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Body surface area (BSA) is an estimation of a person's total surface area, based on a calculation involving weight and height. A simple calculation multiplies the weight by the height, divides by 3,131 if the measurements are in pounds and inches, or by 3,600 if in meters and kilograms, and then takes the square root of the remaining number. This formula is known as the Mosteller formula and it is popular with medical practitioners for its ease of use and accurate results.
Looking at body surface area can provide useful information about deposits of fat on a patient's body, and it is sometimes compared to the body mass index (BMI). Some people feel that body surface area creates a more accurate picture, as people who are unusually heavy for their height will have a correspondingly high surface area, illustrating the deposition of fat on their frames. The calculation is only an estimate, and it is important to consider factors like a patient who exercises a lot and may be heavier as a result of being well-muscled, as muscle tissue is very heavy.
People use the body surface area to determine the dosage of some kinds of medication. Drugs like chemotherapy must be calculated very carefully to ensure the appropriateness of the dosage for a patient. Intravenous fluids are also commonly dosed on the basis of body surface area. Care providers who regularly determine medication dosages may carry a small calculator to make it easier to accurately compute doses for their patients.
This calculation can also be used for other situations. Care providers interested in determining a patient's renal clearance to assess the health of the kidneys can use body surface area to get a clear idea of how much the kidneys are filtering in a given time period. It can also be used in the computation of the cardiac index, comparing cardiac output to a patient's size. This should fall within a stable range, showing that a patient has a healthy heart. If it is unusually low, there may be a problem with the patient's heart function.
Some factors can change body surface area and make a patient's actual surface area differ from the number determined by calculating. People who have lost a lot of weight rapidly, for example, may have large skin folds that increase their surface area but aren't revealed in a simple formula involving their weight and height.