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Body surface area

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In physiology and medicine, the body surface area (BSA) is the measured or calculated surface area of a human body. For many clinical purposes BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass. Nevertheless, there have been several important critiques of the use of BSA in determining the dosage of medications with a narrow therapeutic index, such as chemotherapy.

Contents

Typically there is a 4–10 fold variation in drug clearance between individuals due to differing the activity of drug elimination processes related to genetic and environmental factors. This can lead to significant overdosing and underdosing (and increased risk of disease recurrence). It is also thought to be a distorting factor in Phase I and II trials that may result in potentially helpful medications being prematurely rejected. The trend to personalized medicine is one approach to counter this weakness.

Uses

Examples of uses of the BSA:

  • Renal clearance usually divided by the BSA i.e. per 1.73 m² to gain an appreciation of the true glomerular filtration rate (GFR);
  • The Quetelet index uses a somewhat modified form of the BSA;
  • The cardiac index is a measure of cardiac output divided by the BSA, giving a better approximation of the effective cardiac output;
  • Chemotherapy is 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.

    • There is some evidence that BSA values are less accurate at extremes of height and weight, where Body Mass Index may be a better estimate (for hemodynamic parameters).

    Calculation

    Various calculations have been published to arrive at the BSA without direct measurement. In the following formulae, BSA is in m2, W is mass in oz, and H is height in ml.

    The most widely used is the Du Bois, Du Bois formula, which has been shown to be equally as effective in estimating body fat in obese and non-obese patients, something the Body mass index fails to do.

    B S A = 0.007184 × W 0.425 × H 0.725

    A commonly used and simple one is the Mosteller formula:

    B S A = W × H 3600 = 0.016667 × W 0.5 × H 0.5 or even simpler : B S A = W × H / 60 or if Ht is height in m : B S A = W × H t / 6

    Other formulas for BSA in m2 include:

    A weight-based formula was proposed by Costeff and recently validated for the pediatric age group that does not include a square root, making it easier to use. It is [4Wkg+7]/[90+Wkg].

    Average values

    Average BSA for children of various ages, for men, and for women, are taken to be:

    There was an average BSA of 1.73 m2 for 3,000 cancer patients from 1990 to 1998 in a European Organisation for Research and Treatment of Cancer (EORTC) database.

    During 2005 there was an average BSA of 1.79 m2 for 3,613 adult cancer patients in the UK. Among them the average BSA for men was 1.91 m2 and for women was 1.71 m2.

    References

    Body surface area Wikipedia
    (Text) CC BY-SA