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Protocol - Body Mass Index

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Protocol Name from Source:

This section will be completed when reviewed by an Expert Review Panel.

Availability:

Publicly available

Description:

Standing height is measured from the top of the participant’s head to his or her heels. Measured weight is assessed using a digital floor scale that should be calibrated daily using standardized weights, and a log of calibration results should be maintained. These measurements are taken among participants who are able to stand unassisted.

Protocol:

There are several overarching, critical issues for high-quality data collection of anthropometric measures. These issues include (1) the need for training (and retraining) of study staff in anthropometric data collection; (2) duplicate collection of measurements, especially under field conditions; (3) use of more than one person for proper collection of measurements, where required; (4) accurate recording of the protocols and the measurement units of data collection; and (5) use of required and properly calibrated equipment.

Note: Detailed videos illustrating the measurement procedures can be found on the National Health and Nutrition Examination Survey (NHANES) website.

Standing Height Protocol Ask the participant to remove hair ornaments, jewelry, buns, or braids from the top of the head that interfere with the measurement. Shoes should be removed.

Ask the participant to stand erect against the backboard with the body weight evenly distributed and both feet flat on the stadiometer platform (Exhibit 1). The participant’s feet should be positioned with the heels together and toes pointed slightly outward at approximately a 60 degree angle. Check to be sure that the back of the head, shoulder blades, buttocks, and heels make contact with the backboard of the stadiometer.

Note: Depending on the overall body conformation of the individual, all four contact points-head, shoulders, buttocks, and heels-may or may not touch the stadiometer backboard (Exhibit 2). For example, elderly survey participants may have kyphosis, a forward curvature of the spine that appears as a hump at the upper back. In particular, dowager’s hump is a form of kyphosis that creates a hump at the back of the neck. Additionally, some overweight survey participants cannot stand straight while touching all four contact points to the backboard. In such instances, it is important to obtain the best measurement possible according to the protocol.

Stature measurements are made with the head aligned in the Frankfort horizontal plane (Exhibit 2). The head is in the Frankfort plane when the horizontal line from the ear canal to the lower border of the orbit of the eye is parallel to the floor and perpendicular to the vertical backboard (see Exhibit 2). Many people will assume this position naturally, but for some survey participants the examiner may need to gently tilt the head up or down to achieve the proper alignment. Instruct the survey participant to look straight ahead.

If you cannot position the participant such that his or her trunk remains vertical above the waist, such that the arms and shoulders are relaxed, and such that the head is positioned in the Frankfort plane, be sure to note this in the measurement record. This information might be useful to interpret study findings. In the National Health and Nutrition Examination Study 2007-2008, a comment described as "Not Straight" is noted in the stature record.

Once positioned, lower the stadiometer headpiece so that it rests firmly on top of the participant’s head, with sufficient pressure to compress the hair. Instruct the survey participant to stand as tall as possible, take a deep breath, and hold this position. The act of taking a deep breath helps straighten the spine to yield a more consistent and reproducible stature measurement. Notice that the inhalation will cause the headpiece to rise slightly.

As soon as the participant inhales, record the measurement. After recording the measurement, tell the participant to relax. Once the measurement is taken, raise the stadiometer headpiece and have the participant step away from the stadiometer.

Adjustments for shoes and hair: When participants cannot remove hair braids, buns, and headwear that interfere with the stature measurement, measure the distance from the scalp to the top of the hair with a small ruler to the nearest 0.1 cm. If shoes are worn, measure the height of the shoe heel to the nearest 0.1 cm. A corrected height value can be calculated by subtracting these distances from the original stature measurement, thus yielding an adjusted stature value.

Record standing height in meters (or centimeters) or inches. Repeat height measurement.

[img[bmi_exhibit_1.jpg| Exhibit 1. Stadiometer with a fixed length backboard and an adjustable headpiece]]

[img[bmi_exhibit_2.jpg| Exhibit 2. Body Orientation for Standing Height Measurement and Frankfort Horizontal Plane]]

Current Measured Weight Protocol

Note: Detailed videos illustrating the procedure can be found on the NHANES website.

A digital scale or beam balance is used to weigh participants.

Participants are asked to wear an examination gown and socks or light clothes without shoes. Only undergarments are worn beneath the gown. The procedures for obtaining the weight measurement are as follows:

The examiner briefly informs the participant that his/her weight will be measured. Participants are asked to remove objects such as cell phones, wallets, and toys from their pockets.

