Does a KUB (Kidneys, Ureters, and Bladder) X-ray show kidney stones in a patient suspected of having kidney stones?

KUB X-ray for Kidney Stone Detection

A KUB X-ray has poor sensitivity for detecting kidney stones and should not be used as the primary imaging modality—non-contrast CT is the reference standard with 97% sensitivity for all stone types. 1, 2

Diagnostic Performance of KUB

KUB radiography demonstrates severely limited stone detection capabilities:

• **Detects only 8% of stones <5mm** and 78% of stones >5mm, missing the majority of clinically significant calculi 1, 2
• Overall sensitivity of 29% for stones of any size in any location when compared to non-contrast CT as the reference standard 1
• Sensitivity of 72% for large stones (>5mm) in the proximal ureter, but this drops dramatically for smaller stones and other locations 1
• Sensitivity of 53-62% and specificity of 67-69% for detecting ureteral calculi 3

Why KUB Misses Stones

Multiple factors limit KUB's diagnostic accuracy:

• Stone composition: Uric acid stones are completely radiolucent on plain radiography despite being visible on CT 2
• Stone size: Particularly insensitive for stones <4mm and those in the mid and distal ureters 3
• Patient factors: Body habitus and overlying bowel contents obscure visualization 1
• Location: Cannot reliably distinguish ureteral stones from phleboliths or vascular calcifications on a single 2-D view 1

The Gold Standard: Non-Contrast CT

The American College of Radiology recommends non-contrast CT abdomen/pelvis as the reference standard for detecting kidney stones 1, 2:

• 97% sensitivity for detecting all stone types, including radiolucent uric acid stones 2
• Detects stones as small as 1mm with high accuracy 1
• Provides critical additional information: hydronephrosis, perinephric fluid, stone size, location, and density (Hounsfield units) 1

Limited Role for KUB

KUB has only narrow, specific applications in stone management:

• Long-term surveillance of known stone disease to monitor interval stone growth, acknowledging its inferior sensitivity compared to CT 3
• Adjunct to ultrasound for identifying radio-opaque stones (90% of stones contain calcium), though sensitivity remains poor even for these 3, 4
• Treatment planning after CT diagnosis to determine radio-opacity for ESWL candidacy, as non-contrast CT alone cannot predict whether a stone will be visible fluoroscopically 5

Critical Pitfall to Avoid

Do not order KUB as a standalone test for suspected kidney stones—it will miss the majority of clinically significant pathology 2, 3. Even though 90% of stones are theoretically radio-opaque (calcium oxalate, calcium phosphate, struvite), KUB's sensitivity remains unacceptably low due to size, location, and patient factors 1, 3.

Practical Algorithm

For patients with suspected kidney stones:

1. Order non-contrast CT abdomen/pelvis as the initial diagnostic test 1, 2
2. If CT is unavailable or contraindicated (pregnancy, radiation concerns), use ultrasound as first-line alternative—not KUB 3
3. Consider KUB only after CT diagnosis if planning ESWL treatment to confirm fluoroscopic visibility 5
4. For follow-up of known stones, KUB may be used for surveillance if the stone was previously visible, understanding its limitations 3