Users of the 2015-2016 dual-energy X-ray absorptiometry android/gynoid data (DXXAG_I) are encouraged to read the documentation before accessing the data file. NOTE: missing and invalid android and gynoid data were not multiply imputed.
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). In 2015-2016, whole body DXA scans were administered in the NHANES mobile examination center (MEC).
Android and gynoid (A/G) regions were defined by the Hologic APEX software used in the scan analysis. The android area was defined as the lower trunk area bounded by two lines: the pelvic horizontal cut line on its lower side, and a line automatically placed above the pelvic line. The upper gynoid line was placed 1.5 times of the height of android region below the pelvic line and the lower gynoid line was placed such that the distance between the two gynoid lines was twice the height of the android region. All these lines were automatically placed by Hologic software (Shepherd, 2012).
Android obesity is often referred to as the “apple” shape since the increased fat is in the trunk. Gynoid obesity is referred to as the “pear” shape with increased fat in the hip and thigh areas. Fat deposition in the android region is associated with increased risk of cardiovascular disease, hypertension, hyperlipidemia, insulin resistance, and type 2 diabetes (Kissebah, 1994; Folsom, 2000), while gynoid fat deposition is associated with decreased risk of metabolic and cardiovascular diseases (Folsom, 2000; Ashwell, 1994).
Visceral Adipose Tissue (VAT) and subcutaneous Adipose Tissue (SAT) were defined by the Hologic APEX software used in the scan analysis. Visceral adipose tissue area, mass and volume of fat inside abdominal cavity were measured at the approximate interspace location of L4 and L5 vertebra. Subcutaneous adipose tissue area, mass and volume of fat outside abdominal cavity were measured at the approximate interspace location of L4 and L5 vertebra.
The NHANES whole body scans provide nationally representative data on abdominal soft tissue composition and fat distribution overall and for age, gender, and racial/ethnic groups; and data to study the association between abdominal fat distribution and other health conditions and risk factors, such as cardiovascular disease, diabetes, hypertension, and activity and dietary patterns.
Analysis of the whole body scans provides abdominal soft tissue measurements for the android and gynoid areas of the trunk, VAT and SAT. These measurements include:
DXA whole body scans were administered to eligible survey participants aged 8-59. Pregnant females were ineligible for the DXA examination. Participants who were excluded from the DXA examination for reasons other than pregnancy were considered to be eligible nonrespondents. Reasons for exclusion from the DXA examination were as follows:
The variable DXXAGST indicates the examination status for the android/gynoid region. The codes for DXXAGST are as follows:
1= Whole body scan completed, android/gynoid region is valid
2= Whole body scan completed, but android or gynoid region is 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 implants, excessive X-ray “noise” due to obesity, and jewelry not removed. The “Not scanned, other reason” code includes no time to complete the examination, pregnancy test not completed, and participant refusal.
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 DXXAG_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.
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 as part of the participants’ scan review. The codes documented when the technologist had deviated from acquisition procedures and 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. Constant communication was maintained throughout the year among the UCSF, 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 UCSF when the scanners were serviced or repaired so any changes in measurement, as a result of the work, could be assessed.
Scan Analysis
Each participant scan and phantom scan was reviewed and analyzed by the UCSF 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 2015-16. Expert review was conducted by the UCSF on 100% of analyzed participant scans to verify the accuracy and consistency of the results.
Invalidity Codes
Invalidity codes were applied by the UCSF to indicate the reasons regions of the body could not be analyzed accurately. The invalidity codes for the trunk region are provided in the data file (see Analytic Notes for a description of the invalidity codes).
Quality Control Scans
The quality control phantoms were scanned according to a predetermined schedule. The Hologic Anthropomorphic Spine Phantom associated 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. Another set of phantoms, the Hologic Spine (HSP-Q96), Hologic Block, and Hologic Whole Body Phantoms, circulated among the MECs and was scanned at the start of operations at each survey site.
Air scans, which are 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 could 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 2015-2016, longitudinal monitoring was conducted through the daily spine phantom scans as required by the manufacturer, 3 times weekly whole body slim-line phantom scans, and weekly air scans in order to correct any scanner-related changes in participant data. 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 cross-comparability of the data from each MEC was critical so the data could be pooled for analysis.
The UCSF 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 UCSF.
After applying the correction factors developed by UCSF 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 was 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.
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 regions could not be analyzed accurately. Invalidity codes were applicable to completed scans only (DXXAGST=1 or 2). If a participant was not scanned, all invalidity codes are missing. Objects found in the upper portion of the trunk such as pacemakers or breast implants did not invalidate the scan validity for the android gynoid data.
Values for DXXVATV(VAT and SAT), DXXANTV (Android region) and DXXGYTV (Gynoid
region) invalidity codes
0 = Valid
1 = Invalid
The percentage of eligible survey participants in 2015-2016 with valid DXA data in android/gynoid region 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 valid DXA data in android/gynoid region also decreases with increasing BMI due to weight over 450 pounds and “obesity noise” (Table 2).
Table 1. Percentages of interviewed and examined participants aged 8-59 years with valid DXA data in android/gynoid region by age group, NHANES 2015-2016
| Age group (years) | Interviewed and examined * | Eligible for DXA † | %100 valid DXA data‡ | ||
|---|---|---|---|---|---|
| N | N | % | N | % | |
| 8-11 | 845 | 845 | 100 | 774 | 92 |
| 12-15 | 681 | 681 | 100 | 609 | 89 |
| 16-19 | 589 | 589 | 100 | 540 | 92 |
| 20-29 | 931 | 892 | 96 | 771 | 86 |
| 30-39 | 933 | 906 | 97 | 793 | 88 |
| 40-49 | 913 | 909 | 99 | 795 | 87 |
| 50-59 | 888 | 888 | 100 | 803 | 90 |
| Total | 5,780 | 5,710 | 99 | 5,085 | 89 |
* The number interviewed and examined is the total number of participants in the data file with a SEQN variable. This number includes pregnant females.
† 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 aged 20-59 years with valid DXA data in android/gynoid region by body mass index (BMI) * category, NHANES 2015-16
| BMI group | Interviewed and Examined (N) | Eligible for DXA (N) † | Eligible for DXA (%) † | 100% valid DXA data (N) ‡ | 100% valid DXA data (%) ‡ |
|---|---|---|---|---|---|
| <18 | 41 | 41 | 100 | 36 | 88 |
| 18-24.9 | 1,029 | 1,010 | 98 | 873 | 86 |
| 25-29.9 | 1,095 | 1,074 | 98 | 962 | 90 |
| 30-34.9 | 759 | 744 | 98 | 675 | 91 |
| 35-39.9 | 388 | 378 | 97 | 346 | 92 |
| ≥ 40 | 322 | 318 | 99 | 264 | 83 |
| Total | 3,665 | 3,595 | 98 | 3,162 | 88 |
*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.
The NHANES examination sample weight should be used for any analyses using the DXXAG_I data. Please refer to the NHANES Analytic Guidelines and the online NHANES Tutorial for further details on the use of sample weights and other analytic issues.