The collection of BMI data for surveillance purposes is less controversial than BMI screening because surveillance does not involve the communication of sensitive information to parents, does not require individualized follow-up care for students identified to be at risk, and is therefore not likely to generate negative public response or detract from existing prevention programs.
Ideally, BMI should be derived from actual measurements of height and weight. However, measuring the height and weight of large numbers of students may not be feasible and can be costly and logistically challenging. An alternative approach is to use self-reported height and weight for surveillance among adolescents. The CDC’s YRBS, a national, state, and school district survey of health-risk behaviors among high school students, has reported BMI data every other year since 1999 using self-reported height and weight.103,104 A YRBS validation study found that self-reported height and weight are reliable (ie, the same numbers are consistently reported) and that BMIs derived from self-reports are highly correlated with those derived from actual measurements.105
However, using self-reported data have limitations that should be kept in mind. High school students tend to overestimate their height and underestimate their weight: as a result, BMI tends to be lower and the prevalence of obesity tends to be underestimated.105 Similar results have been found in adults.106 Furthermore, youth who are obese underestimate their weight more than those who are normal weight.107 This self-report bias may further distort results as more individuals become obese, resulting in inaccurate prevalence and trend data.107
Policy makers need to consider many factors in deciding whether to implement school-based BMI screening programs. The AAP has developed criteria to help guide decisions on whether schools should implement a screening program for any pediatric health problem.108 To receive AAP support, all of these criteria must be met (Table 4).
Table 4. AAP Criteria for a Successful Screening Program in Schools108
|Disease||Undetected cases must be common or new cases must occur frequently and the disease must be associated with adverse consequences|
|Treatment||Effective treatment must be available and early intervention must be beneficial|
|Screening test||The test should be sensitive, specific, and reliable|
|Screener||The screener must be well trained|
|Target population||Screening should focus on groups with high prevalence of the condition/disease in question or in which early intervention will be most beneficial|
|Referral and treatment||Those with a positive screening test must receive a more definitive evaluation and, if indicated, appropriate treatment|
|Cost/benefit ratio||The benefit should outweigh the expenses (ie, costs of conducting the screening and any physical or psychosocial affects on the individual being screened)|
|Site||The site should be appropriate for conducting the screening and communicating the results|
|Program maintenance||The program should be reviewed for its value and effectiveness|
BMI screening programs clearly meet some of the criteria: obesity is an important public health problem;13 the prevalence of obesity in the general population of children and adolescents is high;14 a screening test is available, that is, sensitive,34 specific,34 and reliable;33 staff training is available on how to properly conduct screenings;45,49,109 and schools are an appropriate site because they can reach virtually all youth including those without medical coverage.
However, school-based BMI screening programs do not meet other AAP criteria for screening programs. Specifically, effective and available treatments for obesity are not available,23,35,110 no standardized referral system exists,28 and the effectiveness and cost-effectiveness of BMI screening programs over time have not been documented. The AAP specifies that schools and school districts should not implement screening if resources for follow-up do not exist.108 Furthermore, research is needed to better understand any possible psychosocial effects on the individuals being screened, such as increased stigmatization and unsafe weight-control practices.
BMI Measurement program safeguards
Before launching a BMI measurement program for surveillance or screening, decision makers need to consider whether the anticipated benefits outweigh the expected costs. To minimize potential harm and maximize potential benefits, schools should not launch a BMI measurement program unless they have established a safe and supportive environment for students of all body sizes; are implementing a comprehensive set of strategies to prevent and reduce obesity; and have put in place a series of safeguards that address the primary concerns raised about such programs.
Following are some key characteristics of a safe and supportive environment for students of all body sizes:49
- • there is zero tolerance for weight discrimination, disrespectful behavior, and bullying
- • curricula foster acceptance of healthy weight by effectively countering social pressures for excessive thinness
- • teachers, school counselors, school nurses, coaches, and other school staff receive the professional development and resources they need to provide useful guidance to students with weight-related concerns. Staff should be prepared to promote positive body image and body satisfaction; help students overcome barriers to healthy eating and physical activity; and help students enhance their ability to find social support, cope with teasing, set goals, and make decisions.
