Component Description
Users of the 2017-2018 dual-energy X-ray absorptiometry whole body data (DXX_J) are encouraged to read the documentation before accessing the data file.
Dual-energy x-ray absorptiometry (DXA) is the most widely accepted method of measuring body composition due in part to its speed, ease of use, and low radiation exposure (Baran, 1997; Genant, 1996; Heymsfield, 1989; and Njeh, 1999). Whole body DXA scans were administered from 1999-2006 and 2011-2018. The NHANES DXA examination provides nationally representative data on body composition (bone and soft tissue), overall, and for age, gender, and race/ethnic groups, to study the association between body composition and other health conditions and risk factors, such as cardiovascular disease, diabetes, hypertension, and physical activity and dietary intake patterns.
The DXA scans provide bone and soft tissue measurements for the total body, for both arms and both legs, the trunk, and the head. Bone measurements also were obtained for the pelvis, left and right ribs, thoracic spine, and lumbar spine. Values for the total body and regions include:
- Total mass (gm)
- Bone mineral content (BMC) (gm)
- Bone area (cm2)
- Bone mineral density (BMD) (gm/cm2)
- Fat mass (gm)
- Lean mass excluding BMC (gm)
- Lean mass including BMC (gm)
- Percent body fat (%)
Eligible Sample
Participants aged 8-59 years were eligible. Pregnant females were excluded from the DXA examination. Participants who were excluded from the DXA examination for reasons other than pregnancy were eligible nonrespondents. Reasons for exclusion from the DXA examination were as follows:
- Pregnancy (positive urine pregnancy test and/or self-report at the time of the DXA examination);
- Self-reported history of radiographic contrast material (barium) use in past 7 days; and
- Self-reported weight over 450 pounds or height over 6’5” (DXA table limitation).
The variable DXAEXSTS indicates the examination status for the whole body scan. The codes for DXAEXSTS are as follows:
DXAEXSTS – whole body scan examination status variable
1 = Whole body scan completed
2 = Whole body scan completed, but invalid
3 = Whole body not scanned, pregnancy
4 = Whole body not scanned, weight > 450 lbs
5 = Whole body not scanned, height > 6’5”
6 = Whole body not scanned, other reason
The main reasons for completed, but invalid, whole body scans were an insufficient scan area or partial scan. The “Not scanned, other reason” code includes no time to complete the examination, pregnancy test not completed, and participant refusal, as well as exclusion for reasons other than pregnancy, such as a medical test.
Protocol and Procedure
The whole body scans were acquired on the Hologic Discovery model A densitometers (Hologic, Inc., Bedford, Massachusetts), using software version Apex 3.2. The radiation exposure from DXA whole body scans is extremely low at less than 20 uSv. All scans in the DXX_I file were analyzed with Hologic APEX version 4.0 software with NHANES BCA option.
The DXA examinations were administered by trained and certified radiology technologists. Further details of the DXA examination protocol are documented in the Body Composition Procedures Manual located on the NHANES website.
Quality Assurance & Quality Control
A high level of quality control was maintained throughout the DXA data collection and scan analysis, including a rigorous phantom scanning schedule.
Monitoring of Field Staff and Densitometers
Staff from the National Center for Health Statistics (NCHS) and the NHANES data collection contractor monitored technologist acquisition performance through in-person observations in the field. Retraining sessions were conducted with the technologists annually and as needed to reinforce correct techniques and appropriate protocol. In addition, technologist performance codes were recorded by the NHANES quality control center at the University of California, San Francisco (UCSF), Department of Radiology (2017) and at the University of Hawaii (UH) Cancer Center (2018), during review of participant scans. The codes documented when the technologist deviated from acquisition procedures and where scan quality could have been improved. The performance codes were tracked for each technologist individually and a summary was reported to NCHS on a quarterly basis. Additional feedback on technologist performance was provided by the UH when problems were noted during review of the scans. Constant communication was maintained throughout the year among the UH, the NCHS, and the data collection contractor regarding any issues that arose.
