When do we image for renal calculi (kidney stones) after a patient has passed one?

Imaging for Renal Calculi After Passage

After a patient has passed a kidney stone, repeat imaging should be offered prior to any stone intervention if symptoms have changed, as a change in stone position may influence treatment approach (ureteroscopy versus shock-wave lithotripsy versus continued observation). 1

Indications for Repeat Imaging

• Imaging should be considered when symptoms change, as this may indicate a change in stone position or the development of complications 1
• Repeat imaging is particularly important if passage of the stone is suspected but needs confirmation 1
• If medical expulsive therapy (MET) is not successful after 4-6 weeks, imaging should be performed before offering definitive stone treatment 1
• Silent hydronephrosis can occur in approximately 3% of patients with asymptomatic renal stones, necessitating regular follow-up imaging to prevent renal damage 2

Imaging Modalities to Consider

Non-Contrast CT Scan

• Non-contrast CT is the reference standard for kidney stone evaluation with sensitivity up to 97% 1
• Low-dose CT protocols (<3 mSv) maintain high diagnostic accuracy with pooled sensitivity of 97% and specificity of 95% 1
• CT is particularly useful when:
  • Stone size and location need precise determination for surgical planning 1
  • Moderate to severe hydronephrosis is present on ultrasound, as these patients have higher risk of stone passage failure 1

Ultrasound

• Ultrasound is appropriate as first-line imaging to limit radiation exposure, especially in younger patients 1, 3
• Sensitivity for detecting large stones (>5mm) approaches 100%, but accuracy decreases significantly for stones <3mm 1
• Ultrasound tends to overestimate stone size in the 0-10mm range, which could lead to inappropriate management decisions in approximately 22% of cases 4

KUB X-ray

• KUB (kidney, ureter, bladder) X-ray can be used for radiopaque stones but has limited utility alone 1
• CT typically underestimates stone size by approximately 12% compared to KUB films, which may impact management decisions, particularly for stones around 5mm 5

Tailored Imaging Approach

• For stones that have reportedly passed:
  • Confirm passage with appropriate imaging to ensure no residual fragments remain 1
  • Use the least radiation-intensive method appropriate for the clinical scenario 1
  • Consider combination of ultrasound and KUB for initial follow-up to minimize radiation exposure 3

Special Considerations

• Stones >5mm are highly likely to cause obstruction, decrease relative renal function, and require intervention 6
• Lower pole stones are less likely to cause symptoms or pass spontaneously compared to upper/mid renal stones (24.3% vs 40.6% become symptomatic) 2
• CT measurements may differ from KUB measurements, with CT typically showing smaller dimensions, which can affect management decisions 5
• Motion artifacts on CT can sometimes make stones appear larger than they actually are, potentially leading to more invasive treatment than necessary 7

Follow-up Imaging Schedule

• For asymptomatic stones being managed conservatively:
  • Regular imaging is recommended to monitor for silent hydronephrosis, which occurred in 3% of patients in one study with average follow-up of 37 months 2
  • Stones in the lower pole require less frequent monitoring as they are less likely to cause symptoms or pass spontaneously 2

By following these evidence-based guidelines for imaging after stone passage, clinicians can confirm complete stone clearance, detect complications early, and make appropriate management decisions while minimizing unnecessary radiation exposure.