Detecting kidney stones in children - Ultrasound or CT?

Both the American Urological Association (AUA) and the European Society for Pediatric Radiology (ESPR) recommend using ultrasound first to image children with suspected nephrolithiasis or urolithiasis.

However, pediatric radiologists from Cincinnati Children’s Hospital report that ultrasound has suboptimal performance as a screening tool based on their clinical effectiveness approach study using unenhanced CT as a reference standard. The AUA recommends unenhanced CT as the initial imaging examination for adults with suspected kidney stones.

The U.S. incidence of pediatric nephrolithiasis, the development of kidney stones, is increasing. A University of Pennsylvania study analyzing data from a national commercial insurance claims database of all children with suspected kidney stones who underwent imaging between 2003 and 2011 revealed CT as the most frequently ordered initial exam, with 63% of the 9,228 patients receiving a CT exam, and only 24% an ultrasound exam. The study also found extensive regional variations by state in CT usage.¹

The Cincinnati researchers conducted a retrospective study of children under 17 with suspected nephrolithiasis who had both a renal ultrasound and an unenhanced abdominal CT exam within 24 hours.  A total of 69 children were identified over a 5-year time span. Fourteen (20%) had a documented history of nephrolithiasis.  CT and ultrasound reports were reviewed, with indication for imaging and presence, location, and size of kidney stones noted for each.

CT identified 68 renal stones in 30 patients, and ultrasound, 38 stones. Ultrasound had a sensitivity of 67%, specificity of 97%, positive predictive value (PPV) of 95%, and negative predictive value (NPV) of 79% for detecting renal calculi in any kidney. It had a sensitivity of 60%, specificity of 98%, PPV of 72%, and NPV of 95% per renal sector.  Ultrasound performance was lower for overweight and obese children with a body mass index (BMI) greater than the 85th percentile.

There were 16 false-positive renal calculi identified by ultrasound in 10 patients. The most common ultrasound features of false-positive calculi were echogenic focus (75%) and twinkling artifact (50%). There were also 30 ultrasound false-negative calculi.

“Our data show ultrasound performs less than ideally as a diagnostic tool for evaluating suspected nephrolithiasis in a pediatric population,” wrote the authors. Noting that 10 of the 17 patients with false-negative calculi by ultrasound that were confirmed as positive by CT, they stated that “these results suggest that ultrasound commonly fails to identify children with nephrolithiasis as having nephrolithiaisis, thus raising concerns about the ability of ultrasound to serve as an effective screening test.”

Jonathan R. Dillman, MD, associate chief of research in the Department of Radiology, and co-authors recommend that if an ultrasound is negative but clinical suspicion is high, use of unenhanced CT should be considered. However, if the ultrasound is positive, a confirmatory unenhanced CT may not be indicated.

REFERENCES

1. Tasian GE, Pulido JE, Keren R, et al. Use of and regional variation in initial CT imaging for kidney stones. Pediatrics. 2014 134;5:909-915.
2. Robertson NP, Dillman JR, O’Hara SM, et al. Comparison of ultrasound versus computed tomography for the detection of kidney stones in the pediatric population: a clinical effectiveness study. Pediatr Radiol. Published online February 23 2018. doi: 10.1007/s00247-018-4099-7.