closely; failure to respond within 48 hours may indicate an atypical infection requiring further evaluation 8

Don't routinely order imaging studies for every UTI episode in children >6 years with known VUR, as this provides limited additional information 7, 9

The American College of Radiology recommends renal and bladder ultrasound (RBUS) as the primary initial study for most children with first febrile UTI, with routine imaging generally not indicated for children >6 years with uncomplicated first febrile UTI 10, 11
For infants <2 months, RBUS is the essential first-line imaging study, with a high risk of underlying urologic abnormalities and bacteremia 12, 13, 11
The American Academy of Pediatrics recommends RBUS for children 2-24 months with febrile UTI 14
For male infants <2 months, voiding cystourethrography (VCUG) should be strongly considered to exclude posterior urethral valves and detect vesicoureteral reflux (VUR) 15
For children 2 months to 6 years, RBUS is the primary imaging study recommended after first febrile UTI, with VCUG indicated after second febrile UTI or if RBUS shows abnormalities 10, 16, 11

RBUS is usually appropriate to detect underlying abnormalities, calculi, or complications like renal/perirenal abscess in atypical or recurrent UTI 14, 11
VCUG or voiding urosonography (VUS) is usually appropriate given increased VUR prevalence in recurrent UTI 16, 17, 14, 11
CT abdomen/pelvis with IV contrast is reserved for suspected complications such as renal abscess or xanthogranulomatous pyelonephritis 10, 16, 11
DMSA renal scan is recommended 4-6 months after atypical/recurrent infection in children <3 years, or for recurrent infection in children >3 years 16, 17

RBUS every 12 months is recommended to monitor renal growth and parenchymal scarring, detecting ~40% of scarring 18, 11
VUS, VCUG, or nuclear medicine cystography is appropriate for reflux reassessment, with longer intervals between studies for lower-grade VUR and shorter for grades III-V, bladder/bowel dysfunction, or older age 18, 11

The American College of Radiology advises against using RBUS as a screening test for VUR due to its poor sensitivity 10, 19
The American College of Radiology recommends against routinely imaging children >6 years with uncomplicated first UTI, as the yield is extremely low 10, 19
The American College of Radiology advises against skipping VCUG in male infants <2 months, as posterior urethral valves must be excluded 15
The American College of Radiology recommends against ordering CT routinely, reserving it for suspected complications only 10, 16, 11

In children with recurrent cystitis, anatomical anomalies of the urinary tract—particularly vesicoureteral reflux—and functional voiding disorders must be excluded as primary causes. 20
Urinary tract obstruction (e.g., stones, strictures) classifies a urinary infection as complicated. 20
Bladder or urethral diverticula promote urine stasis and bacterial persistence, contributing to recurrent infections. 20, 21

Escherichia coli accounts for roughly 75 % of recurrent urinary tract infections across all pediatric age groups. 22, 21
Other frequent uropathogens include Enterococcus faecalis, Proteus mirabilis, Klebsiella species, and Staphylococcus saprophyticus. 21
Infection with urease‑producing bacteria is a risk factor for a complicated course of urinary tract infection. 22

Reinfection is defined as a new urinary infection occurring more than 2 weeks after successful treatment or caused by a different pathogen. 22, 20
Persistence (relapse) is defined as infection with the same organism arising within 2 weeks after treatment. 20, 21

Rapid recurrence or persistence without symptom resolution should be re‑classified as a complicated urinary tract infection and warrants imaging evaluation. 20