What is the first line of checking for suspected kidney stones?

First-Line Imaging for Suspected Kidney Stones

Low-dose non-contrast CT of the abdomen and pelvis is the first-line imaging test for suspected kidney stones in most patients, with 97% sensitivity and 95% specificity. 1, 2

Primary Recommendation: Low-Dose Non-Contrast CT

The American College of Radiology assigns non-contrast CT an appropriateness rating of 8 (usually appropriate) for suspected stone disease, making it the gold standard imaging modality. 3 This recommendation is based on several critical advantages:

• Detects virtually all stones regardless of composition with >95% sensitivity and specificity 3, 1
• Provides precise stone measurements and exact ureteral location, essential for determining whether conservative management or surgical intervention is needed 3, 1
• Identifies secondary signs of obstruction including hydronephrosis, perinephric stranding, and ureteral dilation 3, 1
• Reveals alternative diagnoses if stones are ruled out, unlike older modalities such as intravenous urography 3

Use low-dose protocols (<3 mSv) rather than conventional dosing to minimize radiation exposure while maintaining excellent diagnostic accuracy. 1, 2 If low-dose CT reduces sensitivity, secondary signs and dual-energy CT can clarify findings. 3

Alternative First-Line Imaging in Special Populations

Pregnant Patients

Ultrasound of the kidneys and bladder is the first-line imaging choice in pregnant patients (appropriateness rating of 8), avoiding ionizing radiation to the fetus. 3 However, recognize these limitations:

• Ultrasound has lower sensitivity for direct stone visualization (24-57%) but excellent sensitivity (up to 100%) for detecting hydronephrosis 1, 2
• Combining ultrasound with plain radiography (KUB) improves diagnostic accuracy to 79-90% sensitivity, providing an acceptable alternative to CT 3, 1, 2
• If ultrasound is inconclusive, consider non-contrast MRU, though it has only 69% sensitivity for identifying stone location compared to CT's 100% 3

Recurrent Stone Disease

For patients with known prior stones presenting with recurrent symptoms:

• Non-contrast CT remains usually appropriate (rating of 7), preferably at reduced doses 3
• Limit scanning to the bladder or area of interest to minimize cumulative radiation exposure 3, 1, 2
• Ultrasound (rating of 7) is appropriate for evaluating hydronephrosis in this population 3
• Large radiopaque stones may appear on KUB radiography (rating of 5), allowing tracking of stone progression 3

What NOT to Use as First-Line Imaging

Plain Radiography (KUB) Alone

Abdominal radiography has an appropriateness rating of only 3 (usually not appropriate) as a standalone test. 3 Its narrow capabilities for visualizing different etiologies of renal colic could lead to repeat imaging, canceling out the benefit of lower radiation exposure. 3

MRI

Magnetic resonance imaging is less accurate for identifying stones (rating of 4, may be appropriate) but highly dependable for depicting hydronephrosis and perinephric edema. 3 It should not be used as first-line imaging except when radiation must be avoided and ultrasound is inadequate. 1, 2

Common Pitfalls to Avoid

• Do not use contrast-enhanced CT as first-line imaging—enhancing renal parenchyma obscures stones within the collecting system, particularly small stones 1, 2
• Motion artifact can make stones appear larger than actual size on CT, particularly in obese patients; correlate with KUB and ultrasound when planning surgical approach 4
• Normal-sized kidneys on ultrasound do not exclude stone disease—focus on detecting hydronephrosis and secondary signs of obstruction 3
• Ultrasound sensitivity decreases significantly for stones <5 mm; do not rely solely on ultrasound for detecting small calculi 1

When Initial CT is Inconclusive

If non-contrast CT cannot definitively confirm whether a calcification is within the ureter versus an adjacent structure (common mimics include phleboliths or arterial calcifications):

• 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, 1
• Alternatively, CT urography (CTU) with urinary tract opacification confirms ureteral location and distinguishes stones from mimics 3