What non‑contrast helical CT (CT‑KUB) scan is recommended to rule out renal or ureteral stones in a 30‑year‑old woman?

Imaging for Suspected Kidney Stones

Non-contrast helical CT of the abdomen and pelvis (CT-KUB) is the definitive imaging modality to rule out kidney stones, with 97% sensitivity for detecting all stone types regardless of size, location, or composition. 1, 2, 3

Why Non-Contrast CT is the Gold Standard

Non-contrast CT directly visualizes calculi throughout the entire urinary tract—from the ureteropelvic junction to the vesicoureteric junction—without requiring contrast administration or bladder distension. 2 This imaging modality:

• Detects even radiolucent uric acid stones that are invisible on plain radiography 2, 3
• Identifies stones as small as 1 mm with 97% sensitivity and 95% specificity 2, 3
• Provides accurate stone size measurements critical for treatment planning using coronal reformations and bone window settings 2
• Reveals secondary signs of obstruction including hydronephrosis, periureteral inflammation, and ureteral dilation 2

Technical Protocol Specifications

The scan should extend from the T10 vertebral body superiorly to the pubic symphysis inferiorly to capture the entire urinary tract while minimizing unnecessary radiation exposure. 4, 5 The American College of Radiology emphasizes that:

• CT imaging must include the pelvis to detect stones in the distal ureters and bladder 1, 2
• Low-dose protocols (<3 mSv) maintain the same 97% sensitivity while substantially reducing radiation exposure 2, 6
• Thin-slice acquisition with multiplanar reconstruction optimizes stone detection 1

Why Other Imaging Modalities Are Inadequate

Plain KUB Radiography

Plain KUB has unacceptably poor sensitivity and should never be used as the sole imaging test for suspected kidney stones. 2, 7 The American College of Radiology reports that KUB:

• Detects only 8% of stones ≤5 mm and 78% of stones >5 mm 2, 7
• Has overall sensitivity of only 29% for stones of any size in any location 2
• Is particularly insensitive for stones <4 mm and those in the mid and distal ureters 7
• May be useful only for long-term surveillance of known stone disease to monitor interval growth, not for acute diagnosis 2, 7

Ultrasound

Ultrasound has poor direct stone detection with sensitivity of only 24-57% compared to CT's 97%. 2, 3 The American College of Radiology notes:

• Sensitivity for ureteral stones is even worse (45-61%), though specificity remains ~100% 2
• Ultrasound is 95-100% sensitive for detecting secondary signs of obstruction (hydronephrosis), but within the first 2 hours of symptom onset these signs may not have developed yet 2
• Ultrasound should be reserved as first-line imaging only for pregnant patients, pediatric patients, or those with renal impairment to avoid radiation or nephrotoxic contrast 2

Intravenous Urography (IVU)

The American College of Radiology explicitly states that IVU provides no benefit for initial evaluation of suspected nephrolithiasis and should not be used as a first-line test. 7, 3 IVU is obsolete in modern practice and provides inferior anatomic detail compared to non-contrast CT. 3

MR Urography

MRU has poor accuracy for detecting small urothelial calculi, detecting only 79% of urinary stones compared to 100% detection by CT. 1, 3

Clinical Algorithm for a 30-Year-Old Woman

For this specific patient population:

1. Order non-contrast helical CT of the abdomen and pelvis (CT-KUB) as the initial and definitive imaging study. 1, 2, 3

2. The American College of Radiology notes that in patients <30 years of age without risk factors for urinary tract malignancy, ultrasound or non-contrast CT may be first-line examinations. 1 However, given the superior sensitivity of CT (97% vs. 24-57% for ultrasound), non-contrast CT remains the preferred initial test when stone detection is the primary clinical question. 2, 3

3. Avoid contrast-enhanced CT specifically for stone detection, as it adds unnecessary cost, radiation exposure, contrast-related risks, and actually reduces sensitivity for small stones compared to non-contrast imaging. 2

Critical Pitfalls to Avoid

• Never rely on a negative KUB to exclude kidney stones—it misses 92% of stones ≤5 mm and 22% of stones >5 mm 2
• Do not assume that 90% of stones being radiopaque means KUB is adequate—the sensitivity remains poor even for radiopaque stones 7
• A negative ultrasound provides only 65% negative predictive value for ruling out urinary stones, so it cannot exclude the diagnosis 7
• Contrast-enhanced CT has slightly lower sensitivity than non-contrast CT for detecting small renal calculi, though stones ≥6 mm are detected with 98% accuracy on contrast studies 2