When should Body Mass Index (BMI) be used as a screening tool in pediatric patients?

When to Use BMI in Pediatric Patients

BMI should be calculated and plotted on age- and sex-specific growth charts at every well-child visit for all children aged 2 years and older to screen for overweight and obesity. 1

Universal Screening Recommendations

All children aged 2-19 years should have BMI calculated, plotted, and interpreted at every well-child visit as the primary screening tool for identifying overweight and obesity. 1, 2 This recommendation is based on expert consensus from the American Medical Association, CDC, and Maternal and Child Health Bureau, who concluded that BMI is sufficient for screening because it:

• Correlates strongly with direct measures of body fat, particularly above the 85th percentile 1
• Can be easily calculated from height and weight 1
• Associates only weakly with height 1
• Identifies children with highest body fat with acceptable accuracy 1

Age-Specific Application

Children Under 24 Months

• BMI should not be routinely used for children under 2 years of age 2
• Use WHO growth standards with weight-for-length measurements instead 2, 3

Children 24 Months and Older

• Begin routine BMI screening at age 2 years 1, 2
• Plot on CDC 2000 growth charts (age- and sex-specific) 1, 2
• Continue screening annually through age 19 years 2

Clinical Thresholds for Action

The following BMI percentile cutoffs trigger specific clinical responses:

• BMI ≥95th percentile = Obesity: Requires comprehensive screening for metabolic complications including lipid panel (starting age 2), blood pressure assessment, liver function tests, fasting glucose (age 10+), and screening for NAFLD (age 9-11), sleep apnea, and orthopedic problems 1, 2, 4

• BMI 85th-94th percentile = Overweight: Warrants counseling on nutrition and physical activity, with closer monitoring 1, 2

• BMI <5th percentile: May indicate undernutrition or underlying medical condition requiring evaluation 2, 3

Critical Implementation Requirements

Accurate Measurement Technique

BMI values are meaningless without proper measurement and age/sex adjustment. 2 To ensure accuracy:

• Weight: Use calibrated scale, light clothing only, record to nearest 0.1 kg 2
• Height: Use fixed wall-mounted stadiometer for children who can stand; child faces away from wall, heels together, back straight, arms down 2
• Calculation: BMI = weight (kg) / height (m)² 2
• Plotting: Must plot on age- and sex-specific CDC growth charts; a single BMI number without percentile context is clinically useless 2

Serial Measurements Are Essential

• Growth velocity over time using multiple data points is more informative than single measurements 2, 3
• Monitor every 3-6 months for tracking weight trajectory and response to interventions 2, 3, 4
• A child consistently tracking along a specific percentile curve may be growing appropriately, whereas crossing upward or downward through percentile lines indicates concerning trajectory 2, 3

Important Limitations and Caveats

When BMI May Be Misleading

• BMI tends to underestimate overweight in tall individuals and overestimate in short individuals or those with high lean body mass 2
• BMI is a screening tool, not a diagnostic instrument for overweight and obesity 1
• Direct body composition measures (DEXA, MRI, CT) are more accurate but reserved for research settings, not routine clinical care 2

Alternative Measures Are Not Recommended

There is little evidence that skinfolds, waist circumference, or bioelectrical impedance provide appreciable added information beyond BMI for identifying overweight/obese children in clinical practice. 5 These measures should not be used routinely in primary care settings. 5

Practical Barriers and Solutions

Despite clear recommendations, only 52% of pediatricians actually assess BMI percentile in practice. 6 Common barriers include:

• Lack of familiarity with BMI charts and screening recommendations 7, 6
• Absence of BMI charts readily available in exam rooms 7
• Inaccurate height/weight data 7
• Time constraints and perceived poor treatment effectiveness 6

Electronic medical records that automatically calculate and plot BMI significantly facilitate routine use and should be implemented wherever possible. 7, 2

Special Screening Considerations

Earlier Lipid Screening

While BMI ≥85th percentile is the standard overweight threshold, significant increases in total cholesterol and prevalence of dyslipidemia begin at the 80th percentile, suggesting consideration of lipid screening for children at this lower threshold. 8

Cardiovascular Risk Assessment

Parents recognize their child's overweight status in fewer than half of cases, making objective BMI screening essential for early identification and intervention. 1