If schools raise student and family awareness about obesity through a BMI measurement program, they need to have in place an environment that helps students make healthy dietary and physical activity choices both in and away from the school setting. The CDC has identified a comprehensive set of 10 strategies that schools can implement to prevent obesity by promoting physical activity and healthy eating (www.cdc.gov/healthyyouth/keystrategies).20,111 Many resources are available to help schools implement these strategies, including the following:
- • the School Health Index: A Self-Assessment and Planning Guide helps schools assess and improve their health and safety policies and practices (www.cdc.gov/HealthyYouth/SHI)112
- • the US Department of Agriculture has dietary guidelines for the national school meals program113
- • the IOM has published nutrition guidelines for foods and beverages offered outside of school meals114
- • schools can assess their physical education curriculum and align it with national standards by using the CDC’s Physical Education Curriculum Analysis Tool (www.cdc.gov/healthyyouth/pecat).115
A number of programs have integrated BMI measurement into more comprehensive approaches to addressing obesity. For example:
- • Arkansas Act 1220 mandated the creation of new programs to promote physical activity and healthy eating.78
- • The results from California’s Fitnessgram physical performance test influenced the California Department of Education to develop statewide grade-specific physical education content standards for student knowledge and ability.116
- • In Pennsylvania, the East Penn School District raised awareness of the importance of student health after implementing a BMI screening program.24 This led to changes in school policies and practices, including replacement of the sweetened drinks with 1% milk and 100% juice in vending machines, elimination of candy and high fat snack sales in vending machines, establishment of walking clubs, and increasing the length of lunch periods.
Following is a list of safeguards that need to be put in place to address the primary concerns that have been raised about school-based BMI measurement programs.21,49 These safeguards are needed to ensure respect for student privacy and confidentiality, protect students from potential harm, and increase the likelihood that the program will have a positive impact on promoting a healthy weight.
1. Introduce the program to parents, guardians, students, and school staff; ensure that there is an appropriate process in place for obtaining parental consent for measuring students’ height and weight.
To help minimize negative response from the public, programs need to involve parents or guardians early in the planning stages.24,117 Before the program begins, all parents should receive a clear description of the program to minimize confusion and anxiety. Communications with parents should focus on the health implications of obesity, overweight, and underweight and make it clear that the school will be measuring weight out of concern for a student’s health, not their appearance or a desire to criticize parenting practices.43,85 Schools should assure parents and students that the screening results will remain confidential. In addition, students and school staff should be informed of the purposes and logistics of height and weight measurement, as well as the school’s policy on sharing results.
Parents must be given the option of declining permission to measure their child’s BMI.24,117 Some programs use passive parental consent; that is, all students have their BMI measured unless parents send a written refusal. For example, at the beginning of each school year, Florida school districts inform parents about the school health program and the screenings that are conducted in each grade.52 Parents can choose not to have their child screened; otherwise, all students are measured in grades K, 1, 3, and 6. Other jurisdictions, such as Michigan, recommend active consent from both parents and students; only students who signed the consent form and whose parents have submitted a signed consent form are screened.49
2. Ensure that staff members who measure height and weight have the appropriate expertise and training to obtain accurate and reliable results and minimize the potential for stigmatization.
Accurate measurements are those that correspond to the youth’s actual height and weight, whereas reliable measurements are those that produce consistent results when they are repeated.109 Measurements are more likely to be accurate and reliable when they are conducted by trained professionals, such as school nurses.23,118 Unfortunately, many schools do not have full-time nurses on campus,54 and many school nurses feel that they cannot add another responsibility to their workload.51 Staff members involved in the program need the appropriate technical training from people who are experienced in conducting height and weight measurements and calculating and interpreting BMI results.119 Conducting repetitive tasks, such as measuring height and weight, can be tedious and may lead an individual to become careless and fail to consistently follow measurement protocols. Quality control checks can be implemented through random visits at measurement sites to oversee the performance of the staff measuring students’ height and weight.
