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Measurements of your upper arm provide a rough correlation to BMI.
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When you can't determine the height of a patient, upper-arm circumference can help calculate his body mass index. Body mass index, or BMI, gives a health provider an estimation of a patient's body fat level. Knowing BMI influences the development of a treatment plan, especially when dealing with a person who is malnourished or obese.

About Body Mass Index

Body mass index is usually calculated by taking a person's weight in kilograms and dividing it by height in meters squared. A BMI that registers below 18.5 is considered underweight. A BMI between 18.5 and 24.9 is normal. When BMI goes above 25, a person is considered overweight, and above 30, obese. (ref4)

According to a study published in May 2019 by PLoS One, BMI is useful as a screening tool for conditions related to being under- or overweight, but it isn't diagnostic. It can help a doctor determine which type of dietary treatment a person under medical care may need and to check weight changes over time.

Read more: BMI Vs. Body Fat Percentage

Calculating BMI From Arm Circumference

In some cases, such as with a person in intensive care or one who is bedridden or in a wheelchair, height can't easily be obtained. Upper-arm circumference roughly correlates with BMI in the average person.

A study of patients aged 18 to 80 found a correlation between arm circumference and BMI, and undernutrition in adults. People with a below normal arm circumference were more likely to have undernutrition and at an increased risk for related health complications.

Using arm circumference to predict BMI can help indicate whether a person is at risk for malnutrition It's a simple and cost-effective way to identify persons most in need of intervention to prevent complications and improve nutritional status.

Measuring Upper-Arm Circumference

Use a person's left arm to measure upper-arm circumference. Ideally, the person is seated with the arm hanging loosely at his side. Bend the elbow to 90 degrees and find the midpoint between the bony part of the shoulder, known as the acromion, and the elbow, known as the olecranon. Measure around this midpoint using a tape measure pulled snug, but not tight.

A measure below 23.5 centimeters, or 9.25 inches, indicates that the person may be underweight or borderline underweight with a BMI of 20 or lower. An upper-arm circumference of 32 centimeters, or 12.6 inches, indicates a BMI of 30 or greater, or obesity.

Read more: What Is a Realistic BMI for Someone Athletic?

Interpreting Upper-Arm Circumference Measures

Upper-arm circumference provides a general idea of BMI, but isn't definitive. Taking two measurements of upper-arm circumference each time you measure and figuring according to the average of the two measures yields a more accurate reading.

One study, published in the journal Public Health Nutrition in May 2019, found a correlation between mid-arm circumference, poor nutritional status and wasting in mothers with children. Arm circumference strongly predicted degree of wasting.

People who are extremely muscular may register with a large arm circumference and high BMI due to developed bicep muscles, but not have an excess of fat. For critically ill patients, though, upper-arm circumference may be the only way to easily monitor health risks due to their size.

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