The new body mass index formula; not validated as a predictor of outcome in a large cohort study of patients undergoing general surgery

Background and aims: A new and interesting body mass index (BMI) formula has been proposed. This formula was designed to provide a more accurate estimation of weight categories, not limited in a two-dimensional manner. The objective of this study was to evaluate the predictive value of the new BMI formula on postoperative complications and long-term survival in a large cohort of patients undergoing general surgery. Methods: 4293 consecutive patients undergoing general surgery in a general teaching hospital were included. Data on comorbidity and demographics were gathered prior to surgery. We also collected data on surgery related characteristics. BMI was calculated using the conventional as well as the new BMI formula. Patients were then divided into four weight categories (BMI < 18.5, 18.5-25, 25-30 and >30 kg/m2) as recommended by the World Health Organization. Results: The study population consisted of 4293 patients. Multivariate regression analyses and the area under the ROC-curve (0.531 ± 0.011 and 0.539 ± 0.011) showed comparable results in predicting outcome between the two formulas. A demographic shift was noticed after complementing the new BMI formula. Male patients were the subjects of this shift, usually towards a lower BMI. According to the conventional BMI formula, 58% of men were overweight BMI > 25 kg/m2, compared to 51.4% according to the new formula. Conclusions: This study showed no difference in prediction of outcome after general surgery when comparing the current BMI formula to the new BMI formula. Thus, despite the fact that the new mathematical proposition seemed more logical and interesting, both calculations can be used in clinical practice. Moreover, our results do not support a change from the conventional BMI formula, currently used and accepted worldwide.

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