What is the recommended initial imaging test for suspected kidney stones?

Imaging for Suspected Kidney Stone

Non-contrast CT (NCCT) of the abdomen and pelvis is the gold standard initial imaging test for suspected kidney stones, with sensitivity of 97% and specificity of 95%. 1, 2

Primary Recommendation: Non-Contrast CT

Low-dose non-contrast CT (<3 mSv) should be used as the first-line imaging modality for suspected kidney stones in most patients. 1, 2 This approach provides:

• Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones without IV contrast 1
• Precise measurement of stone size and exact location within the ureter, which is crucial for determining management 1
• Detection of secondary signs including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1, 2
• Rapid acquisition with high spatial resolution and multiplanar reformation capability 1

Technical Optimization

Use low-dose protocols (<3 mSv) rather than conventional dosing to minimize radiation while maintaining diagnostic accuracy. 1, 2 The evidence shows:

• Low-dose CT maintains pooled sensitivity of 97% and specificity of 95% for detecting urolithiasis 1
• Stone measurements remain equivalent to standard-dose CT 1
• Thin (1-1.5 mm) axial slice images are preferred over thick (5 mm) coronal maximum intensity projection images 1
• Viewing on bone window settings with magnified views and coronal reformations improves accuracy 1

Alternative First-Line Options

Ultrasound (Preferred in Specific Populations)

Ultrasound of kidneys and bladder is the appropriate first-line test for pregnant patients due to radiation concerns. 2 Additionally, ultrasound is reasonable for:

• Pediatric patients 2
• Patients requiring frequent follow-up imaging for recurrent stone disease 2

However, recognize the performance limitations:

• Sensitivity for stone detection is only 24-57% compared to CT 2
• Sensitivity for diagnosing ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) is higher at up to 100%, with specificity of 90% 2
• Color Doppler with twinkling artifact assessment can improve sensitivity for small renal stones 2

Combined Ultrasound and Plain Radiography

If ultrasound is used as the initial test, adding plain radiography (KUB) improves diagnostic accuracy to 79-90% sensitivity. 2 This combination approach:

• Provides better stone detection than either modality alone 3
• Identified 50% of stones detected by CT and 68% of stones requiring urological procedures in one prospective study 3
• Is most useful when CT is not immediately available or radiation exposure must be minimized 3

What NOT to Do: Critical Pitfalls

Do not use CT with IV contrast as first-line imaging for suspected kidney stones. 1, 2 The enhancing renal parenchyma obscures stones within the collecting system, potentially missing small stones 1, 2. This is a common error that can lead to missed diagnoses.

Do not rely on plain radiography (KUB) alone as initial imaging. 3 KUB has limited sensitivity:

• Overall sensitivity of only 29% 2
• Sensitivity of 53-62% with specificity of 67-69% for ureteral calculi 3
• Better performance for stones >5mm in proximal ureter (72% sensitivity) 2
• Most useful for follow-up of known radiopaque stones, not acute diagnosis 3, 4

Avoid intravenous urography (IVU/IVP) as it is considered obsolete in most settings. 3 IVU has lower sensitivity (87%) and specificity (94%) compared to non-contrast CT (96% and 100%) 2.

Special Considerations

MRI/MRU

MRI has limited utility for stone detection but can be considered when radiation must be avoided and ultrasound is inconclusive. 2 The evidence shows:

• MRU has poor accuracy for detecting small urothelial calculi 3
• Noncontrast MRU can detect upper tract obstruction with sensitivity of 84%, specificity of 100%, and accuracy of 86% using secondary signs 3
• MRU is preferred over CT urography in patients with renal impairment 3

Recurrent Stone Disease

For patients with recurrent stones, limit the CT scan to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure. 1, 2 Be aware that ultra-low-dose protocols may miss stones <2mm in size 1.

Clinical Algorithm

1. First-line for most patients: Low-dose non-contrast CT abdomen and pelvis 1, 2
2. First-line for pregnant patients: Ultrasound kidneys and bladder 2
3. If ultrasound inconclusive: Add plain radiography (KUB) to improve sensitivity 2
4. If radiation must be avoided and ultrasound inadequate: Consider MRU 2
5. For follow-up of known radiopaque stones: Plain radiography (KUB) is sufficient 3, 4