First-Line Diagnostic for Kidney Stones
Ultrasound (US) is the primary first-line diagnostic tool for suspected kidney stones, followed by non-contrast CT if ultrasound is insufficient or inconclusive. 1
Recommended Diagnostic Algorithm
Initial Imaging: Ultrasound
- Ultrasound of the kidneys and bladder should be performed first as the primary diagnostic tool for suspected kidney stones 1
- US has 45% sensitivity for stone detection, with 94% specificity for ureteral stones and 88% specificity for renal stones 1
- US is up to 100% sensitive and 90% specific for detecting ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) 1
- Important caveat: Within the first 2 hours of presentation, US sensitivity decreases because secondary signs of obstruction may not have developed yet 1
Definitive Imaging: Non-Contrast CT
- Non-contrast enhanced CT is the standard modality for acute flank pain after ultrasound to definitively assess stone location, burden, density, and anatomy 1
- Non-contrast CT has 97% sensitivity and 95% specificity for stone detection 2, 3
- Low-dose CT protocols (<3 mSv) should be used to reduce radiation exposure while maintaining diagnostic accuracy of 93.1% sensitivity and 96.6% specificity 1
Adjunctive Imaging: KUB Radiography
- Plain KUB radiography (44-77% sensitivity) helps differentiate between radioopaque and radiolucent stone types and aids in follow-up 1
- KUB is most useful for monitoring known stones rather than initial diagnosis 4
Special Population Considerations
Pregnant Women
- Ultrasound is strongly recommended as first-line imaging 1
- MRI is the second-line option 1
- Low-dose CT is the last-line option if US and MRI are insufficient 1
Children
- Ultrasound is strongly recommended as first-line imaging 1
- KUB or low-dose non-contrast CT should follow if US does not provide required information 1
Common Pitfalls to Avoid
- Do not rely solely on ultrasound for small stones (<5 mm) as sensitivity decreases significantly for smaller calculi 2
- Do not use contrast-enhanced CT as first-line imaging as enhancing renal parenchyma may obscure stones within the collecting system 2, 3
- Do not dismiss negative ultrasound in early presentation (within 2 hours) as obstruction signs may not yet be visible; proceed to CT if clinical suspicion remains high 1
- Do not use standard-dose CT when low-dose protocols are available, as radiation reduction is critical for patients who may require multiple scans over their lifetime 1, 5
Key Differences from North American Guidelines
The 2025 European Association of Urology guidelines prioritize ultrasound first, whereas the American College of Radiology considers non-contrast CT the gold standard first-line test 2, 3. However, the European approach of starting with ultrasound reduces radiation exposure and is appropriate for most clinical scenarios, reserving CT for cases where US is inconclusive or when precise anatomic detail is needed for treatment planning 1.