What are the recommended diagnostic tests for a suspected kidney stone?

Diagnostic Testing for Suspected Kidney Stones

Non-contrast CT scan is the gold standard diagnostic test for suspected kidney stones, with sensitivity of 97% and specificity of 95%, and should be the first-line imaging modality when kidney stones are suspected. 1

First-Line Diagnostic Testing

• Non-contrast CT (NCCT) is the reference standard for kidney stone evaluation due to its ability to detect virtually all renal calculi without the need for IV contrast 1
• NCCT provides rapid acquisition with high spatial resolution, allowing for precise measurement of stone size and clear depiction of stone location 1
• Low-dose CT protocols (<3 mSv) should be used to reduce radiation exposure while maintaining excellent diagnostic performance (pooled sensitivity 97%, specificity 95%) 1
• NCCT can also detect secondary signs of urolithiasis including periureteral inflammation, perinephric inflammation, and ureteral dilatation 1

Alternative First-Line Testing

• Ultrasound (US) of kidneys and bladder is a reasonable alternative first-line test, particularly in:
  • Pregnant patients due to radiation concerns 1
  • Children 2
  • Patients with recurrent stone disease to limit cumulative radiation exposure 1
• US demonstrates variable performance compared to CT:
  • Sensitivity of 24-57% for stone detection with decreased sensitivity for smaller stones 2
  • Up to 100% sensitive and 90% specific for diagnosing ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) 2
  • Addition of color Doppler with twinkling artifact assessment can improve sensitivity for small renal stones 3

Combination Approaches

• Combining ultrasound with plain radiography (KUB) improves diagnostic accuracy:
  • Combined sensitivity of 79-90% for detecting stones 2, 3
  • Particularly useful for detecting clinically significant stones 3
  • All missed stones in one study had spontaneous passage, suggesting CT may not have added useful information 2

CT Technique Optimization

• 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 and use of magnified views improves accuracy 1
• Coronal reformations enhance stone detection and measurement 1

Less Recommended Imaging Options

• CT with IV contrast is not appropriate as a first-line test as enhancing renal parenchyma may obscure stones 1
• Intravenous Urography (IVU) has lower sensitivity and specificity (87% and 94%) compared to non-contrast CT (96% and 100%) 2
• MRI has limited utility in stone detection but can be considered when radiation exposure must be avoided and ultrasound is inconclusive 2
• Plain radiography (KUB) alone has limited sensitivity (29% overall, 72% for stones >5mm in proximal ureter) 2

Common Pitfalls to Avoid

• Relying solely on ultrasound for small stones (<5mm) as sensitivity decreases significantly 2
• Using contrast-enhanced CT as first-line imaging, which may lead to missed small stones 1
• Failing to consider hydronephrosis on ultrasound as a marker for possible obstruction, even when stones are not visualized 2
• Overlooking the value of plain radiography in follow-up of known stone disease 4

Special Considerations

• For patients with renal failure, ultrasound is sensitive (81%) and specific (100%) for renal stones but has lower sensitivity for ureteric stones (46%) 5
• Adding plain radiography to ultrasound in renal failure patients increases sensitivity for ureteric stones to 77% 5
• For patients with recurrent stone disease, limiting the CT scan to the area of interest or using ultra-low-dose protocols helps reduce radiation exposure 1