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Urinary tract stones are commonly diagnosed and followed by ultrasound (US),the study highlights several important limitations to the test that should be recognized by the clinician. First, compared with axial unenhanced CT, stone size tends to be overestimated, particularly when up to 10mm. This finding illustrates the danger of using US as the sole diagnostic test when planning treatment. In this setting, surgical intervention could be planned when a more conservative management strategy may be warranted. In part, this effect may be due to the operator-dependent nature of the test and the difficulty in defining the interface between the stone and kidney. Ultrasound measurements should be conducted in a minimum of two orthogonal planes and the maximal length reported. Interestingly, we also found that there was a direct correlation between over-estimation of stone size and distance of the stone from the US transducer. To date, this "lensing" effect has not adequately been explained.

Second, compared to unenhanced CT, we found that US has both poor sensitivity and specificity for detecting stones in the ureter and kidney. In a meta-analysis of all published studies, we found that the sensitivity of US when detecting stones among patients presenting to the emergency department with renal colic was approximately 45% and the specificity approximately 90%. Given these findings, the utility of a negative study should be questioned and US should be considered to be of limited value in the work-up of urolithiasis. Even when stones are detected, with these findings, it is reasonable to question whether the clinician has a complete knowledge of the patient's true stone burden.

It is not our intention to eliminate the use of US entirely. Instead, US should be limited to the routine follow-up of radiolucent calculi as well as a first-line investigative tool for pediatric and pregnant patients with suspected urolithiasis in whom radiation exposure is undesirable. US may also be of benefit in the evaluation of hydronephrosis and should be considered in patients at risk of repetitive CT scans.

Source: UroToday