Can ultrasound detect kidney stones in adults with a history of kidney stones or symptoms suggestive of kidney stones, such as flank pain or hematuria?

Can Ultrasound Detect Kidney Stones?

Ultrasound has poor sensitivity (24-57%) for directly detecting kidney stones and should not be relied upon as the primary imaging modality in adults with suspected urolithiasis, except in specific populations where radiation avoidance is critical. 1

Diagnostic Performance of Ultrasound vs. CT

Direct Stone Detection

• Gray-scale ultrasound demonstrates only 24-57% sensitivity for detecting renal calculi when compared to noncontrast CT as the reference standard 1, 2
• Detection is even worse for ureteral stones, with sensitivity up to 61% (though specificity remains 100%) 1
• Ultrasound tends to miss 30% of small papillary-calyceal stones, with 66% of missed stones measuring ≤2 mm 3
• The addition of color Doppler with twinkling artifact can improve sensitivity for small renal stones (<5 mm) to as high as 99%, but carries a false-positive rate up to 60% 1, 4

Indirect Signs of Stone Disease

While ultrasound performs poorly for direct stone visualization, it excels at detecting complications:

• 95-100% sensitivity and 90-100% specificity for detecting and grading hydronephrosis 1, 5
• The absence of hydronephrosis on ultrasound makes larger stones (>5 mm) less likely (NPV 89%) 4, 5
• However, within the first 2 hours of symptom onset, secondary signs of obstruction may not have developed yet, further limiting ultrasound utility 1, 4

Gold Standard: Noncontrast CT

Noncontrast CT of the abdomen and pelvis is the reference standard for evaluating urolithiasis, with sensitivity as high as 97% 6, 1:

• Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones 6
• Low-dose CT protocols (<3 mSv) maintain 97% sensitivity while reducing radiation exposure 1, 4
• CT directly visualizes calculi throughout the entire ureter independent of bladder filling status 1
• Detection accuracy decreases only with smaller stone size (<3 mm), not with patient factors 1

Clinical Algorithm for Imaging Selection

First-Line Imaging in General Adult Population

Obtain noncontrast CT of abdomen and pelvis for acute flank pain with suspected kidney stones 6, 1:

• Provides comprehensive evaluation of the entire urinary tract 1
• Allows assessment of stone size, location, and secondary complications 6
• Stone size >5 mm predicts need for intervention and 30-day events 4, 5

Ultrasound as First-Line Only in Specific Populations

The American College of Radiology recommends ultrasound as first-line imaging only for 1, 7:

• Pregnant patients (to avoid radiation exposure)
• Pediatric patients (per American Academy of Family Physicians guidelines)
• Patients with renal impairment (to avoid nephrotoxic contrast agents, though noncontrast CT uses no contrast)

When Ultrasound is Performed

If ultrasound is obtained and clinical suspicion remains high despite negative or equivocal findings, proceed to noncontrast CT 1, 4:

• Do not rely on bladder distension protocols, as stone visualization depends on the imaging modality's inherent sensitivity 1
• The combination of ultrasound plus KUB radiography may improve detection (sensitivity 77% for ureteral stones) compared to ultrasound alone (45%), though still inferior to CT 1, 8

Common Pitfalls to Avoid

• Do not use contrast-enhanced CT as first-line for suspected stones, as enhancing renal parenchyma may obscure stones in the collecting system (sensitivity drops to 81% overall, though 95% for stones ≥3 mm and 98% for stones ≥6 mm) 6
• Do not assume negative ultrasound rules out clinically significant stones, particularly in the ureter where sensitivity is only 45-61% 1, 8
• Be aware that ultrasound overestimates stone size, particularly for stones ≤5 mm 4
• Do not wait for hydronephrosis to develop before obtaining definitive imaging, as early presentation may not show obstruction 1, 4