Do radiologists have high accuracy in detecting kidney stones using ultrasound in adult patients?

Radiologist Accuracy for Kidney Stone Detection Using Ultrasound

Radiologists have limited accuracy when using ultrasound to detect kidney stones, with sensitivity ranging from only 24-57% for renal stones and even lower (45-61%) for ureteral stones, making ultrasound inadequate as a standalone imaging modality compared to the gold standard noncontrast CT which achieves 97% sensitivity. 1, 2

Direct Stone Detection Performance

The American College of Radiology guidelines establish that gray-scale ultrasound demonstrates poor diagnostic performance for kidney stone detection regardless of operator expertise 1:

• Renal stone sensitivity: 24-57% when compared to noncontrast CT as the reference standard 1, 2
• Ureteral stone sensitivity: 45-61% with specificity of 100%, though sensitivity improves when secondary signs of obstruction are present 1, 2
• Right kidney detection is superior to left kidney: sensitivity of 52-57% for right kidney versus 32-39% for left kidney, with overall accuracy of 67-77% for right kidney and only 53-54% for left kidney 3

Recent 2025 data confirms ultrasound sensitivity of only 77% with a positive predictive value of 75% for stone detection, and the PPV drops to just 59% for stones >4 mm 4.

Stone Size Measurement Inaccuracy

Ultrasound systematically overestimates stone size, particularly for smaller stones ≤5 mm, which can lead to unnecessary surgical interventions in up to 40% of patients. 1, 5, 4

• Mean stone size measured 8.7 mm on ultrasound versus 5.5 mm on CT (p=0.02) 4
• Size overestimation is more pronounced in patients with higher body mass index 4
• This overestimation tendency creates clinical risk by suggesting stones are larger and less likely to pass spontaneously than they actually are 1, 5

Detection of Secondary Signs vs. Direct Stone Visualization

While ultrasound performs poorly for direct stone detection, it demonstrates better accuracy for identifying complications 1, 2:

• Hydronephrosis detection: 93-100% sensitivity and 90-100% specificity 1, 2, 6
• However, within the first 2 hours of symptom onset, secondary signs of obstruction may not have developed yet, limiting even this utility 1, 2, 5
• The absence of hydronephrosis on ultrasound makes a larger obstructing stone (>5 mm) less likely but does not rule out smaller stones 1, 2
• Hydronephrosis presence does not accurately predict stone presence or absence in up to 25% of patients 1

Enhanced Ultrasound Techniques

The American College of Radiology acknowledges that adding color Doppler with twinkling artifact assessment can improve sensitivity 1, 5:

• Sensitivity as high as 99% for stones <5 mm when twinkling artifact is assessed 1, 5
• However, twinkling artifact has a false-positive rate up to 60%, significantly limiting its clinical utility 1
• Performance is influenced by stone site and diameter 1

Operator and Technical Variability

Individual sonographer performance significantly affects diagnostic accuracy, introducing substantial operator-dependent variability. 4, 7

• Significant differences in stone detection (p=0.01) and size measurement (p=0.03) were associated with the individual performing the ultrasound 4
• Fair agreement between ultrasound and CT for stone location (Kappa = 0.368) and hydronephrosis severity (Kappa = 0.394) 7
• Ultrasound is less sensitive for stones in the middle and upper calyx compared to CT 7

Clinical Implications and Appropriate Use

The American College of Radiology recommends noncontrast CT as the reference standard with 97% sensitivity for evaluating suspected urolithiasis in adults. 1, 2

Ultrasound should be reserved as first-line imaging only for specific populations 2:

• Pregnant patients (to avoid radiation exposure) 2
• Pediatric patients (to minimize radiation) 2
• Patients with renal impairment (to avoid nephrotoxic contrast agents) 2

When ultrasound is used in average-risk adults, combining it with KUB radiography improves sensitivity from 45% to 77% for ureteral stones, though this remains inferior to CT 1, 6.

Critical Pitfall to Avoid

If clinical suspicion for kidney stones remains high despite negative or equivocal ultrasound findings, proceed directly to noncontrast CT rather than relying on ultrasound results alone. 2, 5 The low sensitivity of ultrasound means that a negative study does not exclude clinically significant stones, and the size overestimation may lead to inappropriate treatment escalation 4.