  1. The health technician directs participants to stand in the center of the scale platform with hands at their sides and looking straight ahead.
  2. The weight measurement is recorded in kilograms or pounds.
  3. Special situations:
    • Small children: Infants and toddlers who cannot stand alone on the scale will be weighed with an adult, with an infant’s scale. If an adult is holding the child, then the adult guardian or the health technician will stand alone on the scale so the scale can be tared. This sets the scale readout to zero. The child is then handed to the adult and the child’s weight is measured.
    • If the participant is wearing a cast or medical prosthesis, make a note in the record of the location and place.
    • If the participant wore street clothes instead of the examination gown, make a note of this in the record. It is acceptable for infants to wear diapers or underpants and a t-shirt.
    • Participants should not be weighed if they are wearing shoes.
    • Note that special consideration may be needed for participants whose weight exceeds the capacity of the study scale. For example, weight can be obtained using two portable scales:
      • Have the participant stand with one foot on each portable scale.
      • Combine the two results to approximate the weight.
      • Record the weight.
      • If the weight equals the capacity of both portable scales, note that the weight Equals Capacity (EC) of the scales.

Record current weight in pounds or kilograms. Repeat weight measurement.

Derivation of Body Mass Index (BMI)

BMI calculations are recommended in children (2+ years) and adults, but some references suggest that the calculations are relevant from birth.

Measurement Units

Formula and Calculation

Kilograms and meters (or centimeters)

Formula: BMI = weight (kg) / [height (m)]2
With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters.
Example: Weight = 68 kg, Height= 165 cm (1.65 m)
Calculation: 68÷(1.65)2= 24.98

Pounds and inches

Formula: BMI = weight (lb) / [height (in)]2 x 703
Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.
Example: Weight = 150 lbs, Height = 5’5" (65")
Calculation: [150 ÷ (65)2] x 703 = 24.96

Source: Centers for Disease Control and Prevention website. Body Mass Index page.

Interpretation of BMI in Adults

The following categories are used to interpret BMI in both adult men and women (20 years old and older).

  • Underweight : below 18.5
  • Normal weight: 18.5-24.9
  • Overweight: 25-29.9
  • Obesity: 30 and above

Source: Centers for Disease Control and Prevention website. Body Mass Index page.

Interpretation of BMI in Children and Teens

BMI for children and teens is age- and sex-specific, so it is often referred to as BMI-for-age. After calculating BMI, the BMI number should be plotted on the BMI-for-age growth chart (available for boys and girls, separately, from the Centers for Disease Control and Prevention website) to obtain a percentile ranking. The following categories are used to interpret BMI-for-age in children and teens.

  • Underweight: less than the 5th percentile
  • Normal weight: 5th percentile to less than the 85th percentile
  • Overweight: 85th to less than the 95th percentile
  • Obesity: equal to or greater than the 95th percentile

Personnel and Training Required

Technicians should be trained in the basic techniques of anthropometric measurements.

Equipment Needs

Stadiometer with a fixed-length backboard and an adjustable headpiece.

Digital scale or beam balances. Portable scales have also been used in the National Health and Nutrition Examination Survey. Standard weight to calibrate scale. A tare function is used when weighing infants and children. The tare function is a feature found in clinical scale equipment.

Requirements

Requirement CategoryRequired
Average time of greater than 15 minutes in an unaffected individualNo
Major equipmentNo
Specialized requirements for biospecimen collectionNo
Specialized trainingNo

Mode of Administration

Self-administered

Life Stage:

Toddler, Child, Adolescent, Adult, Senior

Specific Instructions:

Body mass index (BMI) is derived from the Standing Height protocol (under the Height measure) and the Measured Weight protocol (under the Weight measure), both from the Anthropometrics domain. Self-reported values of height and weight are considered to be less accurate than those obtained from measurement protocols, so the PhenX Anthropometrics Working Group strongly recommends supervised measurement of height and weight using instrument protocols.

Under usual field conditions, for reliability, the Anthropometrics Working Group suggests that the measurements are taken in duplicate. A third height measurement should be taken if the first two measurements differed by >1.0 cm (1/4"). A third weight measurement should be taken if the first two measurements differ by >200 grams or 0.25 pounds. In either case, if it is necessary to take a third measurement, the two closest measurements are averaged. Should the third measurement fall equally between the first two measurements, all three should be averaged.

Additionally, when measuring weight, the observer should determine whether the woman is pregnant and, if she is pregnant, request how many weeks pregnant. Pregnancy status may be sensitive information, so participants should be asked this question in a private setting.

