Table of Contents

Locator Record

Title: Dual Energy X-Ray Absorptiometry - Android/Gynoid Measurements (DXXAG_D )
Contact Number: 1-866-441-NCHS
Years of Content: 2005 - 2006
First Published: February, 2013
Revised: NA
Access Constraints: None
Use Constraints: None
Geographic Coverage: National
Subject:Dual-energy x-ray absorptiometry, DXA, Android, Gynoid, Body Composition
Record Source: NHANES 2005 - 2006
Survey Methodology: NHANES 2005 - 2006 is a stratified multistage probability sample of the civilian non-institutionalized population of the U.S.
Medium: NHANES Web site; SAS transport files

Component Description

Users of the 2005-2006 dual-energy X-ray absorptiometry android/gynoid data (DXXAG_D) 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 2005-2006, 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 is roughly the area around the waist between the mid-point of the lumbar spine and the top of the pelvis; the gynoid area lies roughly between the head of the femur and mid-thigh. Note: the android and gynoid regions described here approximate the regions defined by the APEX software.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).

The NHANES whole body scans provide nationally representative data on: 1) abdominal soft tissue composition and fat distribution overall and for age, gender, and racial/ethnic groups; and data 2) 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 soft tissue measurements for the android and gynoid areas of the trunk and android/gynoid ratios. These measurements include:

 

Eligible Sample

Whole body scans were administered to eligible survey participants aged 8-69 years. 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 DXATRST indicates the examination status for the trunk (A/G) region. The codes for DXATRST are as follows:
1= Scan completed, trunk region is valid
2= Scan completed, but trunk region is invalid
3= Not scanned, pregnancy
4= Not scanned, weight> 300 lbs
5= 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 percentage of eligible survey participants in 2005-2006 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 300 pounds and “obesity noise” (Table 2).

 

Protocol and Procedure

The whole body DXA scans were acquired with a Hologic QDR-4500A fan-beam densitometer (Hologic, Inc., Bedford, Massachusetts). Hologic software version 8.26:a3* was used to administer all scans. The DXA technique acquires two low-dose x-ray images at different average energies. The ratio of the attenuation of these two average energies, called an R-factor, is used to distinguish both bone from soft tissue, and the percent fat in soft tissue when bone is not present. The radiation exposure from DXA whole body scans is extremely low at less than 10 uSv.

The DXA examinations were administered by certified radiology technologists. Further details of the DXA examination protocol are documented in the Body Composition Procedures Manual located on the NHANES website (http://www.cdc.gov/nchs/nhanes.htm).

 

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 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 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. While some minor mechanical repairs were made during 2005-2006 survey operations, replacement or realignment of the detectors, apertures, or other major hardware was not required for any of the three densitometers.

Scan Analysis

Each participant and phantom scan was reviewed and analyzed by the UCSF using standard radiologic techniques and study-specific protocols developed for the NHANES. Hologic APEX software, version 3.0 (Fan, 2008), was used to reanalyze the 2005-2006 participant scans to provide data for the android and gynoid areas. The APEX software incorporates a 5% adjustment in lean mass and fat mass to account for the overestimate of lean mass by the Hologic QDR-4500A densitometer (Schoeller, 2005). 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: (http://www.cdc.gov/nchs/data/nhanes/nhanes_05_06/BC.pdf).

In 2005-2006, 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 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 assured that scan analysis was accurate and consistent. The air scan quality assurance tool used to evaluate whole body performance was first used in the NHANES and was subsequently adopted by Hologic as a mandatory scan mode for all whole body scanners.

 

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.

 

Analytic Notes

The NHANES examination sample weight should be used for any analyses using the DXXAG_D 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 (http://www.cdc.gov/nchs/nhanes.htm).

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 (DXATRST=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 DXATRTV (Trunk tissue validity code)

0 = Valid data
1 = Objects not removed
2 = Non-removable objects such as implants
3 = Excessive x-ray "noise" due to obesity, i.e., the DXA beam could not penetrate the layers of abdominal fat to provide an analyzable scan image (applied to the trunk region only)
4 = Arm/hip overlap
5 = Movement
7 = Other

 Table 1. Percentages of interviewed and examined participants aged 8-69 years with valid DXA data by   age group, NHANES 2005-2006

