The Uses and Misuses of Body Surface Area in Medicine

Abstract

The practicing physician uses anthropometry in two ways: (1) to adjust variables for body size in order to define “pathological” values and (2) to dose medicine correctly. Two questions need to be answered in this context. Which background population is to be regarded as “normal”? Which is the best method of indexing? Based largely in nineteenth century principles of physiology, medical variables have traditionally been adjusted for body surface area (BSA). There are a number of problems with this approach. BSA is an essentially indefinable concept, its size varying according to the magnification used. Measurements of BSA have been plagued by methodological problems. The current standard BSA of 1.73 m² bears little resemblance to the BSA of modern Western populations. Empirical studies of physiological variables, even commonly used ones, such as glomerular filtration rate (GFR) and cardiac output (Q), have shown that BSA is not the best indexing parameter, or is simply wrong. Since anthropometric variables, such as weight, height, sex and ethnicity are easily available, it is suggested that medical correction formulae should be of the form

$$ \text{Corrected}\text{variable}=\text{Variable}\times \left(A+B\times {\text{Weight}}^{C}\times {\text{Height}}^{D}\right)\times {f}_{\text{1}}\left(\text{Sex}\right)\times {f}_{\text{2}}\left(\text{Ethnicity}\right)$$

where A, B, C and D are constants, and f are defined functions of sex and ethnicity, all of which are determined empirically. Derived constants, such as BSA and body mass index (BMI) should be abandoned, or their use empirically justified. More sophisticated adjustments using fat-free mass (FFM), total body water (TBW) or extracellular water (ECW) may be indicated, but should be justified empirically, as should more complex formulae.