X-ray diffraction and the results assessed by comparing the mean density (as measured in Hounsfield units, HU) with the number of ESWL sessions and clearance. RESULTSIn all, 82 (76%) patients had complete clearance of stones and 26 (24%) had CIRF. There was a linear relationship between the calculus density and number of ESWL sessions required. Of patients with calculi of £ 750 HU, 41 (80%) needed three or fewer ESWL sessions and 45 (88%) had complete clearance. Of patients with calculi of > 750 HU, 41 (72%) required three or more ESWL sessions, and 37 (65%) had complete clearance. The best outcome was in patients with calculus diameters of < 1.1 cm and mean densities of £ 750 HU; 34 (83%) needed three or fewer ESWL sessions, and the clearance rate was 90%. The worst outcome was in patients with calculus densities of > 750 HU and diameters of > 1.1 cm; 23 (77%) needed three or more ESWL sessions and the clearance rate was only 60%. The calculus density was a stronger predictor of outcome than size alone. CONCLUSIONSThe use of NCCT for determining the attenuation values of urinary calculi before ESWL might help to predict the treatment outcome, and so might help in planning alternative treatment in patients with a likelihood of a poor outcome from ESWL.