Age group (years) Interviewed and examined(N) * Eligible for DXA (N) † Eligible for DXA (%) † 100% valid DXA data (N) ‡ 100% valid DXA data (%) ‡
8-11 740 740 100 697 94.2
12-15 1,089 1,085 99.6 962 88.6
16-19 1,118 1,076 96.2 934 86.8
20-29 1,021 792 77.5 637 80.4
30-39 818 725 88.6 593 81.8
40-49 815 812 99.6 684 84.2
50-59 631 631 100 537 85.1
60-69 661 661 100 546 82.6
Total 6,893 6,522 94.6 5,590 85.7

*The number interviewed and examined is the total number of participants in the data file with a SEQN variable. This number includes pregnant females (n= 371).
† Does not include pregnant females.
‡ 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 2005-2006

BMI Category Eligible for DXA(N)† %100 valid DXA data(N)‡ %100 valid DXA data(%)‡
< 18.5 60 51 85
18.5-24.9 997 865 86.7
25-29.9 1,194 1,034 86.6
30-34.9 739 642 86.8
35-39.9 346 278 80.3
= 40 238 116 48.7
Total 3,574 2,986 83.5

*Measured weight in kilograms divided by measured height in meters squared.
† Does not include pregnant females.
‡ Of those eligible for DXA who successfully completed a scan.

References

Codebook and Frequencies

SEQN - Respondent sequence number

Variable Name:
SEQN
SAS Label:
Respondent sequence number
English Text:
Respondent sequence number.
Target:
Both males and females 8 YEARS - 69 YEARS

DXATRST - Trunk (A/G region) scan status

Variable Name:
DXATRST
SAS Label:
Trunk (A/G region) scan status
English Text:
Trunk (A/G region) scan status
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1 Trunk scan completed, trunk scan is valid 5590 5590
2 Trunk scan completed, but trunk scan is invalid 129 5719
3 Not scanned, pregnancy 371 6090
4 Not scanned, weight > 300 lbs 85 6175
5 Not scanned, other reason 718 6893
. Missing 0 6893

DXATRTV - Trunk tissue invalidity code

Variable Name:
DXATRTV
SAS Label:
Trunk tissue invalidity code
English Text:
Trunk tissue invalidity code
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 Valid data 5590 5590
1 Objects not removed 9 5599
2 Non-removable objects such as implants 57 5656
3 Excessive x-ray noise due to morbid obesity 45 5701
4 Hip arm overlap 3 5704
5 Movement 2 5706
7 Other 12 5718
. Missing 1175 6893

DXXANFM - Android fat mass

Variable Name:
DXXANFM
SAS Label:
Android fat mass
English Text:
Android fat mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
102.9 to 7106.1 Range of Values 5589 5589
. Missing 1304 6893

DXXANLM - Android lean mass

Variable Name:
DXXANLM
SAS Label:
Android lean mass
English Text:
Android lean mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
784.5 to 8182.4 Range of Values 5589 5589
. Missing 1304 6893

DXXANTOM - Android total mass

Variable Name:
DXXANTOM
SAS Label:
Android total mass
English Text:
Android total mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
978.4 to 14971.8 Range of Values 5589 5589
. Missing 1304 6893

DXXGYFM - Gynoid fat mass

Variable Name:
DXXGYFM
SAS Label:
Gynoid fat mass
English Text:
Gynoid fat mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
453.9 to 12528 Range of Values 5589 5589
. Missing 1304 6893

DXXGYLM - Gynoid lean mass

Variable Name:
DXXGYLM
SAS Label:
Gynoid lean mass
English Text:
Gynoid lean mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1564.6 to 15694.3 Range of Values 5589 5589
. Missing 1304 6893

DXXGYTOM - Gynoid total mass

Variable Name:
DXXGYTOM
SAS Label:
Gynoid total mass
English Text:
Gynoid total mass
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
2486.6 to 23971.2 Range of Values 5589 5589
. Missing 1304 6893

DXXAGRAT - Android to Gynoid ratio

Variable Name:
DXXAGRAT
SAS Label:
Android to Gynoid ratio
English Text:
Android to gynoid ratio
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.4 to 1.9 Range of Values 5589 5589
. Missing 1304 6893

DXXAPFAT - Android percent fat

Variable Name:
DXXAPFAT
SAS Label:
Android percent fat
English Text:
Android percent fat
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
6.6 to 58.9 Range of Values 5589 5589
. Missing 1304 6893

DXXGPFAT - Gynoid percent fat

Variable Name:
DXXGPFAT
SAS Label:
Gynoid percent fat
English Text:
Gynoid percent fat
Target:
Both males and females 8 YEARS - 69 YEARS
Code or Value Value Description Count Cumulative Skip to Item
7.7 to 56.6 Range of Values 5589 5589
. Missing 1304 6893