Would a kidney stone be visible on a renal ultrasound?

Renal Ultrasound Detection of Kidney Stones

Yes, kidney stones can show up on renal ultrasound, but ultrasound has significant limitations compared to CT scanning, particularly for smaller stones and precise stone characterization.

Diagnostic Performance of Ultrasound for Kidney Stones

Detection Capabilities

• Ultrasound can detect kidney stones in approximately 93% of patients with radiographically opaque stones, but this detection rate varies significantly by stone characteristics 1
• Ultrasound has a sensitivity of only 45% for detecting stones overall, with specificity of 88% for renal stones specifically 2
• The modality is particularly poor at detecting all stones present in a given kidney—ultrasound detected all stones in only 60% of patients who had multiple calculi 1

Size-Dependent Limitations

• Small stones are frequently missed: 30% of papillary-calyceal stones visible on plain radiography were not detected by ultrasound 1
• 66% of missed stones measured ≤2mm, highlighting ultrasound's poor sensitivity for small calculi 1
• Ultrasound sensitivity ranges from 24-57% for stone detection when compared to CT as the reference standard 3

When Ultrasound Is Appropriate

Guideline-Recommended Scenarios

• Ultrasound is the primary diagnostic tool recommended by the European Association of Urology for initial evaluation of suspected urolithiasis, though it should not delay emergency care 2
• The American College of Radiology recommends ultrasound as first-line imaging for pregnant patients, pediatric patients, and when radiation avoidance is a priority 4, 5
• Ultrasound is "usually appropriate" for evaluating suspected urolithiasis in patients with painful, nontraumatic hematuria 5

Ultrasound's Strengths

• Excellent for detecting hydronephrosis: sensitivity of 95% and specificity of 100% for detecting and grading hydronephrosis 5
• No radiation exposure, making it safer than CT or plain radiography 5
• Can be performed rapidly at the bedside in emergency settings 6

Critical Clinical Context

Hydronephrosis as a Surrogate Marker

• The absence of hydronephrosis on ultrasound makes a larger obstructing stone (>5mm) less likely (negative predictive value 89%) 6
• However, absence of hydronephrosis does NOT rule out symptomatic non-obstructing stones 3
• Hydronephrosis may not be present in the first 2 hours of acute obstruction, creating false-negative results 3

When CT Is Superior and Necessary

• Non-contrast CT is the reference standard for urolithiasis evaluation with sensitivity as high as 97% 2
• CT should be performed after ultrasound when stone disease is suspected and ultrasound is non-diagnostic 2
• Virtually all renal calculi are radiopaque on CT, allowing detection of even very small stones 2

Practical Algorithm for Stone Detection

Initial Imaging Approach

1. Start with ultrasound if the patient is pregnant, pediatric, or when minimizing radiation is a priority 2, 5
2. Proceed directly to non-contrast CT in non-pregnant adults when stone disease is strongly suspected and rapid, definitive diagnosis is needed 2
3. Upgrade to CT if ultrasound is equivocal or negative but clinical suspicion remains high 2, 4

Combining Modalities

• Ultrasound plus plain KUB radiography may improve diagnostic accuracy compared to either modality alone, though this combination still underperforms compared to CT 4
• Low-dose CT protocols (sensitivity 93.1%, specificity 96.6%) can reduce radiation while maintaining excellent diagnostic accuracy 2

Key Pitfalls to Avoid

• Do not rely on ultrasound alone to rule out kidney stones—a negative ultrasound does not exclude stone disease, particularly for small or non-obstructing stones 3, 1
• Do not assume ultrasound can accurately count or size multiple stones—it detected all stones in only 60% of patients with multiple calculi 1
• Do not use ultrasound for precise stone measurements—CT is far superior for determining stone size, which is critical for treatment planning 2
• Do not mistake absence of hydronephrosis for absence of stones—symptoms can develop in 7-77% of cases with small non-obstructing renal stones 3

Bottom line: While ultrasound can detect many kidney stones and is appropriate as first-line imaging in specific populations, CT remains the gold standard when definitive diagnosis, stone characterization, and treatment planning are required 2.