Relative Outcome Measures for Bariatric Surgery. Evidence Against Excess Weight Loss and Excess Body Mass Index Loss from a Series of Laparoscopic Roux-en-Y Gastric Bypass Patients

Bariatric results expressed in the relative measure excess weight loss (%EWL) vary significantly by initial body mass index (BMI): the heavier the patient, the lower the %EWL. We examine if this variation is caused by using a wrong outcome measure and argue that no relative weight loss measure can express bariatric or metabolic goals unequivocally. Nadir weight loss results after laparoscopic gastric bypass in 168 women with initial BMI a parts per thousand yen35 to < 60 kg/m(2) are calculated for %EWL and 61 different relative measures using the formula 100% x (initial BMI -aEuro parts per thousand nadir BMI) / (initial BMI -aEuro parts per thousand a), with a ranging from -30 to +30. Standard deviations are compared mutually and with those reported in the literature. For each relative measure, the significance of any variation by initial BMI is determined with the Mann-Whitney U test. Mean initial BMI was 44.9 +/- 6.7 (35.0-59.7) kg/m(2). Mean nadir BMI was 28.8 +/- 5.8 (18.5-44.4) kg/m(2). Mean nadir excess BMI loss (%EBL; a = 25) was 87.0 +/- 28.0 (19.4-155.1)%. Mean nadir (total) weight loss (%TWL; a = 0) was 35.9 +/- 8.5 (9.5-57.1)%. Mean nadir %EWL was 77.3 +/- 22.8 (17.7-135.2)%. The smallest variation coefficient was 23.7% at a ranging from -1 to +3, including %TWL (a = 0). This is lower than variation coefficients of %EWL results in our series and in the literature. Variation by initial BMI is significant using relative measures with a a parts per thousand yen3, including %EBL and %EWL (both p < 0.0001) and not significant with a < 3, including %TWL (p = 0.13). In contrast to their widespread use, %EBL and %EWL are not suited for comparing different patients or nonrandomized groups. They cause variation by initial BMI, which disappears using %TWL. In general, absolute terms should be preferred for bariatric outcome and goals. The power of bariatric procedures is best represented by their mean %TWL value.