Diagnosis Overview

Body Mass Index

Body Mass Index (BMI) is an indirect way to assess body fat and is calculated by dividing weight by height squared (1). In a clinical setting BMI is a useful tool in assessing children’s weight status, but additional clinical information is useful in making the diagnosis of obesity. For children aged 2 to 20 years, age- and gender-specific references for BMI exist. They were included in the 2000 CDC Growth Charts, which are widely used in clinical practice and are available at www.cdc.gov/growthcharts.

The current recommendation from the American Academy of Pediatrics is to track BMI percentile for children on a yearly basis. The growth curves for children younger than 2 years do not include BMI percentiles, as BMI standards for children in that age group do not exist. Weight-for-height values for these children should be plotted. Intervention and prevention of childhood obesity should begin as early in life as possible. For a more detailed discussion of possible factors and interventions in children's early years, see Perspectives for the Future.

BMI is defined by six categories (Table 1). A child is considered underweight when the BMI < 5th percentile for age and gender. A BMI = 5th percentile and < 85th percentile is considered healthy weight. A child with a BMI > 85th and < 95th percentiles is categorized as overweight; if the BMI = 95th percentile, the child is considered obese. For children younger than 2 years, weight-for-height values above the 95th percentile are classified as overweight. Currently, there are no cutoff points to define obesity in children younger than 2 years. Table 1. BMI Categories for Children 2-18 Years of Age

BMI < 5th percentile Underweight
BMI 5th-84th percentile Healthy Weight
BMI 85th-94th percentile Overweight
BMI ≥ 95th percentile Obese
BMI ≥ 99th percentile* Severe Obesity
Children < 2 years: Weight-for-height > 95th percentile Overweight

*Reported for 5-18 year-olds
BMI Handout

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