What are the indications for a computed tomography (CT) scan with contrast for patients with suspected kidney stones?

Indications for CT with Contrast for Kidney Stones

Non-contrast CT is the gold standard for diagnosing kidney stones, while contrast-enhanced CT is indicated only in specific circumstances such as when collecting system anatomy details are needed or alternative diagnoses are suspected. 1, 2

Primary Imaging Modality for Kidney Stones

• Non-contrast CT scan is the initial imaging test of choice for suspected kidney stones with:
  • Sensitivity and specificity exceeding 95% 2
  • Ability to precisely measure stone size and location
  • Clear depiction of secondary signs of obstruction

Indications for CT With Contrast

1. When detailed collecting system anatomy is required:

   • For surgical planning, particularly for complex stones
   • When anomalous anatomy is suspected

2. When alternative diagnoses are being considered:

   • Unexplained hydronephrosis with unknown cause 1
   • When clinical presentation is atypical for simple urolithiasis
   • When initial non-contrast CT findings are inconclusive

3. In cases of suspected infection with obstruction:

   • To help distinguish pyonephrosis from simple hydronephrosis, though this can be challenging even with contrast 1

When Non-Contrast CT is Sufficient

• Initial diagnosis of suspected kidney stones
• Follow-up of known stones
• Assessment of stone burden
• Determination of stone density/composition
• Evaluation of hydronephrosis

Special Considerations

• Radiation exposure: Non-contrast CT involves radiation exposure, which is a concern especially for young patients and those requiring repeated imaging 3
• Low-dose protocols should be considered to minimize radiation exposure, with a reduction of approximately 50% in radiation dose 2, 3
• Alternative imaging: Ultrasound is recommended as the initial imaging modality for pregnant patients (sensitivity ~45%, specificity ~88%) and children 2

Clinical Decision Algorithm

1. Initial presentation of suspected kidney stone:

   • Start with non-contrast CT (most accurate initial test)
   • If provider is confident in diagnosis and doesn't expect CT to change management, consider forgoing CT 4

2. When to order contrast-enhanced CT:

   • When surgical intervention is being planned and detailed anatomy is needed
   • When initial non-contrast CT is inconclusive
   • When alternative diagnoses are being considered
   • When infection with obstruction is suspected

3. Follow-up imaging:

   • Ultrasound is appropriate for routine follow-up to reduce radiation exposure
   • Consider alternating between ultrasound and non-contrast CT for long-term monitoring 2

Common Pitfalls

• Over-reliance on CT: CT scan doesn't change management when providers don't expect it would 4
• Size misrepresentation: Motion artifacts in CT images can cause warping distortion that makes stones appear larger than they actually are 5
• Inadequate collecting system visualization: Standard non-contrast CT may not provide sufficient distention of the renal collecting system for optimal surgical planning 6
• Radiation concerns: Repeated CT scans expose patients to significant cumulative radiation, especially problematic for recurrent stone formers 3

For optimal visualization of the collecting system when needed, consider protocols such as the DRINK protocol (oral hydration with 1L water and 20mg oral furosemide 30-60 minutes before scan), which has been shown to increase collecting system volume by 63% 6.