What are the recommended tests and treatments for suspected kidney stones?

Testing for Kidney Stones

Low-dose non-contrast CT of the abdomen and pelvis is the gold standard imaging test for suspected kidney stones, with 95-97% sensitivity and specificity, providing precise stone size and location measurements essential for determining management. 1, 2, 3

First-Line Imaging: Non-Contrast CT

Use low-dose helical (spiral) non-contrast CT as the primary imaging modality for most patients with suspected kidney stones. 1, 2, 3

• Low-dose protocols (<3 mSv) should replace conventional dosing to minimize radiation exposure while maintaining 97% sensitivity and 95% specificity 2, 3, 4
• Non-contrast CT detects virtually all renal calculi regardless of composition, unlike plain radiography which misses radiolucent stones 2, 3
• The scan provides critical information: exact stone location within the ureter, precise size measurements, and secondary signs including ureteral dilatation and perinephric stranding 1, 3
• Stone size and location directly determine whether conservative management or intervention is needed—smaller, more proximal stones typically pass spontaneously 1

Technical Optimization

• Use thin (1-1.5 mm) axial slice images rather than thick (5 mm) coronal reformations for optimal stone detection 3
• View images on bone window settings with magnified views and coronal reformations to improve measurement accuracy 3
• Avoid contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system 3

Special Population Considerations

Pregnant Patients

Ultrasound of the kidneys and bladder is the imaging tool of choice for pregnant patients due to radiation concerns. 1, 2, 4

• Ultrasound has 100% sensitivity for detecting hydronephrosis and 90% specificity for ureteral obstruction without exposing the fetus to ionizing radiation 1, 4
• If ultrasound is inconclusive, MRI without contrast (MRU) is the second-line option, with 84% sensitivity and 100% specificity for detecting upper tract obstruction using secondary signs 1, 2, 4
• Low-dose CT is the last-line option only if absolutely necessary after ultrasound and MRI prove insufficient 1, 4

Pediatric Patients

Ultrasound is strongly recommended as first-line imaging for children, followed by plain radiography (KUB) or low-dose non-contrast CT only if ultrasound is inconclusive. 2, 4, 5

• Children with urolithiasis face high risk for multiple recurrences throughout life, making cumulative radiation exposure a critical concern 5
• Non-contrast CT significantly increases the risk for abdominal and pelvic cancer in children 5
• Ultrasound is highly reliable for diagnosing urolithiasis in children without any harm 5

Recurrent Stone Disease

For patients with known recurrent stones, use ultra-low-dose CT limited to the kidneys or review previous imaging to minimize cumulative radiation exposure. 1, 2, 3, 6

• Ultra-low-dose renal CT (mean effective dose 0.57 mSv) represents a 92% dose reduction compared to standard low-dose CT 6
• Ultrasound (appropriateness rating 7) is an acceptable option for evaluating hydronephrosis in recurrent cases 1
• Large stones may appear on plain radiography depending on composition and patient body habitus, allowing tracking of stone progression 1

Alternative Imaging Modalities

Ultrasound Combined with Plain Radiography

• Combining ultrasound with plain radiography (KUB) achieves approximately 79% sensitivity for identifying clinically significant stones, providing an acceptable alternative to low-dose CT for select patients 1, 2
• However, ultrasound alone has limited sensitivity (24-57%) compared to CT 2
• If conservative management fails or surgery is anticipated, non-contrast CT is recommended for definitive evaluation 1

Imaging Modalities to Avoid

Plain abdominal radiography alone has an appropriateness rating of 3 (usually not appropriate) due to narrow visualization capabilities that often necessitate repeat imaging. 1

• Magnetic resonance imaging has an appropriateness rating of 4 (may be appropriate)—it is less accurate for identifying stones but highly dependable for depicting hydronephrosis and perinephric edema 1
• Intravenous urography also has an appropriateness rating of 4 and cannot guide physicians toward alternative diagnoses if stones are ruled out 1

Common Pitfalls to Avoid

• Do not use contrast-enhanced CT as first-line imaging—the enhancing renal parenchyma may obscure small stones within the collecting system 3
• Be aware that ultra-low-dose protocols may miss stones <2mm in size, though these small stones rarely require intervention 2, 3
• Ultrasound sensitivity decreases within the first 2 hours of presentation because secondary signs of obstruction may not have developed yet 4
• Avoid repetitive standard-dose CT in patients with recurrent stones—use ultra-low-dose protocols or ultrasound for follow-up imaging 1, 2, 6

When CT is Inconclusive

If non-contrast CT is inconclusive for stone presence (e.g., unclear whether calcification is within ureter or adjacent structure), CT with IV contrast can help differentiate ureteral stones from phleboliths or arterial calcifications by demonstrating the "soft tissue rim" sign and opacifying iliac vessels. 1