Can a kidney stone be visualized on a plain abdominal X‑ray (KUB)?

Can You See a Kidney Stone with an X-ray?

Yes, you can sometimes see a kidney stone on a plain abdominal X-ray (KUB), but this imaging modality has poor sensitivity and should never be used as the sole diagnostic test for suspected kidney stones. 1

Understanding Radio-Opacity vs. Detectability

While approximately 90% of kidney stones are radio-opaque (primarily calcium oxalate, calcium phosphate, and struvite), this does not mean KUB can reliably detect them 2:

• KUB detects only 8% of stones ≤5 mm, even when they are radio-opaque 2
• KUB detects 78% of stones >5 mm 2
• Overall sensitivity of KUB for detecting stones of any size in any location is only 29% 2
• Sensitivity for ureteral stones specifically ranges from 44-77% 2

This critical distinction means that even though most stones contain calcium and are theoretically visible on X-ray, the practical detection rate is unacceptably low for clinical decision-making.

Factors That Reduce Stone Visibility on KUB

Several factors make stones difficult or impossible to visualize 1, 2:

• Stone size: Stones <4 mm are particularly difficult to detect 1
• Stone location: Mid and distal ureteral stones are harder to visualize than renal or proximal ureteral stones 1, 2
• Patient body habitus: Increased body mass significantly reduces stone visibility 2, 3
• Overlying bowel contents: Can obscure visualization 3
• Stone composition: Uric acid and certain drug-related stones are radiolucent and completely invisible on X-ray 2

What Imaging Should Be Used Instead

Non-contrast CT is the gold standard for kidney stone diagnosis, with 93-97% sensitivity and 95-100% specificity 2:

• The American College of Radiology explicitly recommends CT scan (non-contrast) as the initial diagnostic test when urolithiasis is suspected, not KUB 1
• CT dramatically outperforms KUB with 93-96% sensitivity and 93-100% specificity for confirming obstruction 1
• Low-dose CT protocols maintain 93.1% sensitivity and 96.6% specificity while reducing radiation exposure 2

Ultrasound is recommended as the first-line imaging test in certain populations 2:

• In pregnant women, ultrasound is first-line, followed by MRI, with low-dose CT only as last resort 2
• In children, ultrasound is first-line, followed by KUB or low-dose CT only if ultrasound is insufficient 2
• Ultrasound has 45% sensitivity and 94% specificity for ureteral stones, and 88% specificity for renal stones 2

Limited Acceptable Uses of KUB

KUB has only narrow clinical applications 1, 4:

• Long-term surveillance of known radio-opaque stone disease to monitor interval stone growth 1
• As an adjunct to ultrasound for identifying radio-opaque stones when ultrasound findings are equivocal 1, 2
• Combining KUB with ultrasound improves sensitivity compared to either modality alone, identifying approximately 50% of stones that would be detected by CT 3

Critical Clinical Pitfall

Do not assume that 90% of stones being radio-opaque means KUB is adequate—the sensitivity remains poor even for radio-opaque stones 1. In practical terms, if you order only a KUB for suspected kidney stones, you will miss the majority of clinically significant pathology 1. One study found that plain abdominal X-ray is more sensitive than CT scout films (48% vs. 17%), but this still means KUB misses more than half of stones 5.

Special Consideration for Follow-Up

If a stone is visible on the initial CT scout film, it will also be visible on KUB, making KUB appropriate for follow-up imaging in that specific patient 6. However, all stones visible on CT scout are also visible on KUB, but not vice versa—KUB detects stones that CT scout misses 5, 6. This means you should check the CT scout film first; if the stone is visible there, KUB can be used for subsequent monitoring to minimize radiation exposure 6.