Presented by: Ryan Sun MD
Stanford University

Introduction

Renal ultrasound is an imaging modality used for kidney stone surveillance that does not expose patients to the risks of radiation associated with computed tomography (CT). However, US has lower sensitivity and specificity for the diagnosis of kidney stone disease compared with CT. Additionally, US may overestimate stone size, which could prompt unnecessary surgical treatments. To date, studies have reported varying results regarding US performance for kidney stone surveillance and it remains unclear if US is sufficient to inform clinical-decision making for patients with known kidney stone disease.  In this study, we aim to assess the accuracy of US for kidney stone surveillance, using CT as reference, and to identify factors that affect US performance. 

Materials

This was a retrospective review at a single large volume academic medical center of patients with known kidney stone disease  who were seen in the Urology clinic and who underwent both renal US and CT within 90 days for surveillance from January 2022 to December 2022. Patients with stone passage and interventions were excluded. Stone characteristics were recorded based on formal radiology reports, and statistical analysis was performed comparing the diagnostic accuracy of US and CT.  

Results

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A total of 107 patients and 128 stones were included, with a mean time difference of 25.7 ± 23.7 days between US and CT. The sensitivity was 77% and positive predictive value (PPV) was 75% for detection of stone by US. The PPV of stone diagnosis by US was only 59% for stones that are measured to be >4mm by CT. The mean stone size on US was 8.7 ± 6.2mm, compared with 5.5 ± 6.4mm on CT (p=0.02). This overestimation was more pronounced in patients with smaller stones and greater BMI (p<0.05). Subgroup analysis showed no significant difference in US performance when stones were categorized by location, laterality, or time between US and CT scans. The individual technologist who performs the US exam, and not the radiologist who interprets the US exam, was associated with differences in stone detection (p=0.01) or stone size (p=0.03). 

Conclusion

US performance is limited by worse detection of kidney stones as compared with CT and is dependent on stone size, patient BMI, and technologist. US significantly overestimated stone size, and if used alone to guide management, up to two in five patients with stones measuring >4mm may undergo unnecessary intervention.  

Funding

None

Co-Authors

Elijah Sommer,
Stanford Univeristy

Calyani Ganesan,
Stanford University

Alan Pao,
Stanford University

John Leppert,
Stanford University

Helena Chang,
Kaiser

Simon Conti,
Stanford Univeristy

Timothy Chang,
Stanford University