Staff members need to ensure that each student takes off his or her shoes and jacket or other heavy clothing items and removes all items from his or her pockets before being weighed.120 Similarly, staff members must make sure that hair styles do not interfere with an accurate measurement of height.120 Each measurement should be taken twice and the youth should be repositioned prior to each measurement.109 If the 2 measurements do not agree within one fourth of a pound for weight or one fourth of an inch for height, then 2 additional measures should be taken until there is an agreement.109,119 Height errors, in particular, reduce the validity of BMI substantially.109
Staff also need appropriate training to measure height and weight in a sensitive and caring manner. This training should address procedures to maintain student privacy during measurement,49 increase awareness of groups at increased risk of stigmatization (ie, larger students, shorter boys, and taller girls), provide information about body size acceptance and the dangers of unhealthy weight-control practices, and help staff identify indications of student problems related to weight or body image (eg, eating disorders). Staff should be prepared to respond to questions or comments by students. For example, if a student makes a negative comment about his or her own weight, staff members need to be able to respond with supportive statements such as “Kids’ bodies come in lots of different sizes and shapes. If other kids are teasing you about your body, let’s talk and see what we can do about it.”21 Staff members also need to know how to respond to questions about what the school will do with the measurement results and referrals.
Resources that can assist with training on height and weight measurement include:
3. Ensure that the setting for data collection is private.
Height and weight measurements must not be conducted within sight or hearing distance of other students. The trained staff member conducting the measurement should be the only person to see the results and should not announce them out loud.49 To maintain anonymity when collecting data for surveillance purposes, school staff should remove identifying information, including the student’s name, from the data collection form as soon as record keeping is complete and prior to calculating BMI and aggregating and analyzing the data.122
4. Use equipment that can accurately and reliably measure height and weight.
The preferred equipment to assess students’ weight is an electronic or beam balance scale that is properly calibrated to the nearest one-fourth pound according to the manufacturer’s directions.109 Spring balance scales, such as bathroom scales, are not sufficiently accurate. The preferred equipment to assess height is a stadiometer, a wall-mounted or portable unit solely designed to measure height to the nearest one-eighth inch.109 The stadiometer should include a vertical board, metric tape, and horizontal headpiece that slides down to measure height. All equipment should be maintained and calibrated regularly.109
5. Ensure that BMI is calculated and interpreted correctly.
The formula for calculating BMI is as follows:
Schools should establish the BMI-for-age percentile using the CDC growth charts, available on the CDC’s Web site (www.cdc.gov/growthcharts).123 Staff must collect the student’s correct age in years and months as well as their gender to properly plot the BMI on the CDC growth charts. Schools conducting BMI screening programs should refer youth categorized as underweight, overweight, and obese to a medical care provider for diagnosis and possible weight management counseling.123
6. Develop efficient data collection procedures.
To facilitate efficient and accurate data collection, BMI measurement programs should coordinate data collection times with school administrators and employ a sufficient number of staff members to minimize disruptions to class time. In Florida, some districts use software that automatically calculates BMI after the necessary variables are entered.52,102 The software substantially reduces the time it takes staff to conduct screenings. In addition, the software can aggregate the data and produce health report cards.52,102
7. Do not use the actual BMI-for-age percentiles of the students as a basis for evaluating student or teacher performance (eg, in physical education or health education class).
Many factors beyond physical education and health education courses influence a student’s weight, so it is not appropriate to hold students or teachers accountable for changes in BMI percentiles. Using BMI results to evaluate performance might heighten attention to weight and increase stigmatization and harmful weight-related behaviors. Knowledge, skills, and changes in dietary, physical activity, and sedentary behaviors are more appropriate as performance measures.
