In an adult with acute flank pain and possible urolithiasis, is a KUB (plain abdominal X‑ray) the preferred initial imaging study?

KUB for Kidney Stone: Not Recommended as Initial Imaging

KUB (plain abdominal X-ray) should not be used as the initial imaging study for suspected kidney stones—non-contrast CT of the abdomen and pelvis is the gold standard and should be ordered first. 1, 2

Why KUB Fails as Initial Imaging

The diagnostic performance of KUB is unacceptably poor for acute stone evaluation:

• KUB detects only 8% of stones ≤5 mm and 78% of stones >5 mm when compared to CT as the reference standard 1, 3
• Overall sensitivity is only 29% for stones of any size in any location, though it improves to 72% for large (>5 mm) proximal ureteral stones 1, 4
• Multiple factors limit KUB sensitivity: stone composition, location, size, patient body habitus, and overlying bowel contents 1
• KUB cannot distinguish phleboliths from ureteral stones on a single 2-D view, leading to diagnostic confusion 1

The Gold Standard: Non-Contrast CT

Non-contrast CT dramatically outperforms KUB and should be your first-line imaging:

• Sensitivity of 97% and specificity of 95% for stone detection 2
• Detects stones as small as 1 mm throughout the entire urinary tract 2
• Provides accurate stone size measurements critical for treatment planning using coronal reformations and bone window settings 3, 2
• Shows secondary signs of obstruction including hydronephrosis, periureteral inflammation, and ureteral dilation 2
• Low-dose CT protocols maintain 97% sensitivity while reducing radiation exposure to <3 mSv 2

When to Consider Alternative Imaging

Ultrasound may be used as first-line imaging only in specific populations:

• Pregnant patients: Ultrasound is preferred due to absence of radiation exposure 3, 2
• Pediatric patients: Ultrasound avoids radiation in children 3
• Patients with renal impairment: Ultrasound avoids nephrotoxic contrast (though CT for stones requires no contrast) 3
• Recurrent stone formers requiring frequent surveillance: Ultrasound limits cumulative radiation 2

However, be aware that ultrasound has poor sensitivity (24-57%) for direct stone detection and even worse performance for ureteral stones (up to 61% sensitivity) 1, 3. If clinical suspicion remains high despite negative ultrasound, proceed immediately to non-contrast CT 3.

The Limited Role of KUB

KUB has only one acceptable indication in stone disease:

• Long-term surveillance of known stone disease to monitor interval stone growth in patients with previously documented radio-opaque stones 4, 5

Even in this limited role, KUB is acknowledged to be less sensitive than CT 4.

Critical Pitfalls to Avoid

• Do not order KUB as a standalone test for suspected acute kidney stones—it will miss the majority of clinically significant stones 4
• Do not assume that 90% of stones being radio-opaque means KUB is adequate—sensitivity remains poor even for radio-opaque stones 4
• A negative KUB does not exclude a stone: KUB fails to detect approximately 22% of stones >5 mm and 92% of stones ≤5 mm 3
• Do not add IV contrast to CT for stone detection—it reduces sensitivity for small stones and adds unnecessary cost, radiation, and contrast-related risks 3

Practical Imaging Algorithm

For an adult with acute flank pain and suspected urolithiasis:

1. Order low-dose non-contrast CT of abdomen and pelvis as the initial test 1, 2
2. Ensure the CT includes the pelvis to detect distal ureteral and bladder stones 3, 2
3. If CT is unavailable or contraindicated, use ultrasound—not KUB—as the alternative 4
4. If ultrasound is negative but clinical suspicion remains high, proceed to CT 3

The evidence is unequivocal: KUB has no role in the initial evaluation of suspected kidney stones in modern practice 1, 4, 2.