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New England Section of the American Urological Association

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Does CT scan after ultrasound change surgical planning for nephrolithiasis?
Adam Ludvigson, MD, Piruz Motamedinia, MD.
Yale New Haven Hospital, New Haven, CT.

BACKGROUND: Ultrasound (US) is often used to diagnose nephrolithiasis, but it is less accurate than CT scan, potentially affecting surgical plans. We examined how often obtaining a CT scan after US changed the indicated management of nephrolithiasis, to see if risk factors that determine inaccurate US scans could be identified. METHODS: Approval was obtained through our institutional IRB. From those who presented to our health system over the past 3 years with suspected nephrolithiasis, we selected patients who had undergone a retroperitoneal US, and then a CT scan within 30 days. We recorded stone size and location for all studies. We recorded stone density and skin-to-stone distance for each CT scan. Using current AUA guidelines, we determined the indicated procedure based on findings of each imaging study: extracorporeal shockwave lithotripsy (ESWL), ureteroscopy (URS), or percutaneous nephrolithotomy (PCNL). We used Minitab software to perform all statistical analysis, i.e. general linear models and two-tailed Student's t-tests. RESULTS: 305 patients met inclusion criteria. Of these, the CT scan changed the indicated procedure 108 times (35.4%). 26 US studies indicated ESWL; CT changed 19 of these (73%). 129 US studies indicated URS; CT changed 62 of these (48%). 24 US studies suggested PCNL; CT changed 9 of these (37.5%). Using CT as the gold standard, 18 US studies were false positive for stones (sensitivity=0.72). 51 US studies were false negative for stones (specificity=0.86). Average body mass index (BMI) was significantly lower in the patients with a false positive US as compared to those with a true positive US (26.59 versus 29.13 kg/m2, t(24)=2.33, p=0.029). When comparing patients who had their indicated procedure changed by the CT scan to those who did not, there were no significant differences in age, BMI, or skin-to-stone distance. However, those who had their procedure changed had significantly lower density stones (566.1 versus 747.5 HU, t(154)=2.92, p=0.004). When patients were examined in subgroups based on what their indicated procedure was before and after the CT scan, there were also no significant differences based on age, BMI, or skin-to-stone distance. There was a significant difference based on HU, with stone density explaining 22.88% of the variation (F(11,154)=4.15, p<0.001). CONCLUSIONS: US has many advantages over CT, but cannot always be used in place of it. Our study demonstrates that a CT scan changes the indicated stone treatment in more than 1/3 of cases, and that lower-density stones are a risk factor for inaccurate US findings. US should be used with particular caution in those with a history of low-density stone types, and for ESWL surgical planning.


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