8. Evaluate the BMI measurement program by assessing the process, intended outcomes, and unintended consequences of the program.
Data should be collected on concerns about the program, such as stigmatization, cost, parental responses, and displacement of other health-related initiatives. Schools can use the evaluation results to guide improvements to their program. The results should be shared with key stakeholders, parents, the community, school administrators, and policy makers to inform their decisions about school-based BMI measurement. The CDC’s Division of Adolescent and School Health Web site provides program evaluation resources:124www.cdc.gov/healthyyouth/evaluation/resources.htm.
Additional safeguards for BMI screening programs
1. Ensure that resources are available for safe and effective follow-up.
Because BMI screening programs are not intended to diagnose weight status, schools should refer students who need follow-up to appropriate local medical care providers. Before initiating a screening program, schools should work with the local medical community to ensure that adequate diagnostic and treatment services are available, staffed by employees with appropriate training, and accessible to all students, including those with low family incomes or without insurance. Schools should also identify school- or community-based health promotion programs that encourage physical activity and healthy eating. School nurses should be educated, trained, and equipped with the appropriate resources to respond to parents requesting guidance.125 School nurses can be a valuable resource during the follow-up period because they can provide parents with a clear explanation of the results and health risks associated with obesity, develop an action plan for behavior change, and connect the family to medical care in the community.125
2. Provide all parents with a clear and respectful explanation of the BMI results and a list of appropriate follow-up actions.
Student BMI results should be sent to parents by secure means, such as by mail, and not brought home by students. To reduce the risk of stigmatizing students, letters should be sent to all parents.24,117 To avoid giving the impression that a diagnosis has been made, the letters to parents about students who need further evaluation—those classified as underweight, overweight, or obese—should avoid definitive statements about the student’s weight category.22 For example:
- 1Letters might state that the student’s BMI result “suggests” that he/she “might be” overweight.47
- 2Letters might simply identify the student’s height, weight, and BMI-for-age percentile and include a table defining BMI-for-age percentile categories.48
- 3Letters might state that the student’s weight was found to be low/normal/high for his/her height and age.120
All letters should strongly encourage parents to consult a medical care provider to determine if the student’s weight presents a health risk.35
Letters to all parents, including those whose children have been classified as normal weight, should include scientifically sound and practical tips designed to promote health-enhancing physical activity and dietary behaviors. For example, the letters might summarize the US Dietary Guidelines for Americans, which recommend that youth include a variety of fruits and vegetables, whole-grain products, and fat-free or low-fat milk in their diet each day.126 Parents should also be aware that youth should engage in at least 60 minutes of physical activity on most, preferably all, days of the week.126 The letters should be written in appropriate languages and at appropriate reading levels to be understood by parents; the tone should be neutral to avoid making parents feel that they are being blamed for their child’s weight status.46 Motivational messages included in the letters should be guided by sound communication and health behavior change theories. To ensure comprehension and effectiveness, the letters can be tested with representative parents in advance.
If the safeguards described above are implemented, BMI results may also be shared directly with older students—the Michigan Department of Education recommends that results not be shared with students below grade 4—as long as staff ensure that this communication remains private and does not stigmatize or label the students.49 Because these letters could have a significant impact on the students, the school nurses and school counselors should be prepared to deal with such reactions as anxiety and despair.
The letters should include (a) contact information for the school nurse or other school-linked medical care provider; (b) educational resources for weight, nutrition, and physical activity; (c) contact information for community-based health programs or medical care providers who treat weight-related problems (including programs for those without health insurance); and (c) information on school- and community-based programs that promote nutrition and physical activity.
Screening programs have developed standardized letters tailored to the weight status of the child.47,48,119 Examples are available at:
panaonline.org/programs/khz/screening; www.achi.net/BMI_info/health_letter.asp; and www.cnr.berkeley.edu/cwh/PDFs/color_weighing.pdf.
Additional guidance on BMI measurement safeguards is available in: