What investigation should be done to look for kidney scarring in a patient with recurrent Urinary Tract Infections (UTIs)?

DMSA Scan is the Optimal Investigation for Kidney Scarring in Recurrent UTIs

DMSA (dimercaptosuccinic acid) renal scan is the gold standard investigation for detecting kidney scarring in patients with recurrent urinary tract infections. 1

Rationale for DMSA Scan

DMSA renal scan offers significant advantages over other imaging modalities for detecting renal scarring:

• Superior sensitivity and specificity: DMSA has 90% sensitivity and 95% specificity for detecting pyelonephritis and subsequent scarring 1
• Gold standard status: Multiple guidelines recognize DMSA as the reference method for evaluating renal scarring 1
• Comparative advantage: Other imaging modalities have significant limitations:
  • Ultrasound (US): Despite good specificity (98.3%), US has extremely poor sensitivity (5.2%) for focal renal scarring and only 47.2% sensitivity for diffuse scarring 2
  • VCUG (Voiding Cystourethrography): Primarily detects vesicoureteral reflux rather than parenchymal scarring 1
  • MCUG (Micturating Cystourethrogram): Similar to VCUG, focuses on reflux rather than scarring 1

Clinical Application in Recurrent UTIs

The American College of Radiology (ACR) Appropriateness Criteria specifically recommends:

• A delayed DMSA renal scan (4-6 months after infection) to evaluate for renal scarring in high-risk patients with atypical or recurrent UTI 1
• DMSA imaging when renal ultrasound is abnormal or when there is greater concern for scarring due to breakthrough UTI 1

The American Urological Association (AUA) guidelines state:

• "DMSA imaging is recommended when a renal ultrasound is abnormal, when there is greater concern for scarring due to breakthrough UTI or VUR grade III–V or if there is an elevated serum creatinine" 1
• "DMSA may be considered for follow-up of children with VUR to detect new renal scarring, especially after a febrile urinary tract infection" 1

Timing of DMSA Scan

For optimal detection of renal scarring:

• Perform DMSA scan 4-6 months after the acute infection episode 1, 3
• This timing allows acute inflammatory changes to resolve, revealing permanent scarring 1

Important Clinical Considerations

1. Risk stratification: The risk of renal scarring varies by:

   • Age (highest in infants)
   • UTI type (higher in upper tract/febrile UTIs)
   • Recurrence pattern (higher with multiple episodes) 4

2. Limitations to consider:

   • DMSA requires specialized equipment and expertise
   • Involves small radiation exposure (approximately 1 mSv) 1
   • May require sedation in young children 1

3. Clinical pathway:

   • Initial evaluation with ultrasound to detect gross abnormalities
   • DMSA for definitive assessment of renal scarring in recurrent UTI cases
   • Consider VCUG only if DMSA shows evidence of pyelonephritis (top-down approach) 1

In conclusion, while other imaging modalities have their place in the evaluation of UTIs, DMSA renal scan remains the definitive investigation for detecting renal scarring in patients with recurrent urinary tract infections.