What is the best way to diagnose a kidney stone?

How to Diagnose Kidney Stone

Non-contrast CT scan of the abdomen and pelvis is the gold standard for diagnosing kidney stones, with 97% sensitivity and 95% specificity, and should be your first-line imaging test. 1, 2

First-Line Imaging: Non-Contrast CT

• Use low-dose CT protocols (<3 mSv) to minimize radiation exposure while maintaining excellent diagnostic accuracy (pooled sensitivity 97%, specificity 95%). 1, 2
• Non-contrast CT detects virtually all kidney stones regardless of composition, accurately measures stone size, and precisely localizes stones within the ureter. 2
• CT also identifies secondary signs of obstruction including hydronephrosis, periureteral inflammation, perinephric fluid, and ureteral dilation. 1, 2
• Request thin (1-1.5 mm) axial slice images viewed on bone window settings with coronal reformations for optimal stone detection and measurement accuracy. 2

Alternative First-Line: Ultrasound

Use ultrasound as first-line imaging in pregnant patients and children to avoid radiation exposure. 1

• Ultrasound has lower sensitivity for direct stone visualization (24-57%) but excellent sensitivity (up to 100%) for detecting ureteral obstruction through hydronephrosis and ureterectasis. 1
• Combine ultrasound with plain radiography (KUB) to improve diagnostic accuracy to 79-90% sensitivity when CT is not appropriate. 1
• Adding color Doppler with twinkling artifact assessment improves detection of small renal stones. 1

When CT is Inconclusive

If initial non-contrast CT is inconclusive (e.g., uncertain whether calcification is in ureter versus adjacent phlebolith):

• Obtain CT with IV contrast to visualize the "soft tissue rim" sign around ureteral stones, confirm iliac arterial calcifications, and detect delayed nephrogram from obstruction. 3
• The portal or nephrographic phase with contrast helps differentiate true ureteral stones from mimics. 3

What NOT to Do: Common Pitfalls

• Do not use contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system, particularly small stones. 1, 2
• Do not rely solely on ultrasound for detecting small stones (<5 mm) as sensitivity decreases significantly for smaller calculi. 1
• Do not ignore hydronephrosis on ultrasound even when stones are not directly visualized—this indicates possible obstruction requiring further evaluation. 1
• Do not order MRI for routine stone diagnosis—it has poor accuracy for detecting small urothelial calculi and is costly. 3, 1

Less Useful Imaging Options

• Plain radiography (KUB) alone has poor sensitivity (29% overall, 72% for stones >5mm in proximal ureter only) and cannot distinguish ureteral stones from phleboliths or vascular calcifications. 3, 1
• Intravenous urography (IVU) has inferior sensitivity (87%) and specificity (94%) compared to non-contrast CT (96% and 100%). 1
• MRI/MRU cannot directly visualize small stones and should only be considered when radiation must be avoided and ultrasound is inconclusive. 3, 1

Special Populations

Pregnant patients:

• Start with ultrasound of kidneys and bladder as first-line imaging. 3, 1
• If ultrasound is inconclusive, consider MRU without contrast (though sensitivity for stone detection is only 69% compared to 100% for CT). 3
• Reserve low-dose CT for cases where diagnosis remains uncertain and clinical suspicion is high. 3

Recurrent stone formers:

• Use ultra-low-dose CT protocols or limit scanning to the area of interest to reduce cumulative radiation exposure. 1, 2