Research Domain Information

Release Date:

February 24, 2012

Definition

Body mass index (BMI) is derived from height and weight measurements. Height is the distance from the top of the participant’s head to the heels of his or her feet (i.e., the vertical length). Current measured weight is the weight in kilograms or pounds of the participant. BMI is calculated through a formula in which weight is divided by height squared.

Purpose

Body mass index (BMI) is a measure of body fat, based on height and weight. Height or stature is used to assess body size and bone length. Current weight is used to assess participant’s growth, development, and current health status. Higher BMI levels are associated with various health-related co-morbidities.

Selection Rationale

The National Health and Nutrition Examination Survey (NHANES) 2007-2008 protocols were selected as best-practice methodology and are the most widely used protocols to assess measured height and weight. Both protocols are used to calculate body mass index, assess nutritional status, and predict morbidity and mortality.

Language

English, Spanish

Standards

StandardNameIDSource
Common Data Elements (CDE)Person Body Mass Index Value2006410CDE Browser

Process and Review

This section will be completed when reviewed by an Expert Review Panel.

Source

Centers for Disease Control and Prevention (CDC), National Center for Health Statistics (NCHS). (2007-2008). National Health and Nutrition Examination Survey anthropometrics procedure manual. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention.

General References

None

Protocol ID:

570101

Variables:

Export Variables
Variable NameVariable IDVariable DescriptionVersiondbGaP Mapping
PX570101_Standing_Height_1_FeetsPX570101010100Standing Height measured in centimeter, first measurement4N/A
PX570101_Standing_Height_1_InchesPX570101010200Standing Height measured in centimeter, first measurement4N/A
PX570101_Standing_Height_1_MetersPX570101010300Standing Height measured in centimeter, first measurement4N/A
PX570101_Standing_Height_2_FeetsPX570101020100Standing Height measured in centimeter, second measurement4N/A
PX570101_Standing_Height_2_InchesPX570101020200Standing Height measured in centimeter, second measurement4N/A
PX570101_Standing_Height_2_MetersPX570101020300Standing Height measured in centimeter, second measurement4N/A
PX570101_Standing_Height_3_FeetsPX570101030100Standing Height measured in centimeter, third measurement4N/A
PX570101_Standing_Height_3_InchesPX570101030200Standing Height measured in centimeter, third measurement4N/A
PX570101_Standing_Height_3_MetersPX570101030300Standing Height measured in centimeter, third measurement4N/A
PX570101_Standing_Height_Average_FeetsPX570101040100Standing Height measured in centimeter, average4N/A
PX570101_Standing_Height_Average_InchesPX570101040200Standing Height measured in centimeter, average4N/A
PX570101_Standing_Height_Average_MetersPX570101040300Standing Height measured in centimeter, average4N/A
PX570101_Cast_Or_Medical_ProsthesisPX570101050100Record whether the participant is wearing a cast or prosthesis.4N/A
PX570101_Cast_Or_Medical_Prosthesis_LocationPX570101050200If the participant is wearing one, record the location of a cast or medical prosthesis4N/A
PX570101_Street_ClothesPX570101060000Record whether or not the participant is wearing street (as opposed to a gown) clothes during measurement.4N/A
PX570101_Measured_Weight_1_PoundsPX570101070100Measured weight in lbs, first measurement4Variable Mapping
PX570101_Measured_Weight_2_PoundsPX570101070200Measured weight in lbs, second measurement4Variable Mapping
PX570101_Measured_Weight_3_PoundsPX570101070300Measured weight in lbs, third measurement4Variable Mapping
PX570101_Measured_Weight_Average_PoundsPX570101070400Measured weight in lbs, average4Variable Mapping
PX570101_Measured_Weight_1_KilogramsPX570101070500Measured weight in kg, first measurement4Variable Mapping
PX570101_Measured_Weight_2_KilogramsPX570101070600Measured weight in kg, second measurement4Variable Mapping
PX570101_Measured_Weight_3_KilogramsPX570101070700Measured weight in kg, third measurement4Variable Mapping
PX570101_Measured_Weight_Average_KilogramsPX570101070800Measured weight in kg, average4Variable Mapping
PX570101_BMI _Kilogram_Per_SquaremeterPX570101080100calculated BMI in kg/m24N/A
PX570101_BMI _Pound_Per_SquareinchPX570101080200calculated BMI in lb/in24N/A