What is the initial test for a 6-month-old baby with a urinary tract infection (UTI) when further investigation is desired?

Initial Imaging for 6-Month-Old with UTI

For a 6-month-old infant with UTI, renal and bladder ultrasound is the initial test of choice. 1, 2, 3

Rationale for Renal Ultrasound as First-Line Imaging

The American College of Radiology (ACR) 2024 guidelines explicitly recommend renal and bladder ultrasound (RBUS) as the primary imaging modality for infants under 6 months of age with UTI, rating it 9/9 (usually appropriate). 1, 2 This recommendation is reinforced by NICE guidelines, which specify that ultrasound should be performed within 6 weeks of the UTI if it's a typical infection, or during the acute infection if atypical. 1

The high yield of ultrasound in this age group is substantial:

• Hydronephrosis is found in 45% of neonates with UTI 1
• Infants under 6 months have increased incidence of renal anomalies and sepsis compared to older children 1
• Ultrasound can detect structural abnormalities, hydronephrosis, renal size asymmetry, and collecting system duplications 4

Why Not the Other Options

MCUG/VCUG (Option C) - Not Initial Test

VCUG is not the first-line imaging study. 1, 2, 3 The 2024 ACR guidelines indicate that VCUG should be considered in specific circumstances for infants under 2 months, particularly males (to exclude posterior urethral valves), but even then, ultrasound comes first. 1, 2 More importantly, recent evidence shows that in children under 3 months with first febrile UTI caused by E. coli and normal renal ultrasound, VCUG can be safely avoided. 1, 3

Abdominal Ultrasound (Option D) - Too Broad

While "abdominal ultrasound" might technically include the kidneys and bladder, the specific test indicated is renal and bladder ultrasound, not a general abdominal survey. 1 The focused renal and bladder study is the appropriate imaging protocol.

Chest X-ray (Option A) - Not Indicated

There is no role for chest radiography in the initial evaluation of pediatric UTI. 1

Clinical Algorithm for This 6-Month-Old

1. Obtain renal and bladder ultrasound within 6 weeks (or during acute infection if atypical features present) 1, 3

2. If ultrasound is normal AND the infection is E. coli AND the infant responds well to treatment within 48 hours: VCUG can likely be avoided 1, 3

3. If ultrasound shows abnormalities: Proceed to VCUG to evaluate for vesicoureteral reflux 2, 3

4. If atypical features are present (seriously ill, poor urine flow, non-E. coli organism, failure to respond within 48 hours, elevated creatinine): Consider VCUG regardless of ultrasound findings 1, 2

Important Caveats

Ultrasound has limitations: It has poor sensitivity (76.5%) for detecting vesicoureteral reflux and renal scarring. 1 However, it has excellent negative predictive value (98.5%) for excluding high-grade (IV-V) VUR in infants under 2 months. 4 The sensitivity for detecting any VUR is only 32.7%, but for clinically significant grades IV-V VUR, sensitivity improves to 86.7%. 4

Don't skip ultrasound thinking VCUG is more definitive - these are complementary studies providing different information, and ultrasound must come first to guide further management. 5, 4

Answer: B - Renal ultrasound is the correct initial test for this 6-month-old infant with UTI.