BSA (Du Bois)

About BSA (Du Bois)

Body surface area (BSA) is the measured or calculated surface area of the human body. It is used in clinical medicine to dose chemotherapy and other medications with narrow therapeutic windows, to index cardiac output and renal function (glomerular filtration rate), to estimate burn surface area, and to scale physiological parameters across body size. The Du Bois formula, published by Delafield Du Bois and Eugene Du Bois in 1916 in the Archives of Internal Medicine, is the oldest and most enduring BSA equation, derived from direct body-surface measurements of nine subjects including the researchers themselves.

How the formula works

The Du Bois formula is BSA (m^2) = 0.007184 x weight(kg)^0.425 x height(cm)^0.725. The exponents 0.425 and 0.725 reflect the allometric scaling of body surface area with mass and stature: surface area scales with mass to roughly the two-thirds power and with height to a slightly lower power because of the body's non-cylindrical geometry. The constant 0.007184 calibrates the result to square meters. For a 175 cm, 75 kg adult, the Du Bois formula gives BSA = 0.007184 x 75^0.425 x 175^0.725 = approximately 1.90 m^2, which is a typical adult value.

Evidence and comparison with other formulas

The Du Bois formula was derived from only nine subjects, a limitation that motivated the development of later formulas (Mosteller 1987, Haycock 1978, Gehan and George 1970, Boyd 1935) from larger datasets. Despite its small derivation sample, the Du Bois formula agrees with the larger-sample formulas to within a few percent for most adults and remains the reference standard against which other formulas are compared. The Mosteller formula, which is algebraically simpler and gives nearly identical results, has largely supplanted Du Bois in clinical practice because of its ease of computation. A 2003 review by Verbraecken and colleagues in the International Journal of Obesity compared 25 BSA formulas and found that they agree to within about 5 percent for normal-weight adults but diverge more at the extremes of body size.

Clinical use

BSA is used throughout clinical medicine. Chemotherapy dosing is most commonly based on BSA, with drugs such as carboplatin, paclitaxel, and doxorubicin dosed in mg per m^2. Cardiac index (cardiac output divided by BSA) normalizes heart function across body size. The GFR estimate from serum creatinine (Cockcroft-Gault and the MDRD and CKD-EPI equations) is indexed to 1.73 m^2 BSA. Burn surface area is estimated using the rule of nines but can be refined with BSA for fluid resuscitation calculations.

Interpretation

Average adult BSA is about 1.7 to 1.9 m^2, with men typically slightly higher than women because of greater height and weight. BSA rises with both height and weight, so very large individuals can exceed 2.5 m^2. BSA is used less often as a stand-alone anthropometric index than BMI or waist circumference, but it is essential for the dosing and indexing applications noted above.

Limitations

BSA formulas, including Du Bois, were derived in adults and may be inaccurate in children (for whom specific pediatric formulas exist) and in the obese (in whom BSA is overestimated because the allometric exponents assume average body proportions). The differences among BSA formulas are small for most adults but can matter for narrow-therapeutic-index drugs. Some modern oncology guidelines are moving away from BSA-based dosing for certain drugs toward flat dosing or dosing based on renal function or body composition, because BSA does not account for the variability in drug clearance that body composition, age, and organ function produce.