Body surface area prediction in normal-weight and obese patients

None of the equations frequently used to predict body surface area (BSA) ha s been validated for obese patients. We applied the principles of body size scaling to derive an improved equation predicting BSA solely from a patien t's weight. Forty-five patients weighing from 51.3 to 248.6 kg had their he ight and weight measured on a calibrated scale and their BSA calculated by a geometric method. Data were combined with a large series of published BSA estimates. BSA prediction with the commonly used Du Bois equation underest imated BSA in obese patients by as much as 20%. The equation we derived to relate BSA to body weight was a power function: BSA (m(2)) = 0.1173 x Wt (k g)(0.6466). Below 10 kg, this equation deviated significantly from the BSA vs. body weight curve, necessitating a different set of coefficients: BSA ( m(2)) = 0.1037 x Wt (kg)(0.6724). Covariance of height and weight for patie nts weighing <80 kg reduced the Du Bois BSA-predicting equation to a power function, explaining why it provides good BSA predictions for normal-size p atients but fails with obesity.