Protocol - Body Composition - Body Composition by Dual-Energy X-Ray Absorptiometry
Dual-energy X-ray absorptiometry (DXA) is one of the most widely used methods to assess body composition in a laboratory environment. A densitometer scanner device is used to scan the entire body or specific body sites such as the hip, spine, and limbs. DXA uses X-ray energy to measure body fat, muscle, and bone mineral and as a result, a small amount of radiation is delivered to the body during the DXA scan procedure. The results from a whole body DXA scan can be used to estimate total mass (gm), bone mineral content (BMC) (gm), bone area (cm2), bone mineral density, fat mass, lean mass excluding BMC (gm), lean mass including BMC (gm), and percent body fat (%). These measures can also be subdivided from the whole body to regional measures of the arms, legs, trunk, android, gynoid, and head. Furthermore, special algorithms have been created to estimate visceral and subcutaneous adipose tissue. Source: Centers for Disease Control and Prevention, National Center for Health Statistics, National Health and Nutrition Examination Survey (NHANES 2001-2002) DXA file documentation. URL: http://www.cdc.gov/nchs/data/nhanes/dxa/dxx_b.pdf (accessed January 22, 2009).
Study-specific protocols and equipment descriptions should be reviewed carefully. In the National Health and Nutrition Examination Survey (NHANES), a certified radiologic technician positions the participant and operates the dual-energy X-ray absorptiometry (DXA) scanner device. Due to concerns about radiation exposure to the unborn fetus, urine pregnancy tests or self-reported pregnancy history were used to exclude pregnant females from the NHANES DXA component. Equipment limitations excluded NHANES participants who weighed more than 300 lbs or were more than 6’5" tall. However, current generation DXA systems, such as the Hologic Horizon and GE-Lunar iDXA can tolerate participant weights up to 450 lbs (Hangartner, Warner, Braillon, Jankowski, & Shepherd, 2013). Individuals with amputations other than fingers and toes were also excluded from the NHANES DXA exam. The technician should determine if the participant is wearing metallic items such as jewelry or eyeglasses, has implanted medical devices such as a pacemaker or artificial joint, or has undergone certain types of medical tests that might affect the scan results. The NHANES DXA Procedures Manual describes the protocol, exclusions, and screening questions in detail. Rigorous quality control procedures are needed to monitor the performance of the DXA scanner, the performance of the certified radiology technologists who performed the DXA scan, and the quality of the DXA scans. The NHANES DXA Procedures Manual describes the quality assurance and quality control measures that were used in NHANES.
A downloadable PDF of the full National Health and Nutrition Examination Survey (NHANES) Body Composition Procedures Manual is available here: http://www.cdc.gov/nchs/data/nhanes/nhanes_11_12/Body_Composition_Procedures_Manual.pdf The following is a summary version of the full NHANES protocol. There are several overarching, critical issues for high-quality data collection of anthropometric measures which optimize the data in gene-environment etiologic research. 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: Dual-energy X-ray absorptiometry (DXA) measurement is not complex, but does require costly hardware, specialized software to analyze the DXA scans, rigorous quality control, certified examiners, and continuous equipment calibration and monitoring. Although excerpts of the National Health and Nutrition Examination Survey (NHANES) protocol are provided for informational purposes, investigators should review all facets of the protocol prior to undertaking DXA examinations in a study. A detailed description of the NHANES protocol is available from the Centers for Disease Control and Prevention (CDC) website. The following excerpts from the NHANES 1999-2006 protocols refer to a specific make and model DXA scanner. Several brands of DXA scanners are available and manufacturer-specific software, calibration, and quality control practices should be followed. DXA Equipment: Several DXA densitometer devices are marketed commercially. The Hologic™ 4500A fan beam X-ray bone densitometer was used in NHANES 1999-2009 protocol. In 2011, the NHANES systems were replaced with the Discovery/A systems. Neither of these systems are made anymore, but this protocol is applicable to the current Hologic Horizon whole body models (models A or W.)
Whole Body DXA Scan:
- Record the participant information and relevant information about the DXA machine and software. At a minimum, height, weight, sex, and ethnicity should be entered since these are used in some of the DXA calculations.
- Check to see if the participant is positioned correctly for the whole body scans. NHANES used the positioning recommended by the International Society for Clinical Densitometry as outlined in detail in Hangartner et al. (2013).
- Check one more time to ensure that no objects will interfere with the movement of the table or the runner belt.
Conducting the Scan:
- Conduct a whole body scan according to the directions specified by the International Society for Clinical Densitometry (Hangartner, 2013).
- Once initiated, the DXA system will scan until complete.
- Insure that the participant does not speak or move during the exam.
- After the scan is completed, a screen displaying the DXA image appears. This image should look like a whole body X-ray with bones and soft tissue clearly visible, without any distortion from movement, and with the whole body in the field. The scan may be repeated up to three times if the scan quality can be improved. (The conditions under which a scan may or may not be repeated are outlined below in the section entitled Ensuring Quality of Scan.)
Ensuring Quality of Scan: The conditions under which a scan may or may not be repeated include:
- Participant movement during the exam: If the participant moves during the exam, tell them to remain still and complete the scan. If the conditions of the scan can be improved such that you think a second scan would improve the quality, complete a second exam. Explain the procedure again with an emphasis on the importance of remaining quiet and still throughout the scan.
- Positioning problem: If you complete a scan and upon review, notice a problem with the position of the participant that can be corrected (i.e. body part off scanner, hip overlapping the hands, etc.), get permission from the participant and repeat the scan. In some cases, you may not be able to correct the positioning problem. For example, if the participant has a curvature of the spine or other physical condition that might limit ideal positioning, repeating the scan will not improve the positioning problem.
- Hands positioned along sides, not flat: If the participant is too wide for the table and the hands cannot be positioned flat by their side, then you should use offset scanning. Ideally, you would determine this before the first scan. Offset scanning is the positioning of the participant offset to the table axis such that the left arm is off the table but the entire torso is still in the scan field. Both Hologic and GE systems have special analysis routines that will reflect the right arm values to the left to create a complete scan.
- Too tall for the table, feet cut out of the scan: If the participant is too tall for the table and you cannot get the entire body in the scan, there are two options. First, if only one scan is to be acquired, make sure you get the entire head in the scan. Missing feet should be noted in the comments field. Missing feet will alter the whole body mass but will not impact the total body bone mineral density (BMD) or percentage of body fat (%BF) very much. If a complete scan is desired, move the patient further up the table such that the head is resting off the table and the feet are within the scan field. Using the two scans, one can generate a whole scan by using the head, arms, and torso from the first scan and the legs from the second. This has to be done manually in post analysis.
- Pillow used for head support: If the participant’s head is not positioned optimally because of excess neck fat or back problems, a radiolucent pillow can be used to support the head. The use of a pillow should be noted in the comments field. If the pillow is used correctly, the scan is valid.
- Equipment failure: If you have an equipment problem during a scan, use the emergency stop button to insure that the participant can get off the scanner without it accidently starting. If the problem can be resolved during the visit, repeat the scan.
Completing the Second Scan
Second scans are acquired in the same way as a first scan with repositioning. Focus on the cause for repeating the scan and make the necessary modifications (reposition, remove jewelry or objects, etc.). Before starting the scan, make sure that all corrections have been made. If a problem occurs with this scan, you will only be able to repeat one more time for a total of three attempts.
DXA is the preferred method for assessing bone mineral content (BMC) and BMD. The posterior-anterior (PA) lumbar spine and PA proximal femur scans are typically acquired for all osteoporosis assessments of BMD in adults. For children, the PA lumbar spine and total body less head BMD and BMC are used to assess low bone density. Soft tissue measures can only be acquired from whole body scans and may be helpful in evaluating patients with chronic conditions associated with malnutrition (such as anorexia nervosa, inflammatory bowel disease, cystic fibrosis), or with both muscle and skeletal deficits (such as idiopathic juvenile osteoporosis, or sarcopenic osteoporosis). Adults, BMD status is compared to the young adults reference data by deriving the T-score. In children with linear growth or maturational delay, spine and TBLH BMC and areal BMD results should be adjusted for absolute height or height age (Zemel et al., 2011) in addition to age and gender using Z-scores. SOURCE: International Society for Clinical Densitometry. (2007). 2007 official positions and pediatric official positions of the International Society for Clinical Densitometry (p. 25). Retrieved from http://www.iscd.org/wp-content/uploads/2012/10/ISCD2007OfficialPositions-Combined-AdultandPediatric.pdf
Protocol Name from Source:
2011-2012 National Health and Nutrition Examination Survey Body Composition Procedures Manual
Personnel and Training Required
Technicians should be trained in the specific use of DXA systems. In all states, certified radiology technologists can operate DXA systems without further certification. In some states such as California, a limited license can be obtained after DXA-specific training and testing has occurred. Lastly, in a few states such as Nevada, anyone can operate a DXA system. DXA technologist courses are offered by the International Society for Clinical Densitometry. Additionally, study personnel should receive specialized training in the basic techniques of anthropometric measurements, the operation of the specific make and model of the dual-energy X-ray absorptiometry (DXA) machine used in the study, and the study protocol.
Device-specific education and training should be provided to the operators and interpreters prior to clinical use. Quality control procedures should be performed regularly.
Source: International Society for Clinical Densitometry. (2007). 2007 official positions and pediatric official positions of the International Society for Clinical Densitometry (p. 18). Retrieved from http://www.iscd.org/wp-content/uploads/2012/10/ISCD2007OfficialPositions-Combined-AdultandPediatric.pdf
Several dual-energy X-ray absorptiometry (DXA) machines are marketed commercially. According to the International Society for Clinical Densitometry, "Bone density measurements from different devices cannot be directly compared." Because there are many manufacturers, models, and software versions of the DXA machine, investigators must report those they are using. Regular (i.e., weekly at a minimum for large studies) scans using DXA phantoms are recommended as an independent assessment of system calibration. Studies that use more than one scanner device should also implement a plan to monitor inter-scanner differences at baseline and at regular intervals during the study. Note: Manufacturers of DXA equipment are constantly striving to improve and update both their equipment and software with the latest technological advances. The recommended equipment for this protocol in PhenX is current as of January 2015. Please refer to the DXA specifications for your model number to ensure the level of information collected is compatible with the PhenX protocol.
SOURCE: International Society for Clinical Densitometry. (2007). 2007 official positions and pediatric official positions of the International Society for Clinical Densitometry (p. 9). Retrieved from http://www.iscd.org/wp-content/uploads/2012/10/ISCD2007OfficialPositions-Combined-AdultandPediatric.pdf
Mode of Administration
Noninvasive radiologic assessment
Dual-energy X-ray absorptiometry (DXA) methods have been used in clinical and epidemiological studies with infants, children, and adults. There is no lower limit to age, but DXA measures are challenging for infants, younger children, and some persons with disabilities who are unlikely to remain still and in position during measurement, and even small amounts of radiation exposure are usually not allowed for pregnant women. In the National Health and Nutrition Examination Survey, Individuals between 8 and 59 years of age years in 2011-2012 were eligible for the DXA component. However, reference data exists from other sources for infant spine bone mineral density (BMD) and for children down to 3 years old for whole body BMD and BMC for Hologic systems.
The PhenX Anthropometrics Working Group considers dual-energy X-ray absorptiometry (DXA) to be the current gold standard for obtaining a measurement of a person’s percentage body fat and muscle mass.
|Common Data Elements (CDE)||Person Dual X-ray Absorptometry Body Composition Value||2936520||CDE Browser|
Bone Mineral Density (BMD), Bone Mass, Body Fat, Lean Body Mass; and Bone, Fat, and Body Mass of specific anatomical regions
Process and Review
The Expert Review Panel #1 reviewed the measures in the Anthropometrics, Diabetes, Physical Activity and Physical Fitness, and Nutrition and Dietary Supplements domains.
Guidance from the ERP includes:
Updated the protocol (same source)
Back-compatible: there are changes to the Data Dictionary, previous version of the Data Dictionary and Variable mapping in Toolkit archive (link)
Centers for Disease Control and Prevention (CDC), National Center for Health Statistics (NCHS). (2011-2012). National Health and Nutrition Examination Survey body composition procedures manual. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention.
Hangartner, T. N., Warner, S., Braillon, P., Jankowski, L., & Shepherd, J. (2013). The official positions of the international society for clinical densitometry: Acquisition of dual-energy X-ray absorptiometry body composition and considerations regarding analysis and repeatability of measures. Journal of Clinical Densitometry, 16(4), 520-536.
International Society for Clinical Densitometry. (2007). 2007 official positions and pediatric official positions of the International Society for Clinical Densitometry. Retrieved from http://www.iscd.org/wp-content/uploads/2012/10/ISCD2007OfficialPositions-Combined-AdultandPediatric.pdf
Kalkwarf, H. J., Zemel, B. S., Yolton, K., & Heubi, J. E. (2013). Bone mineral content and density of the lumbar spine of infants and toddlers: influence of age, sex, race, growth, and human milk feeding. Journal of Bone and Mineral Research, 28(1), 206-212.
Lohman, T. G., & Chen, Z. (2005). Dual-energy X-ray absorptiometry. In S. Heymsfield, T. G. Lohman, Z. Wang, & S. B. Going (Eds.), Human body composition (pp. 63-78). Champaign, IL: Human Kinetics.
National Institute of Diabetes and Digestive and Kidney Diseases. (2007). Body composition. Retrieved from http://www2.niddk.nih.gov/Research/ClinicalResearch/MCRU/MCRUBodyComposition.htm
Tufts University Nutritional Collective, Center for Drug Abuse and AIDS Research (TNC-CDAAR). (2003). Dual energy X-ray absorptiometry (DEXA) for evaluation of body composition: Protocol. Retrieved April 8, 2009, from http://www.tufts.edu/med/nutrition-infection/tnc-cdaar/protocols/DEXA2.pdf
University of Vermont, Department of Nutrition and Food Sciences. (n.d.). Dual energy X-ray absorptiometry (DEXA). Retrieved April 8, 2009 from http://nutrition.uvm.edu/bodycomp/dexa/ NOTE: This protocol uses single beam DXA rather than the fan beam DXA used in NHANES. The fan beam DXA scan used in NHANES takes only 3 minutes and has less radiation exposure.
Zemel, B. S., Kalkwarf, H. J., Gilsanz, V., Lappe, J. M., Oberfield, S., Shepherd, J. A., . . . , Winer, K. K. (2011). Revised reference curves for bone mineral content and areal bone mineral density according to age and sex for black and non-black children: Results of the bone mineral density in childhood study. Journal of Clinical Endocrinology and Metabolism, 96(10), 3160-9.
|Variable Name||Variable ID||Variable Description||Version||dbGaP Mapping|
|PX020302_BodyComposition_Dxa_Adequate_Staffing||PX020302080000||Is the team adequately staffed so that more than one person is available for proper collection of measurements where required?||N/A|
|PX020302_BodyComposition_Dxa_Amputations||PX020302120000||Does the subject have any amputations other than fingers and toes?||N/A|
|PX020302_BodyComposition_Dxa_Calibration_Training||PX020302100000||Has the staff been properly trained in the use of and have access to required and properly calibrated equipment?||N/A|
|PX020302_BodyComposition_Dxa_ConductingScan_Image_Quality||PX020302200000||Does the DXA image appear, similar to a whole body X-ray with bones and soft tissue clearly visible, without any distortion from movement, and with the whole body in the field?||N/A|
|PX020302_BodyComposition_Dxa_ConductingScan_Patient_Instructions||PX020302190000||Has the subject been instructed to not speak or move during the whole body examination?||N/A|
|PX020302_BodyComposition_Dxa_ConductingScan_Repeat_Scan||PX020302210000||If not, was the scan repeated (up to 3x)?||N/A|
|PX020302_BodyComposition_Dxa_Duplicate_Measurements||PX020302070000||Will duplicate measurements taken?||N/A|
|PX020302_BodyComposition_Dxa_EnsuringQuality_Patient_Width||PX020302220000||Is the subject too wide for the table so that his/her hands cannot be positioned flat by their side?||N/A|
|PX020302_BodyComposition_Dxa_EnsuringQuality_Pillow||PX020302230000||Was a radiolucent pillow used to support the subject's head?||N/A|
|PX020302_BodyComposition_Dxa_Height65||PX020302050000||Is the subject taller than 6'5"?||N/A|
|PX020302_BodyComposition_Dxa_Pregnancy||PX020302010000||Did the subject report or did urine pregnancy tests indicate the the subject is pregnant?||N/A|
|PX020302_BodyComposition_Dxa_Quality_Assurance_AndControl||PX020302130000||Are there quality assurance and quality control measures in place, such as those described in the NHANES DXA Procedures Manual?||N/A|
|PX020302_BodyComposition_Dxa_Quality_Training||PX020302090000||Has the staff been properly trained to acquire an accurate recording of the protocols and measurements units of data collection?||N/A|
|PX020302_BodyComposition_Dxa_Staff_Training||PX020302060000||Have the study staff undergone any training or retraining in anthropometric data collection?||N/A|
|PX020302_BodyComposition_Dxa_Subject_Prohibited_Items||PX020302110000||Has the staff checked whether the patient is wearing metallic items such as jewelry or eyeglasses, has implanted medical devices such as a pacemaker or artificial joint, or has undergone certain types of medical tests that might affect the scan results?||N/A|
|PX020302_BodyComposition_Dxa_System||PX020302020000||Which DXA system is being used for this procedure?||N/A|
|PX020302_BodyComposition_Dxa_Weight300||PX020302040000||Does the subject weight more than 300 lb?||N/A|
|PX020302_BodyComposition_Dxa_Weight450||PX020302030000||Does the subject weight less than 450 lb?||N/A|
|PX020302_BodyComposition_Dxa_WholeBody_Correct_Poisitioning||PX020302180000||Is the subject's body being correctly position for the whole body scans as recommended by the International Society for Clinical Densitometry?||N/A|
|PX020302_BodyComposition_Dxa_WholeBody_Ethnicity||PX020302170000||What is the subject's ethnicity?||Variable Mapping|
|PX020302_BodyComposition_Dxa_WholeBody_Gender||PX020302160000||What is the subject's gender?||Variable Mapping|
|PX020302_BodyComposition_Dxa_WholeBody_Height||PX020302140000||What is the subject's height?||Variable Mapping|
|PX020302_BodyComposition_Dxa_WholeBody_Weight||PX020302150000||What is the subject's weight?||Variable Mapping|
October 1, 2015
Body composition defined most broadly refers to the proportions of fat mass (FM) and fat-free mass (FFM) or lean body mass (LBM) but also encompasses a related concept of regional body fatness. With an increase in FM or adiposity, there may be changes in the relative distribution of fat, for example, toward visceral or dorsal deposits and away from limb fat. Regional distribution of fat also changes normally with maturation and differentially between sexes; changes that may be aggravated by overweight or obesity. Early identification of patterns of regional fatness that may be associated with risky profiles is also encouraged.
Scientific and practical guidance on which protocol may be best suited for a study’s needs are offered here: Body Composition Guidance
The study of body composition looks at the differences in bone, muscle, organs, and fat. Body composition analysis is an indicator of overall health as determined by a person's percentage of fat and lean mass. Body composition tests are designed to give a "whole picture" of the body, but measures can also be used to estimate regional fat distribution. This information is useful to help develop nutrition and exercise programs to benefit the individual and to assess risk for later-life chronic diseases.
Anthropometrics, body fat, body mass index, BMI, obesity lean body mass, muscle mass, fat body mass, diabetes, bone density, bone mineral density, BMD, body fat, bone mass, fat mass, skinfold thickness, BIA, metabolic syndrome, DEXA, DXA, NHANES