Will a renal ultrasound (US) show a kidney stone?

Renal Ultrasound Detection of Kidney Stones

Renal ultrasound can detect kidney stones, but it has significant limitations with only 24-57% sensitivity for direct stone visualization compared to CT, though it performs much better (78-81% sensitivity) for detecting at least one stone per kidney and is highly sensitive (93-100%) for identifying secondary signs of obstruction like hydronephrosis. 1, 2

Direct Stone Detection Performance

Renal ultrasound's ability to directly visualize kidney stones varies considerably based on several factors:

• Gray-scale ultrasound alone has modest sensitivity of 24-57% for detecting individual renal calculi when compared to non-contrast CT as the reference standard 1
• Per-kidney sensitivity improves to 78-81%, meaning ultrasound can identify at least one stone in a kidney even if it misses others 1, 2
• Specificity remains high at 91-100%, so when ultrasound identifies a stone, it is usually correct 3, 2

Stone Size Matters Significantly

The detection rate is heavily influenced by stone size:

• Stones <5 mm are detected in only 8-24% of cases 1
• Stones >5 mm have detection rates of 70-78% 1
• Color Doppler with twinkling artifact can improve sensitivity to as high as 99% for small stones (<5 mm), though this technique has a false-positive rate up to 60% 1, 4

Stone Location Affects Detection

• Renal stones are detected more reliably than ureteral stones 1, 2
• Ureteral calculi have particularly poor sensitivity of only 45-61% on ultrasound alone 1, 5
• The left upper calyx has lower detection rates than other renal locations 2

Critical Limitation: Size Overestimation

A major pitfall is that ultrasound significantly overestimates stone size, particularly for stones ≤10 mm, which can lead to inappropriate management decisions. 1, 3

• Ultrasound overestimates size in stones 0-10 mm range, with more pronounced overestimation in smaller stones and patients with higher BMI 3, 6
• This size discrepancy leads to inappropriate counseling in approximately 20-40% of patients, where ultrasound suggests intervention for stones that CT would classify as appropriate for observation 3, 6
• Stones measured as 5-10 mm on ultrasound have the highest probability (43%) of having management recommendations changed when CT is performed 3

Indirect Detection: Hydronephrosis as a Marker

Ultrasound excels at detecting secondary signs of obstruction, which can be more clinically useful than direct stone visualization:

• Sensitivity of 93-100% and specificity of 90-100% for detecting hydronephrosis/ureteral obstruction 1, 5
• Moderate or greater hydronephrosis on ultrasound is 94.4% specific for presence of symptomatic renal stone in patients with renal colic 1
• Important caveat: Within the first 2 hours of presentation, secondary signs may not have developed yet, reducing sensitivity 1, 4
• Absence of hydronephrosis makes a larger obstructing stone (>5 mm) less likely 1, 4

Improving Ultrasound Performance

Combining ultrasound with plain radiography (KUB) improves detection rates:

• Ultrasound plus KUB achieves 77-90% sensitivity for stone detection versus 79-93% for CT alone 1, 5
• This combination is particularly helpful since 90% of stones are radio-opaque 7
• However, KUB is still comparatively insensitive for stones <4 mm and those in the mid/distal ureters 1, 7

When Ultrasound is Appropriate

According to the American College of Radiology guidelines, ultrasound is appropriate as initial imaging in specific scenarios:

• Pregnant patients (to avoid radiation exposure) 1, 7
• Known stone disease surveillance (though CT remains more accurate) 7, 8
• When CT is unavailable or contraindicated 7
• Pediatric patients (to minimize radiation) 7

When CT is Superior and Necessary

Non-contrast CT remains the gold standard with 96-97% sensitivity and 98-100% specificity for stone detection. 1, 4

The American College of Radiology recommends CT as initial imaging when:

• Acute stone disease is suspected and diagnosis will change management 1, 7
• Ultrasound findings are equivocal 4
• Precise stone size measurement is needed for treatment planning 3, 6
• Detection of small stones (<5 mm) or ureteral stones is critical 1

Practical Clinical Algorithm

For suspected kidney stones:

1. If radiation exposure is acceptable: Non-contrast CT is first-line 1, 7
2. If radiation must be avoided (pregnancy, pediatrics): Ultrasound with color Doppler is first-line 1, 7, 4
3. If ultrasound shows hydronephrosis in symptomatic patient: This is highly specific (94.4%) for obstructing stone; may not need CT 1
4. If ultrasound is negative but clinical suspicion remains high: Proceed to CT 4
5. For surveillance of known stones: Ultrasound is reasonable but recognize 20-40% may be inappropriately counseled on size 3, 6

Key Pitfalls to Avoid

• Do not rely on ultrasound alone for treatment planning decisions, as size overestimation occurs in up to 40% of cases with stones >4 mm 6
• Do not assume negative ultrasound rules out stones, especially small (<5 mm) or ureteral stones 1
• Recognize operator dependence: Differences in stone detection and size measurement are associated with the individual performing the ultrasound 6
• Consider timing: Early presentation (<2 hours) may not show hydronephrosis even with obstructing stones 1, 4