A prospective study of risk factors for nephrolithiasis after Roux-en-Y gastric bypass surgery.

PURPOSE

Roux-en-Y gastric bypass surgery has become an increasingly common form of weight management. Early retrospective reviews have suggested that new onset nephrolithiasis develops in some patients after undergoing Roux-en-Y gastric bypass. We present a prospective longitudinal study to assess risk factors for nephrolithiasis after Roux-en-Y gastric bypass.

MATERIALS AND METHODS

A total of 45 morbidly obese patients scheduled to undergo Roux-en-Y gastric bypass surgery were enrolled in this prospective study between November 2006 and November 2007. Exclusion criteria included history of nephrolithiasis or inflammatory bowel disease. Serum uric acid, parathyroid hormone, calcium, albumin, and creatinine and 24-hour urine collections were obtained within 6 months before Roux-en-Y gastric bypass, and at 6 to 12 months postoperatively. A Wilcoxon signed-rank test was used to compare preoperative and postoperative serum laboratory values and 24-hour urine values. McNemar's test was used to determine if the percent of abnormal values underwent a statistically significant change after Roux-en-Y gastric bypass. For both statistical methods a p value was calculated for the change in each variable with p <0.05 considered statistically significant.

RESULTS

Statistically significant changes included increased urinary oxalate and calcium oxalate supersaturation, and decreased urinary citrate and total urinary volume postoperatively. A statistically significant percentage of patients exhibited decreased urinary calcium, while a statistically significant percentage of patients experienced increased urinary oxalate and calcium oxalate supersaturation.

CONCLUSIONS

Our prospective study demonstrated multiple factors that increase the relative risk of nephrolithiasis after Roux-en-Y gastric bypass. These changes may make stone formation after Roux-en-Y gastric bypass increasingly likely and pose an ongoing challenge in the realm of urology.