Will kidney stones be visible on a renal ultrasound (ultrasound of the kidney)?

Will Kidney Stones Show Up on Renal Ultrasound?

Kidney stones will sometimes show up on renal ultrasound, but ultrasound has poor sensitivity (24-57%) for directly detecting stones compared to CT scan, which is the gold standard with 97% sensitivity. 1

Direct Stone Detection Performance

Ultrasound demonstrates highly variable and generally poor performance for visualizing kidney stones themselves:

• Gray-scale ultrasound has an overall sensitivity of only 24-57% for detecting renal calculi when compared to noncontrast CT as the reference standard 1
• Detection is even worse for ureteral stones, with sensitivity up to 61% (though specificity remains 100%) 1
• Sensitivity drops further for smaller stones, with ultrasound particularly limited for stones <5mm 1, 2
• One study found ultrasound accuracy of only 52-57% for detecting stones in the right kidney and 32-39% for the left kidney 3

What Ultrasound Actually Detects Well

The real clinical utility of ultrasound is detecting hydronephrosis (kidney swelling from obstruction), not the stones themselves:

• Ultrasound has 95% sensitivity and 100% specificity for detecting and grading hydronephrosis 4
• Emergency physician-performed bedside ultrasound shows 78.4% sensitivity for detecting hydronephrosis in patients with CT-proven stones 2
• Sensitivity improves to 90% when stones are ≥6mm (which are clinically more significant) 2
• The combined finding of either hydronephrosis OR visualized stones on ultrasound reaches 82.4% sensitivity 2

Clinical Decision-Making Algorithm

When to use ultrasound as first-line imaging:

• Pregnant patients - ultrasound is the imaging modality of choice due to absence of radiation 1, 4
• Pediatric patients - to avoid radiation exposure 4
• Patients with renal impairment - avoids nephrotoxic contrast agents 4
• Initial screening in low-risk presentations - particularly when combined with urinalysis 5

When ultrasound findings should prompt CT:

• Negative ultrasound does NOT rule out stones - it has a negative predictive value of only 65% for ureteral stones 6
• However, absence of hydronephrosis on ultrasound makes stones >5mm less likely (NPV 89%) 6
• If clinical suspicion remains high despite negative ultrasound, proceed to noncontrast CT 5

Critical Pitfalls to Avoid

Do not rely on ultrasound alone for definitive stone diagnosis:

• Ultrasound will miss approximately 43-76% of kidney stones that are present 1, 3
• Small stones (<5mm) are particularly likely to be missed, yet these account for the majority of cases 1
• Stone size estimation on ultrasound is limited and tends to overestimate compared to CT 1

The absence of hydronephrosis does not exclude stones:

• Hydronephrosis may not develop in the first 2 hours of presentation 7
• Non-obstructing stones (which may still cause symptoms) will not produce hydronephrosis 7
• Up to 7-77% of small non-obstructing stones can cause symptoms without obstruction 7

Recommended Imaging Strategy

The American College of Radiology recommends:

• Noncontrast CT is the reference standard for evaluating suspected urolithiasis, with sensitivity as high as 97% 1
• Low-dose CT protocols maintain 97% sensitivity while reducing radiation exposure 1
• Ultrasound should be used as first-line only in specific populations (pregnant, pediatric, renal impairment) or when combined with clinical findings 1, 4
• The combination of ultrasound plus KUB radiography may improve detection compared to either alone, though still inferior to CT 1

Bottom line: If you need to definitively detect or rule out kidney stones, order a noncontrast CT scan. Ultrasound is excellent for detecting complications (hydronephrosis) but will miss the majority of stones themselves. 1, 5