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

Recommended Initial Diagnostic Test for Kidney Stones

Non-contrast CT scan of the abdomen and pelvis is the gold standard first-line diagnostic test for suspected kidney stones, with 97% sensitivity and 95% specificity, and should be performed using low-dose protocols (<3 mSv) to minimize radiation exposure while maintaining excellent diagnostic accuracy. 1, 2

Primary Recommendation: Low-Dose Non-Contrast CT

• Non-contrast CT is the definitive imaging modality because it detects virtually all kidney stones regardless of composition, accurately measures stone size (critical for determining if spontaneous passage is likely), and precisely localizes stones within the ureter 1, 3

• Low-dose CT protocols should always be used instead of conventional dosing, delivering <3 mSv radiation while maintaining pooled sensitivity of 97% and specificity of 95% 1, 2

• CT provides essential clinical information beyond stone detection, including secondary signs of obstruction (hydronephrosis, periureteral inflammation, perinephric fluid, ureteral dilation) that guide management decisions 1, 2

• The American College of Radiology assigns non-contrast CT an appropriateness rating of 8 (usually appropriate) for acute onset flank pain with suspected stone disease 3

Alternative First-Line Approach: Ultrasound

Ultrasound is the preferred first-line test in pregnant patients and children to avoid radiation exposure entirely 1, 4, 2

• Ultrasound has lower sensitivity (24-57%) for direct stone visualization but excellent sensitivity (up to 100%) and specificity (90%) for detecting ureteral obstruction through hydronephrosis and ureterectasis 3, 2

• The European Association of Urology recommends ultrasound as primary first-line imaging, followed by non-contrast CT if ultrasound is insufficient or inconclusive 4

• Combining ultrasound with plain radiography (KUB) improves diagnostic accuracy to 79-90% sensitivity, providing an acceptable alternative to CT when radiation exposure is a concern 1, 2

Critical Pitfalls to Avoid

• Do not rely solely on ultrasound for detecting small stones (<5 mm) as sensitivity decreases significantly for smaller calculi 1, 2

• Ultrasound findings within the first 2 hours of presentation may be falsely negative because secondary signs of obstruction may not have developed yet 3, 4

• Never use contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system, particularly small stones 1

• Plain radiography (KUB) alone is inadequate, with only 29% overall sensitivity for stones of any size and 72% sensitivity for large (>5 mm) proximal ureteral stones 3, 2

Algorithmic Approach

For non-pregnant adults with acute flank pain:

• Order low-dose non-contrast CT as first-line test 1, 2
• If CT is inconclusive, obtain CT with IV contrast to visualize the "soft tissue rim" sign around ureteral stones 1

For pregnant patients:

• Start with ultrasound of kidneys and bladder as first-line imaging 1, 4
• Consider MRI without contrast if ultrasound is inconclusive 1, 4
• Use low-dose CT only as last-line option if absolutely necessary 4

For children:

• Use ultrasound as first-line imaging 4, 2
• Follow with KUB or low-dose non-contrast CT if ultrasound does not provide required information 4

For recurrent stone formers:

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

Obsolete Modalities

• Intravenous urography (IVU) is no longer recommended, with inferior sensitivity (87%) and specificity (94%) compared to non-contrast CT (96% and 100% respectively) 3, 2

• MRI has limited utility in stone detection but can be considered when radiation exposure must be avoided and ultrasound is inconclusive 2