Accurate Measurement and Interpretation of BMI in Pediatric Patients

Introduction to BMI Measurement

To accurately measure BMI in children, precise height and weight measurements should be obtained using standardized equipment and technique, and then plotted on age- and sex-specific growth charts, according to the American Academy of Pediatrics 1, 2, 3

For infants and young children unable to stand, an infant scale should be used with the child completely undressed, and weight recorded to the nearest 0.1 kg, as recommended by the American Medical Association 4
For children who can stand independently, weight should be measured with the child wearing light clothing, standing on a calibrated scale with feet flat and heels touching, and recorded to the nearest 0.1 kg, following guidelines from the American Medical Association 4
Recumbent length should be measured in children up to 24 months or unable to stand, using an infant stature board with a fixed headboard and movable footboard, with two measurers, as suggested by the American Academy of Pediatrics 4
Standing height should be measured in children able to stand unassisted, using a fixed stadiometer attached to the wall, with the child standing facing away from the wall, heels together, back straight, and arms down, as recommended by the Centers for Disease Control and Prevention 4

BMI should be calculated using one of the following formulas: BMI = weight (kg) / height (m)², BMI = weight (kg) / height (cm) / height (cm) × 10,000, or BMI = weight (lb) / height (in) / height (in) × 703, according to the World Health Organization 1
BMI values are meaningless without age and sex adjustment in children, as BMI varies dramatically with age, and should be plotted on age- and sex-specific growth charts, as recommended by the American Academy of Pediatrics 1, 2
Children under 24 months should be plotted on WHO growth standards, while children 24 months and older should be plotted on CDC 2000 growth charts, according to the Centers for Disease Control and Prevention 3, 5, 6

A BMI ≥95th percentile is classified as obese, a BMI 85th-94th percentile is classified as overweight, and a BMI <5th percentile may indicate undernutrition or an underlying medical condition, as defined by the American Medical Association 1, 2, 3, 5
The American Medical Association, CDC, and Maternal and Child Health Bureau expert committee concluded that BMI is sufficient for screening children because it correlates strongly with direct measures of body fat, particularly above the 85th percentile, as stated by the American Medical Association 3

BMI tends to underestimate overweight in tall individuals and overestimate overweight in short individuals and those with high lean body mass, as noted by the American Academy of Pediatrics 1, 2
BMI generally correlates well with precise adiposity measures in individuals with BMI ≥95th percentile, but may not be valid for children younger than 17 years due to complex multi-scaling properties of weight-for-height relationships during growth, as reported by the World Health Organization 1, 2
Direct body composition measures, such as DEXA, underwater weighing, MRI, and CT, provide accurate fat mass estimates but are used mainly in research settings, not routine clinical care, as stated by the American Heart Association 1, 2, 7

Growth velocity over time using multiple data points is more informative than single measurements and should be used in conjunction with medical and family history to assess appropriate growth, as recommended by the American Academy of Pediatrics 5
Serial measurements every 3-6 months are recommended for tracking weight trajectory and response to interventions, and a child consistently tracking along a specific percentile curve may be growing appropriately, whereas crossing downward through percentile lines indicates true growth faltering, as suggested by the American Academy of Pediatrics 5