Body Mass Index (BMI) Measurement (Ambulatory) - CE/NCPD

The content in Clinical Skills is evidence based and intended to be a guide to clinical practice. Always follow your organization’s practice.

ALERT

The body mass index (BMI) score may overestimate body fat in patients with muscular stature and underestimate body fat in older adults with decreased muscle tone.^{undefined#ref4">4} Take care to consider these situations when interpreting BMI scores.

OVERVIEW

BMI provides an estimation of body fat, resulting in a numeric score. The BMI is calculated by a formula using weight in kilograms and height in meters squared.¹ The BMI score is then interpreted using the appropriate category range, which includes underweight (less than 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), and obesity (30 and higher) (Table 1).⁵ Online calculators are available to input height and weight measurements for simplified results.

An increased BMI is associated with increased risk for cardiovascular diseases, dyslipidemia, diabetes, stroke, gallbladder disease, respiratory impairment, arthritis, mental illnesses, decreased mobility, cancer, and decreased quality of life.³ Prevention of these diseases and risk reduction can occur with just 5% to 10% excess weight loss in those with higher BMI scores.⁵

The BMI score is calculated the same for both children and adults (Table 2), but the interpretation is different.² Adult BMI scores are not age or sex specific, unlike children and adolescent BMI scores. The Centers for Disease Control and Prevention (CDC) growth charts contain BMI for age percentiles using age and BMI plot points for both male and female patients ages 2 to 19.² Obesity is defined as a BMI score at or exceeding the 95th percentile for children 2 to 19 years old.²

SUPPLIES

See Supplies tab at the top of the page.

EDUCATION

• Give developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, preferred learning style, and overall neurologic and psychosocial state.
• Teach the patient how to interpret a BMI score.
• Encourage questions and answer them as they arise.

PROCEDURE

1. Clean hands and don appropriate personal protective equipment (PPE) based on the risk of exposure to bodily fluids or infection precautions.
2. Determine if the patient has health literacy needs or requires tools or assistance to effectively communicate. Be sure these needs can be met without compromising safety.
3. Weigh the patient in kilograms.⁴ Enlist assistance from an additional health care team member if needed to ensure patient safety.
4. Get the patient’s actual height in meters. Do not rely on stated height because this could result in an inaccurate calculation.
5. Calculate the BMI score using online calculators or a mathematical formula (Table 2). The BMI calculation may be done with online tools or with a mathematic equation using kilograms and meters squared.³

   Be sure the correct measurement units are used depending on the method for calculating the BMI score.

EXPECTED OUTCOMES

• Height and weight are obtained correctly for calculating a BMI.
• BMI is interpreted per child or adult categories.

UNEXPECTED OUTCOMES

• Patient’s weight exceeds the maximum capacity of the scale.

DOCUMENTATION

• Method of calculating patient BMI
• Patient’s BMI
• Education
• Unexpected outcomes and related interventions

PEDIATRIC CONSIDERATIONS

• Children and adolescents 2 to 19 years old² should have BMI interpreted using an age-appropriate growth chart.

OLDER ADULT CONSIDERATIONS

• Older adults may have decreased muscle tone and body fat that can be underestimated.

REFERENCES

1. Centers for Disease Control and Prevention (CDC). (2024). About body mass index (BMI). Retrieved December 4, 2024, from https://www.cdc.gov/bmi/about/?CDC_AAref_Val=https://www.cdc.gov/healthyweight/assessing/bmi/index.html
2. Centers for Disease Control and Prevention (CDC). (2024). BMI frequently asked questions: Child and teen BMI questions. Retrieved December 4, 2024, from https://www.cdc.gov/bmi/faq/?CDC_AAref_Val=https://www.cdc.gov/healthyweight/assessing/bmi/childrens_bmi/about_childrens_bmi.html
3. Centers for Disease Control and Prevention (CDC). (2024). BMI frequently asked questions: Common BMI questions. Retrieved December 4, 2024, from https://www.cdc.gov/bmi/faq/?CDC_AAref_Val=https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
4. Institute for Safe Medication Practices (ISMP). (2024). 2024-2025 Targeted medication safety best practices for hospitals. Retrieved December 4, 2024, from https://www.ismp.org/guidelines/best-practices-hospitals
5. National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (NHLBI). (n.d.). Assessing your weight and health risk. Retrieved December 4, 2024, from https://www.nhlbi.nih.gov/health/educational/lose_wt/risk.htm

Clinical Review: Margaret E. Williams, PhD, RN, CRRN, FNP, DDP, FABDA, FARN

Published: January 2025