Body Mass Index and Body Surface Area: What’s the Difference?

Body mass index (BMI) and body surface area (BSA) are two measures often utilized in health care, but they usually are not well understood. Although each are indicators of body size, they supply very different information. What exactly does each of those metrics tell us and the way should they be used?

Body mass index (BMI)

is a measure used to find out how chubby an individual is. Calculated based on height and weight,¹ BMI is straightforward to measure, reliable and correlated with body fat percentage. This is a more accurate estimate of total body fat in comparison with body weight alone.² BMI may help assess your risk of heart disease, hypertension, type 2 diabetes, gallstones, respiratory problems and a few cancers. The drawback of measuring BMI alone is that it might overestimate body fat in athletes or individuals with a muscular construct. Conversely, it might underestimate body fat in older people or in individuals who have lost muscle.

BMI formula

BMI is calculated by dividing your weight in kilograms (kg) by the square of your height in meters (m). While there are programs and mobile apps that may calculate your BMI for you, it is vital to know the formula and derive the reply. Let’s practice!

Example 1: What is the BMI of Mr. Jones, who weighs 210 kilos and is 6 feet 3 inches tall?

1. Convert kilos to kilograms: 210 kilos ÷ 2.2 kg/lb = 95.45 kg
2. Calculate height in meters:
   1. 6 feet 3 inches = 75 inches
   2. 75 inches x 2.54 cm/inch = 190.5 centimeters (cm)
   3. 190.5 cm ÷ 100 cm/m = 1.905 meters
   4. Square height (1.905 x 1.905) = 3.63 m²
3. Calculate BMI by dividing weight by height (m)²
   1. 95.45 kg ÷ 3.63 m² =

Example 2: What is the BMI of Mr. Smith, who weighs 210 kilos and is 5 feet 4 inches tall?

1. 210 lbs = 95.45 kg
2. Calculate height in meters:
   1. 5 feet 4 inches = 64 inches
   2. 64 in x 2.54 cm/in = 162.56 cm
   3. 162.56 cm = 1.625 meters
   4. Square height (1.625 x 1.625) = 2.64 m²
3. Calculate BMI:
   1. 95.45 kg ÷ 2.64 m² =

Example 3: What is the BMI of Mrs. Williams, who weighs 50 kilos and is 5 feet 8 inches tall?

1. 110 lbs = 50 kg
2. Calculate height in meters:
   1. 5 feet 8 inches = 68 inches
   2. 68 inches x 2.54 cm/inch = 172.72 cm
   3. 172.72 cm = 1.727 m
   4. Square height = 2.98 m²
3. Calculate BMI:
   1. 50 kg ÷ 2.98 m² =

What does the BMI result mean?

What do these results mean? According to the BMI classification scale adopted by the National Institutes of Health (NIH) and the World Health Organization (WHO), Mr. Jones might be considered barely chubby, Mr. Smith might be classified as obese and Ms. Williams might be considered underweight.

BMI scores are classified based on heart problems risk and might be applied to Caucasian, Hispanic and African-American people. However, these standards may underestimate the chance of obesity and diabetes in people of Asian and South Asian descent. A lower threshold needs to be used for these populations.

Measuring waist circumference combined with BMI can provide additional details about risk that is just not captured by BMI. The NIH recommends measuring waist circumference in chubby and obese adults to evaluate abdominal obesity. Waist circumference > 40 inches (102 cm) for men and > 35 inches (88 cm) in women may indicate an increased risk of cardiovascular and metabolic diseases.²

Upon further evaluation of the above patients, Mr. Jones was found to have a waist circumference of 37 inches and was considered to be at lower risk for obesity. In Mr. Smith’s case, measuring waist circumference is just not essential because most patients have a BMI > 35 kg/m² are already considered to be at high cardiometabolic risk.

Body surface area (BSA)

measures total body surface area and is used to calculate drug doses and medical indicators or rankings. The first formula was developed by Du Bois in 1916, and several other others have been developed since then. The easiest to calculate and remember, Mosteller’s formula is essentially the most commonly used formula in practice and clinical research.²

BSA formula

Mosteller’s formula is predicated on the square root of height (cm) multiplied by weight (kg) divided by 3,600.

Let’s use the identical examples above and calculate each patient’s BSA.

Example 1: Mr. Jones

1. Calculate the load in kilograms: 210 kilos ÷ 2.2 = 95.45 kg
2. Calculate height in centimeters: 6 feet 3 inches = 75 inches x 2.54 cm/inch = 190.5 cm
3. Multiply your height by your weight and divide by 3,600
   1. (190.5 cm x 95.45 kg) ÷ 3600 = 5
4. Take the square root of 5 =

Example 2: Mr. Smith

1. Weight in kg = 95.45 kg
2. Height in cm: 5 feet, 4 inches = 64 inches x 2.54 cm/inch = 162.56 cm
3. (162.56 cm x 95.45 kg) ÷ 3600 = 4.3
4. Take the square root of 4.3 =

Example 3: Mrs. Williams

1. Weight in kg = 50 kg
2. Height in cm: 5 feet, 8 inches = 68 inches x 2.54 cm/inch = 172.72 cm
3. (172.72 cm x 50 kg) ÷ 3600 = 2.39
4. Take the square root of two.39 =

How to make use of your BSA rating

The average adult BSA is 1.7 m² (1.9 m² for adult males and 1.6 m² for adult women). This number is used to calculate doses of cytotoxic anticancer drugs. To minimize differences in patient size, the dosage of most chemotherapy agents is mg of drug per m2² body surface.² Although this technique has not been rigorously validated, BSA-based dosing has grow to be the usual for prescribing most cytotoxic agents and a few therapeutic monoclonal antibodies. Theoretically, BSA mitigates patient size variability and abnormal body fat to assist optimize drug efficacy, improve drug clearance, and minimize or prevent toxicity.²

BSA can be used for more precise measurements of hemodynamic parameters comparable to cardiac index (CI = cardiac output divided by BSA), stroke volume index (SVI = stroke volume divided by BSA), systemic vascular resistance index (SVRI = systemic vascular resistance divided by BSA) and pulmonary vascular resistance index (PVRI = pulmonary vascular resistance divided by BSA). Additionally, BSA is used to regulate creatinine clearance when comparing it with normal values ​​to evaluate the presence and severity of kidney disease.²

Let’s have a look at the cardiac index. If Mr. Jones, whose BSA is 2.24 m²he has a cardiac output of 4.3 L/min, his cardiac index could be 1.92 L/min/m² (4.3 l/min divided by 2.24 m²). If Mrs. Williams, whose BSA is 1.55 m²has the identical cardiac output of 4.3 L/min, her cardiac index could be 2.77 L/min/m². While 4.3 L/min is inside the traditional cardiac output range, Mr. Jones’ cardiac index of 1.92 L/min/m² is below the traditional range of two.5 – 4.0 l/min/m². Further evaluation is essential to find out the reason for low cardiac output and plan treatment options. May need fluid bolus for dehydration and tachycardia or inotrope for heart failure.

I hope this review of BMI vs BSA formulas has been helpful. We’d love to listen to your thoughts on how you should utilize BMI and BSA in your on a regular basis practice.

BMI and BSA references

1. U.S. Department of Health and Human Services. National Institute of Health. Health and weight risk assessment. Downloaded on 18/07/17 from https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm

2. UpToDate: Obesity in adults: prevalence, screening and assessment. Downloaded on 18/07/17 from https://www.uptodate.com/contents/obesity-in-adults-prevalence-screening-and-evaluation?source=search_result&search=bmi&selectedTitle=1~150