SAFE AND ACCURATE dosage calculations are critically important responsibilities for nurses who administer medications and I.V. fluids. Accurate dosage calculation often requires an estimate of the patient's body surface area (BSA) as a body size indicator. This article defines BSA and discusses when and why BSA calculations are needed, how BSA compares with body mass index (BMI), and how to calculate BSA.
What is BSA?
As a calculated estimate of a patient's total body surface area, BSA is a simple and useful biometric two-dimensional measure of a person's body size.1 BSA is calculated in meters squared, a score that combines the patient's weight and height and reflects an estimate of the patient's total surface (outer layer) area.2 For example, a BSA of 2, written as 2 m2, means that person's skin surface could be placed into a two-meter-by-two-meter box.3 A patient's estimated and calculated BSA is related to physiologic and metabolic processes such as heat exchange, blood volume, and the size of vital organs such as the heart.4,5
How BSA compares with BMI
Another indicator of body size, BMI is used to measure the body's degree of overweight or underweight.1,6 Like BSA, BMI is based on the patient's height and weight, and both measurements tell nurses more about the patient than weight alone.1 However, the type of information they convey is different. Based on the patient's weight/height ratio, BMI is an estimate of body fat used to assess obesity and overweight in adults.1,6,7 This easy and useful estimate of body fat derives from the patient's body mass (weight in kg) divided by the square of the patient's height in meters (meter squared).
As an estimated measure of body fat, a patient's BMI score helps clinicians determine the patient's weight category, the degree of obesity if present, and morbidity risk, using established norms (see Interpreting BMI). The greater the BMI score, the greater the risk of obesity-related diseases such as heart disease and type 2 diabetes.7
However, BMI scores have limitations. For example, they may overestimate the amount of body fat in physically fit, athletic, and muscular patients and underestimate body fat levels in older adults, inactive adults, and neurologically impaired patients who have lost muscle mass.1,7
Following a large retrospective study, Roy and colleagues noted that elevated BSA calculations predicted elevated coronary artery calcium levels, but elevated BMI levels did not.8 They asserted that the BSA estimate is a better predictor of coronary artery disease because it better differentiates between muscle and fat. Muscle tissue is heavier and denser than fatty tissue, so a kg of muscle weighs the same as a kg of fat but takes up far less space. In addition, established morbidity norms for BMI scores may underestimate morbidity and obesity risk in Asian populations.1 For these reasons, some consider the BMI measure to be a biased and less useful tool than BSA, especially for adults with weights outside the 55- to 90-kg range.8,9
Obtaining an accurate BSA
Before calculating BSA, nurses must obtain an accurate and current weight and height, preferably in metric units.10 Although the patient's height is unlikely to change over the course of treatment, weight may change significantly. Therefore, current and accurate height and weight measures need to be routinely repeated and BSA measures adjusted accordingly.2,10
BSA can be calculated using one or more of various possible equations, formulas, or nomograms. At least five different validated BSA calculation formulas are in clinical use, with slight differences among them. Though nearly all the BSA calculation methods are based on the patient's current and carefully assessed height and weight, systemwide single-formula adherence is strongly recommended among clinicians and researchers alike to ensure consistency.2,10,11
In 1987, Mosteller published a simplified calculation of body-surface area that has been widely used and accepted.10,12-15
To use this formula, clinicians need only a basic calculator with a square root key (see Calculating BSA is easy). Based on National Health Statistics of average height and weight for US adults, the calculated average adult BSA is 2.09 m2 for men and 1.86 m2 for women.16
The Mosteller BSA formula is accurate, reliable, easy to use, easy to remember, and requires no additional printed nomogram, software, or specialized purpose-built calculator.11,12 It also is more convenient, less complex, applies well to children and adults, removes possible error if height and weight numbers are transposed, and eliminates nomogram printing errors.10,11,13
Calculating BSA gives the clinician a better estimate of patient size than BMI alone. Consider this example: Patient A is a short, physically fit adult male 5'2” (157.5 cm) in height and weighing 180 lb (81.6 kg). Patient A has a BMI of 32.92 kg/m2 (obese) and a BSA of 1.89 m2, which is below the 2.09 m2 average for men (see Calculated BSA averages for adults according to age).
In contrast, Patient B is an obese 6' (182.8 cm) adult male who weighs 243 lb (110.2 kg). His BMI is 32.96 kg/m2—nearly identical to Patient A's BMI. However, his BSA is 2.37 m2, which is well above the 2.09 m2 average BSA score for adult males.14,16,17 This example illustrates why BSA is considered a better body size estimate.1,6,18,19
In healthcare settings, BSA estimate scores are used when calculating medication dosages, especially when medications have a narrow therapeutic index as with chemotherapeutic drugs.12 The greater the BSA, the greater the patient's fluid and nutrient needs. Thus, BSA measures are appropriate considerations for accurate and safe infusion rates.5 Using accurate BSA measures decreases the risk of under- or overdosing, especially when the consequences of both error types are significant.
BSA estimates are also used to determine physiologic measures such as cardiac index (cardiac output divided by BSA), stroke volume, thermoregulation, and cardiac dimensions.14,15,18 Pettersen and colleagues used BSA as a measure to help calculate the size of cardiac structures and dimensions in children and adolescents.4 Additionally, a recent study identified BSA scores as excellent categorical predictors of surgical complications following lung cancer surgery in Chinese patients.20
Improving patient care
Using a systemwide formula for calculating BSA, such as the Mosteller formula, will reduce medication and fluid administration errors and improve drug and fluid infusion efficacy. Clinicians should make BSA calculations an integral part of patient care.
Calculating BSA is easy12,15
Using a standard calculator with a square root key, multiply the patient's height (cm) times weight (kg); then hit the ÷ (divide) key and divide by 3600. As that answer appears on the calculator screen, hit the √ (square root) key. The answer appearing on the screen is the patient's BSA (m2).
The BSA can also be calculated using the patient's height and weight in inches (in) and lb according to this formula:
BSA (m2) = the square root of ht (in) × wt (lb) divided by 3131.
Using a 73-in-tall, 175-lb patient as an example, calculate BSA as follows: 73 × 175 = 12,775 ÷ 3131 = 4.08; then calculate the square root of 4.08, which is 2.02 m2.
Though some facilities want the m2 rounded to the nearest thousandth, the standard is rounding to the nearest hundredth. Importantly, information entered into the calculator is never rounded; rounding occurs only after the final m2 answer is obtained.
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