Can ultrasound of the kidneys detect renal calculi (kidney stones)?

From the Guidelines

Ultrasound can detect renal calculi, but its sensitivity varies depending on the clinical scenario and stone size, with reported sensitivity ranging from 24% to 57% for stone detection, and up to 100% sensitive and 90% specific for the diagnosis of ureteral obstruction. The effectiveness of ultrasound in detecting renal calculi is influenced by several factors, including stone size, location, and patient characteristics such as obesity. According to a recent study published in the Journal of the American College of Radiology 1, the addition of color Doppler and assessment of twinkling artifact can improve the sensitivity of ultrasound for detecting small renal stones, with reported sensitivity as high as 99% for stones <5 mm. However, this technique is prone to false-positives, with a false-positive rate reported up to 60% 1.

Key Points to Consider

• Ultrasound is a useful initial screening tool for detecting renal calculi, with the advantage of no radiation exposure, lower cost, and wider availability compared to CT scans.
• The sensitivity of ultrasound for detecting renal calculi is lower than that of CT scans, which remain the gold standard for kidney stone detection with sensitivity above 95% 1.
• The combination of ultrasound and radiography can improve stone detection, with reported sensitivity of 79% to 90% for detecting stones, although this may not be as effective as CT scans for detecting smaller stones 1.
• Patient factors such as obesity can limit the effectiveness of ultrasound in detecting renal calculi.
• Ultrasound is highly sensitive and specific for the diagnosis of ureteral obstruction, with reported sensitivity up to 100% and specificity up to 90% 1.

Clinical Implications

• Ultrasound can be used as an initial screening tool for patients with suspected renal calculi, particularly in situations where CT scans are not readily available or are contraindicated, such as in pregnant women or children.
• CT scans should be considered for patients with suspected renal calculi who have a high risk of complications or who require accurate stone sizing and location for treatment planning.
• The use of color Doppler and assessment of twinkling artifact can improve the sensitivity of ultrasound for detecting small renal stones, but this technique should be used with caution due to the risk of false-positives.

From the Research

Ultrasound Detection of Renal Calculi

• The sensitivity and specificity of ultrasonography (US) for detecting renal calculi were found to be 54% and 91%, respectively, in a study published in 2017 2.
• The same study noted that US significantly overestimated the size of stones in the 0-10 mm range, which could lead to inappropriate counseling for patients.
• Another study from 2009 recommended US as the initial imaging modality for suspected renal colic in pregnant women and children, but suggested that low-dose CT scans may be safe in pregnancy 3.
• A 2018 study compared the sensitivity of noncontrast computed tomography (CT) with endoscopy for detection of renal calculi and found that CT had a higher sensitivity, but may underreport the number of small stones 4.
• A 2016 review of kidney stone imaging techniques noted that ultrasonography has a lower sensitivity and specificity than CT, but does not require radiation 5.
• The use of US to guide clinical decision-making for residual or asymptomatic calculi is limited by its low sensitivity and inability to size the stone accurately, with one study suggesting that up to 22% of patients may be inappropriately counseled when using US alone 2.

Comparison with Other Imaging Modalities

• Noncontrast CT scans are considered the gold standard for diagnosing renal calculi, with sensitivities of 95-100% reported in the literature 4, 5.
• A 2008 study found that arterial phase CT scans could detect 75% of renal calculi, including all calculi larger than 5 mm, although noncontrast scans are still recommended for detection of smaller stones 6.
• Plain radiography (KUB) is most helpful in evaluating for interval stone growth in patients with known stone disease, but is less useful in the setting of acute stones 3, 5.