Renal Ultrasound is the Most Appropriate Investigation
For a 6-month-old infant after a first febrile UTI treated successfully with antibiotics, renal and bladder ultrasound is the recommended initial imaging study. This approach is supported by current guidelines that prioritize detection of anatomic abnormalities while avoiding unnecessary invasive procedures and radiation exposure in young children 1, 2.
Evidence-Based Imaging Algorithm
First-Line Investigation: Renal Ultrasound (Answer B)
The American College of Radiology assigns renal/bladder ultrasound a rating of 7-9/9 ("usually appropriate") for children 2 months to 6 years after a first febrile UTI with good treatment response 3, 1.
Ultrasound detects clinically significant anatomic abnormalities including hydronephrosis, obstruction, renal duplication, and structural anomalies that require further evaluation 1, 4.
This modality involves no ionizing radiation, making it the safest imaging option for infants 1.
The American Academy of Pediatrics specifically recommends renal and bladder ultrasonography for all febrile infants <2 years with first UTI 1, 2, 5.
Why NOT DMSA Scan (Answer A)
DMSA renal cortical scintigraphy is rated 3/9 ("usually not appropriate") for immediate post-treatment evaluation after a first febrile UTI 1.
DMSA should be reserved for 4-6 months after infection to assess for renal scarring, not for acute post-treatment work-up 3, 1.
The top-down approach (DMSA followed by VCUG if abnormal) has limited evidence supporting routine use after first UTI, with studies showing it does not reliably predict which children need VCUG 3.
DMSA delivers higher radiation exposure (~1 mSv effective dose) compared to ultrasound's zero radiation 1.
Why NOT MCUG/VCUG (Answer C)
VCUG receives a rating of only 4/9 ("may be appropriate") after a first febrile UTI in this age group, indicating it is not routinely required 1.
The American Academy of Pediatrics explicitly states that VCUG is NOT recommended routinely after the first UTI 1, 2.
VCUG should be performed only if:
VCUG is invasive and carries moderate radiation exposure, making it inappropriate as a first-line study 1.
Clinical Context and Rationale
Why Imaging is Indicated at All
Children <2 years with first febrile UTI require imaging to detect anatomic abnormalities that may predispose to recurrent infections or renal damage 1, 2, 5.
Approximately 15% of children develop renal scarring after first UTI, which can lead to hypertension (5%) and chronic kidney disease (3.5% of ESRD cases) 1, 4.
The Shift in Imaging Philosophy
Recent evidence has shifted from aggressive imaging (routine VCUG for all) to a more targeted approach based on ultrasound findings and clinical course 3, 6.
The RIVUR trial demonstrated that prophylactic antibiotics reduce recurrent UTI by 50% in children with VUR but do NOT reduce renal scarring, indicating that early VUR detection after first UTI does not improve long-term renal outcomes 1.
Many renal abnormalities previously attributed to post-infection scarring are actually congenital kidney and urinary tract abnormalities identified on prenatal ultrasound 1, 6.
Common Pitfalls to Avoid
Do not order VCUG routinely after first UTI – this exposes the child to unnecessary radiation and invasive catheterization without proven benefit 1, 2.
Do not order DMSA scan in the acute or immediate post-treatment phase – it should only be considered 4-6 months later if there are concerns about scarring 3, 1.
Do not skip ultrasound in infants <2 years – this is the one imaging study with strong evidence supporting its use after first febrile UTI 1, 2.
Ensure the ultrasound is performed with the patient well-hydrated and bladder distended for optimal visualization 1.