Validation of Body Surface Area Equations for Estimating Fat-Free Mass by Dual X-Ray Absorptiometry in a Regional Chilean Sample Aged 4 to 85 Years

Background/Objectives: Body surface area (BSA) is an important metric that represents human dimensionality and could provide a more accurate representation of body composition. The objectives were (a) to verify the validity of a set of equations based on BSA to estimate lean body mass (LBM), using dual X-ray absorptiometry (DXA) as a reference method and (b) to propose reference values of BSA by anthropometry and LBM by DXA in a regional sample of Chile aged 4 to 85 years. Methods: A descriptive cross-sectional study was performed. The sample size was 5493 participants. Weight and height were measured. BSA was calculated using seven equations. LBM was assessed by DXA. Results: Only three BSA equations (Dubois–Dubois, 1916, Fujimoto, Watanabe, 1969, and Mattar, 1981) best explained LBM. The explanatory power for males was R² = 83 to 84%, and that for females was R² = 69%. The standard error of estimation (SEE) of the three equations showed acceptable values in both sexes. These values ranged from 0.049 to 0.080 in males and from 0.035 to 0.088 in females. The Bland–Altman concordance analysis showed adequate limits of agreement. In men, they ranged from −0.092 to 0.069 m². In females, they ranged from −0.064 to 0.084 m². Reference values for BSA and LBM were constructed using percentiles. Conclusions: This study demonstrated the validity of three equations for estimating LBM in a Chilean sample aged between 4 and 85 years. These results show consistent behavior and acceptable accuracy, especially in the Mattar equation for all ages. However, the Dubois & Dubois and Fujimoto equations could also be an alternative in females. Reference values were generated for BSA and LBM according to age and sex. The results suggest their applicability and usefulness in clinical and public health contexts.