To create a model for prediction of stone radiopacity based on computed tomography (CT) parameters.

We reviewed the medical records of 513 patients referred to our department for consultation for urolithiasis between March 2011 and December 2012. CT scan and kidney-ureter-bladder (KUB) film were reviewed to identify the value of scout film in revealing radiopaque stones and to identify parameters predicting radiopacity in scout-negative stones.

Of 375 patients who met inclusion criteria and were finally analyzed, all 206 visible stones in scout film were KUB radiopaque. Analyzing scout-negative stones, we found that 92 stones (54.4%) were radiopaque in KUB. Multivariate analysis showed that stone size >9.7 mm, non-midureteral stone location, anterior abdominal wall fat thickness ≤23.9 mm, and Hounsfield units >772 are all independent predictors of stone radiopacity in stones that were not visible in scout film, and the aforementioned parameters were used for the creation of a Web-based calculator.

Scout film can identify radiopaque stones in KUB with high specificity, and thus, KUB can be used for following-up stones which are visible in CT scout film. For stones that are not visible in scout film, the probability of a stone to be radiopaque in KUB can be calculated trough our predictive model.