To determine whether body mass index (BMI) and Hounsfield units (HUs) could be used as independent predictors of stone-free status after extracorporeal shock wave lithotripsy (ESWL). No detailed studies have assessed BMI as an independent predictor of ESWL outcome. Some studies have suggested that HUs of urinary calculi on noncontrast computed tomography may predict the stone-free rate after ESWL.

We evaluated 100 patients who had undergone ESWL for 5-mm to 10-mm upper urinary tract stones. The BMI was calculated for each patient. HU determination and chemical analysis was performed for each stone. Radiographic assessment at 6 weeks categorized patients into a stone-free (SF) or residual-stone (RS) group. Statistical analysis, using BMI and HUs as predictors, was performed to assess the stone-free rate after ESWL.

Of 100 patients, 72 were in the SF and 28 in the RS group. The mean BMI for the SF group was 26.9 ± 0.5 versus 30.8 ± 0.9 in the RS group (P <0.05). The difference in the mean HU values for the SF and RS groups was statistically significant (577.8 ± 182.5 versus 910.4 ± 190.2, respectively; P <0.05). A successful outcome was significantly related to BMI (odds ratio 1.34, 95% confidence interval 1.09 to 1.65, P <0.01) and HUs (odds ratio 1.01, 95% confidence interval 1.007 to 1.019, P <0.01). The following equation was formulated to compute the probability of treatment failure: 1/1 + 2.7^−z, where z = 0.294 BMI + 0.13 HU − 18.98.

BMI and HUs were statistically significant independent predictors of stone-free rates after ESWL. We believe a predictive model of ESWL outcome may be formulated on the basis of these findings and may be used to select the preferred treatment option for patients with urinary calculi.