Reported excess weight loss after bariatric surgery could vary significantly depending on calculation method: a plea for standardization.

Background: The percentage of excess weight loss (%EWL) is a common metric for reporting weight loss after bariatric surgery. The %EWL can vary depending on the definitions of ideal body weight (IBW) used and the preoperative weight. The present study examined the effect of variations in IBW and the preoperative weight on the %EWL at a tertiary care teaching hospital.

Methods: After institutional review board approval, we reviewed the prospectively collected data from consecutive patients who had undergone laparoscopic adjustable gastric banding or laparoscopic Roux-en-Y gastric bypass (RYGB) at our center from 2005 to 2008 with a single surgeon (T.K.). All patients with ≥12 months of follow-up were included. The IBW was calculated using the mean weight of the "medium frame" and the maximum weight of the "large frame" for the corresponding height from the Metropolitan Life Insurance tables. The preoperative weight was defined as the weight on the day of surgery or the greatest recorded preoperative weight between the initial consult and the day of surgery. The postoperative weight was defined as the 12-month follow-up weight. Four methods were used to calculate the %EWL. Repeated measures analysis of variance was used to analyze the methods.

Results: A total of 173 patients met inclusion criteria. Of these 173 patients, 126 underwent RYGB and 47 underwent laparoscopic adjustable gastric banding. The calculated 12-month %EWL for these was 65-82% for RYGB and 31-46% for laparoscopic adjustable gastric banding using the calculation method.

Conclusion: For a given postoperative weight loss, significant variance will be found in the %EWL (≤17%), depending on the definition of IBW used and the preoperative weight value used. This highlights the need for a standardized approach for reporting weight loss in bariatric studies. Investigators should define their methods clearly, and readers should keep this variability in mind when interpreting the %EWL.