What percentage of nephrolithiasis (kidney stones) is radio-opaque?

Radio-Opacity of Nephrolithiasis

Approximately 90% of kidney stones are radio-opaque on plain radiography (KUB X-ray), consisting primarily of calcium oxalate, calcium phosphate, and struvite stones. 1

Stone Composition and X-Ray Characteristics

The radio-opacity of kidney stones varies significantly by their mineral composition:

• Radio-opaque stones (90% of all stones) include calcium oxalate, calcium phosphate, and struvite, which are visible on plain KUB radiography 1
• Radiolucent stones (10% of all stones) include uric acid and certain drug-related stones that are not visible on plain X-ray 2
• Stone appearance on plain X-ray varies by mineral composition and can affect treatment decisions 2

Critical Limitation: Radio-Opacity Does Not Equal Detectability

A common and dangerous pitfall is assuming that because 90% of stones are radio-opaque, KUB X-ray is an adequate diagnostic tool—this is incorrect. Despite most stones being radio-opaque, KUB has poor sensitivity:

• KUB detects only 8% of stones ≤5 mm even when they are radio-opaque 3
• KUB detects 78% of stones >5 mm 3
• Overall sensitivity of KUB for detecting stones of any size in any location is only 29% 3
• KUB sensitivity for ureteral stones specifically ranges from 44-77% 2 and 53-62% 1, 4

Factors Affecting Radio-Opaque Stone Visibility

Even when stones are radio-opaque, multiple factors reduce their detection on KUB:

• Stone location: Mid and distal ureteral stones are more difficult to visualize than renal or proximal ureteral stones 1
• Patient body habitus: Increased body mass reduces stone visibility 1
• Overlying bowel contents: Can obscure stone visualization 1
• Stone size: Dramatically affects detection rates as noted above 1, 3

Recommended Diagnostic Approach

Non-contrast CT is the gold standard for kidney stone diagnosis, not KUB, with 93-97% sensitivity and 95-100% specificity. 4, 3

The appropriate imaging algorithm is:

• First-line: Ultrasound for initial evaluation (45% sensitivity, 94% specificity for ureteral stones; 88% specificity for renal stones) 2
• Second-line: Non-contrast CT after ultrasound to assess stone location, burden, density, and anatomy 2
• Limited role for KUB: Helps differentiate radio-opaque from radiolucent stone types and aids in follow-up of known radio-opaque stones 2, 1
• Low-dose CT protocols: Maintain 93.1% sensitivity and 96.6% specificity while reducing radiation exposure 2

Special Populations

• Pregnant women: Use ultrasound first-line, MRI second-line, and low-dose CT only as last resort 2
• Children: Use ultrasound first-line, followed by KUB or low-dose non-contrast CT only if ultrasound is insufficient 2

Clinical Application

KUB should never be ordered as the sole diagnostic test for suspected kidney stones. 4 Its appropriate uses are limited to:

• Follow-up imaging for known radio-opaque stones to monitor interval growth 1, 3
• As an adjunct to ultrasound to help identify radio-opaque stones when ultrasound findings are equivocal 4
• Differentiating between radio-opaque and radiolucent stone types after diagnosis is established 2