Body surface area: BSA. The total suface area of the human body. The BSA is used in many measurements in medicine, including the calculation of drug dosages and the amount of fluids to be administered IV.

A number of different formulas have been developed over the years to calculate the BSA and they give slightly different results. The most commonly used formula now is that of Mosteller, published in 1987 in The New England Journal of Medicine. According to Mosteller's "simplified calculation of body-surface area In metric terms" the BSA = the square root of product of the weight in kg times the height in cm divided by 3600.

The "normal" BSA is generally taken to be 1.7 m2 but, in actual fact, the BSA depends on more than just height and weight. Other influential factors include the age and gender of the individual. For example:

• Average BSA for adult men: 1.9 m2
• Average BSA for adult women: 1.6 m2
• Average BSA for children (9 years): 1.07 m2
• Average BSA for children (10 years): 1.14 m2
• Average BSA for children (12-13 years): 1.33 m2

Renal function is measured by the glomerular filtration rate (GFR) which is calculated in rearg to the BSA. The cardiac index is a measure of cardiac output divided by the BSA, giving a better approximation of the required cardiac output. Chemotherapy and pharmacotherapies are often dosed according to the patient's BSA. Glucocorticoid dosing is also expressed in terms of BSA for calculating maintenance doses or to compare high dose use with maintenance requirement. And so on.

Reference: Mosteller RD. Simplified calculation of body-surface area. N Engl J Med 1987;317:1098.