Objective: Because a majority of urinary tract stones (UTSs) pass spontaneously and clinically significant alternative pathology is rare, we hypothesize that many computed tomographic (CT) scans to diagnose them are likely unnecessary. We sought to measure the impact of renal CT scans on resource use and to justify a prospective study to derive a score that predicts an emergent diagnosis in patients with suspected UTS by doing so in our retrospective series. Methods: We conducted a

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... study of ED patients who had noncontrast CT of the abdomen for suspected UTS. A split-sample was used to derive and validate a score to predict the presence of an emergent diagnosis on CT. Results: Of the 2,315 patients (50.8% female, mean age 45 years), 49 (2.1%) had an emergent outcome observed on CT. An additional 12 (0.5%) patients had an urgent outcome and 239 (10.6%) had a urologic procedure within 8 weeks of the CT. Serum white blood cell count, highest temperature, urine red blood cell count, and the presence of abdominal pain were significant predictors of the primary outcome. A score derived using these predictors had a potential range of 22 (0.26% predicted risk, 0.5% actual risk of the outcome) to 6 (52% predicted risk). The score was moderately discriminatory with c-statistics of 0.752 (derivation) and 0.668 (validation) and accurate with Hosmer-Lemeshow statistics of 10.553 (p = 0.228, derivation) and 9.70 (p = 0.286, validation). Conclusions: A sensible, relevant score derived and validated on all patients presenting with symptoms suggestive of renal colic could be useful in reducing abdominal CT scan ordering.