Hologic service engineers performed all routine densitometer maintenance and repairs. Copies of all reports completed by the manufacturer’s service engineers were sent to the quality control center when the scanners were serviced or repaired so any changes in measurement, as a result of the work, was assessed.
Scan Analysis
Each participant scan and phantom scan was reviewed and analyzed by the UH using standard radiologic techniques and study-specific protocols developed for NHANES. The Hologic software, APEX v4.0 (Hologic) was used to analyze whole body scans acquired in 2017-18. Expert review was conducted by the UH on 100% of analyzed participant scans to verify the accuracy and consistency of the results.
Invalidity Codes
Invalidity codes were applied by the UH to indicate the reasons that regions of the body could not be analyzed accurately. The invalidity codes are provided in the data file (see the Data Processing and Editing section for a more detailed description of the invalidity codes).
Quality Control Scans
The quality control phantoms were scanned according to a predetermined schedule. The Hologic Anthropomorphic Spine Phantom that traveled with each MEC was scanned daily, as required by the manufacturer, to ensure accurate calibration of the densitometer. Other MEC-specific phantoms, such as the Hologic Whole Body Slim-line Phantom and Hologic Tissue Step Phantom, were scanned 1 to 3 times weekly. A Hologic Spine (HSP-Q96) Phantom, Hologic Block, and Hologic whole body Phantoms circulated among the MECs and were scanned at the start of operations at each survey site.
Air scans, phantom-less scans using the whole body scan mode, were used to describe and monitor the systems’ radiographic uniformity across the entire scan field. Poor uniformity may be caused by poor aperture alignment, incorrect gantry rotation, non-uniform gain in detectors, etc., that result in localized inaccuracies in the attenuation values.
The complete phantom scanning schedule is described in the Body Composition Procedures Manual located on the NHANES website.
In 2017-2018, longitudinal monitoring was conducted through daily spine phantom scans as required by the manufacturer, whole body slim-line phantom scans and air scans in order to correct any scanner-related changes in participant data were conducted 3 times per week. The circulating HSP-Q96, block and whole body phantoms, which were scanned at the start of operations at each site, provided additional data for use in longitudinal monitoring and cross calibration.
The UH used the Cumulative Statistics method (CUSUM) and the MEC-specific phantom data to determine breaks in the calibration of the densitometers over the course of the survey (Lu, 1996). Multiplicative correction factors were used to correct the phantom data back to the baseline calibration. The type, frequency, and magnitude of calibration problems detected in the NHANES data were similar to those in other studies using stationary densitometers that were being monitored by the UH.
After applying the correction factors developed by the UH from the cross-calibration and longitudinal phantom data to the NHANES participant data, the adjusted participant data were compared to unadjusted data. The magnitude of the changes and reduction in standard errors between the adjusted and unadjusted data were found to be small and correction of the participant data not required.
A number of data quality issues were addressed through the quality control program. Direct feedback given to the technologists regarding acquisition problems affecting the quality of the scans and yearly refresher training resulted in improved technologist performance. The rigorous schedule of quality control scans provided continuous monitoring of machine performance. The expert review procedures helped to ensure that scan analysis was accurate and consistent.
Data Processing and Editing
During the editing process, data were reviewed for completeness, consistency, and outliers. Back-end edits of the data were performed when errors were identified. The NHANES BCA option was enabled for whole body scan analysis, this option adds 5% of lean mass to the fat mass. The correction is based on the results of an analysis of QDR- 4500A DXA data from seven research laboratories indicating that the QDR-4500A algorithm underestimated fat mass and overestimated lean mass (Schoeller, 2005).
Invalidity Codes
Invalidity codes were included in the data file to indicate the reasons that regions of the body could not be analyzed accurately. Invalidity codes were applicable to completed scans only (DXAEXSTS= 1 or 2). If a participant was not scanned, all invalidity codes are missing.
The invalidity codes are provided in the data file as follows:
Invalidity codes
DXAHEBV = head bone
DXAHETV = head tissue
DXALABV = left arm bone
DXALATV = left arm tissue
DXALLBV = left leg bone
DXALLTV = left leg tissue
DXARABV = right arm bone
DXARATV = right arm tissue
DXARLBV = right leg bone
DXARLTV = right leg tissue
DXATRBV = trunk bone, includes thoracic and lumbar spine, left and right ribs, and pelvis
DXATRTV = trunk tissue
Values for invalidity codes DXAHEBV and DXAHETV:
0 = Valid data
1 = Removable or non-removable objects
5 = Body parts out of scan region
6 = Positioning problem
7 = Other
Values for invalidity codes DXALABV, DXALATV, DXARABV, DXARATV, DXALLBV, DXALLTV, DXARLBV and DXARLTV:
0 = Valid data
1 = Removable or non-removable objects
4= Arm/leg overlap
5= Body parts out of scan region
6= Positioning problem
7= Other
Values for invalidity codes DXATRBV and DXATRTV:
0 = Valid data
1 = Removable or non-removable objects
3 = Excessive x-ray noise due to morbid obesity
4= Arm/leg overlap
5= Body parts out of scan region
6= Positioning problem
7= Other
If any region was coded as invalid all bone and soft tissue measurements for that region were set to missing.
The percentage of eligible survey participants in 2017-2018 with 100% valid data (all analyzed regions were valid) is shown by age group in Table 1. The percentage of participants with valid data decreases with increasing age. The decrease in valid data with age was due primarily to an increase in the number of participants with implants, such as stents and hip replacements and higher rates of obesity resulting in invalid truncal data from “obesity noise.” The percentage of participants with 100% valid data also decreases with increasing BMI due to weight over 450 pounds and “obesity noise” (Table 2).
Table 1. Percentages of interviewed and examined participants 8-59 years of age with valid DXA data by age group, NHANES 2017-18.
| Age group (years) |
Interviewed and Examined (N)* |
Eligible for DXA (N) † |
Eligible for DXA (%) † |
100% valid DXA data (N) ‡ |
100% valid DXA data (%) ‡ |
| 8-11 |
731 |
731 |
100 |
574 |
79 |
| 12-15 |
573 |
573 |
100 |
413 |
72 |
| 16-19 |
563 |
563 |
100 |
421 |
75 |
| 20-29 |
776 |
750 |
97 |
547 |
73 |
| 30-39 |
813 |
788 |
97 |
543 |
69 |
| 40-49 |
778 |
774 |
99 |
553 |
71 |
| 50-59 |
880 |
880 |
100 |
606 |
69 |
| Total |
5114 |
5059 |
99 |
3657 |
72 |
* The number interviewed and examined is the total number of participants in the data file with a SEQN variable. This number includes pregnant women.
†The total number eligible for DXA does not include pregnant women.
‡ Of those eligible for DXA who successfully completed a scan..
Table 2. Percentages of participants 20 years and older with valid DXA data by body mass index (BMI) * category, NHANES 2017-18.
| BMI group |
Interviewed and Examined (N) |
Eligible for DXA (N) † |
Eligible for DXA (%) † |
100% valid DXA data (N) ‡ |
100% valid DXA data (%) ‡ |
| |
|
|
|
|
|
| < 18 |
42 |
42 |
100 |
26 |
62 |
| 18-24.9 |
839 |
830 |
99 |
644 |
78 |
| 25-29.9 |
959 |
942 |
98 |
699 |
74 |
| 30-34.9 |
675 |
663 |
98 |
472 |
71 |
| 35-39.9 |
353 |
343 |
97 |
231 |
67 |
| > 40 |
344 |
337 |
98 |
167 |
50 |
| Total |
3247 |
3192 |
98 |
2249 |
70 |
*Measured weight in kilograms divided by measured height in meters squared.
†The total number eligible for DXA does not include pregnant women.
‡ Of those eligible for DXA who successfully completed a scan.
Analytic Notes
The NHANES examination sample weights should be used for any analyses using the DXX_G data. Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for further details on the use of sample weights and other analytic issues. Both of these are available on the NHANES website.