Table of Contents

Locator Record

Title: Dual Energy X-ray Absorptiometry - Femur (DXXFEM_E )
Contact Number: 1-866-441-NCHS
Years of Content: 2007 - 2008
First Published: December, 2009
Revised: NA
Access Constraints: None
Use Constraints: None
Geographic Coverage: National
Subject:Personal examination data on proximal femur bone mineral content, bone area, and bone mineral density.
Record Source: NHANES 2007 - 2008
Survey Methodology: NHANES 2007 - 2008 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 2007-2008 dual-energy X-ray absorptiometry femur bone data (DXXFEM_E) 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 bone density due in part to its speed, ease of use, and low radiation exposure (Genant and Heymsfield and Njeh). Beginning in 2005, DXA scans of the proximal femur have been administered in the NHANES mobile examination center (MEC).

The femur scans provide bone measurements for the total femur, femoral neck, trochanter, intertrochanter, and Ward’s triangle. Measurements include:

• Bone mineral content (BMC) (gm)
• Bone area (cm2)
• Bone mineral density (BMD) (gm/cm2)

The NHANES DXA femur bone measurements component provides nationally representative data on 1) femur bone mineral content, bone area, and bone mineral density and 2) estimates of the prevalence of osteoporosis.

 

Eligible Sample

DXA scans were administered to eligible survey participants 8 years of age and older. 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:

• Pregnancy (positive urine pregnancy test and/or self-report at the time of the DXA examination). Females between the ages of 12–59 years and menstruating 8–11 year olds were not permitted to take the DXA examination without a negative MEC pregnancy test result. In addition, females aged 12–59 years were excluded from the examination if they said they were pregnant at the time of the exam, even if the pregnancy test was negative.
• Self-reported history of radiographic contrast material (barium) use in past 7 days.
• Self-reported nuclear medicine studies in the past 3 days.
• Self-reported weight over 300 pounds (DXA table limitation).

The left hip was routinely scanned unless the participant self-reported a fractured left hip, a left hip replacement, or a pin in the left hip. The right hip was scanned in this situation. Participants were excluded from the femur scan if they had fractured both hips, had replacements of both hips, or had pins in both hips.

The variable DXAFMRST indicates the examination status for the femur scan; the codes for DXAFMRST are as follows:

DXAFMRST – femur scan examination status variable
1 = Femur scan completed
2 = Femur scan completed, but all data are invalid
3 = Femur not scanned, pregnancy
4 = Femur not scanned, weight > 300 lbs
5 = Femur not scanned, other reason

The main reason for completed, but invalid, femur scans was panniculus, an “apron” or redundant layer of fat tissue at the lowest portion of the abdominal wall. 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 femur scans were performed with a Hologic QDR 4500A fan-beam densitometer (Hologic, Inc., Bedford, Massachusetts). Hologic software Discovery v12.4 was used to acquire all scans. The radiation exposure from DXA for both the femur or spine scan is extremely low at less than 20 uSv.

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 during review of participant scans. The codes documented when the technologist had deviated from acquisition procedures and where 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. Additional feedback on technologist performance was provided by the UCSF when problems were noted during review of the scans. 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. No mechanical repairs were required for any of the three densitometers in 2007-08.

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 Discovery software, version 12.4, was used to analyze all scans acquired in 2007 and 2008. The Discovery analysis software incorporates the Auto Whole Body application, which was developed to improve bone detection in children participating in the NHANES and other studies of children (Kelly and Fan).
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 femur and spine regions of interest (ROI) could not be analyzed accurately. The invalidity codes are provided in the data file (see 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. The Hologic Femur Phantom was scanned once each week. A single Hologic Spine (HSP-Q96) and Hologic Block Phantoms circulated among the MECs and were scanned at the start of operations at each survey site.

The complete phantom scanning schedule is described in the Body Composition Procedures Manual located on the NHANES website.

In 2007-2008, longitudinal monitoring was conducted through the daily spine phantom scans as required by the manufacturer and the once weekly femur phantom scans in order to correct any scanner-related changes in participant data. The circulating HSP-Q96 and block phantoms, which were scanned at the start of operations at each site, provided additional data for use in longitudinal monitoring and cross calibration.

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). There were no break points for any of the three MECs in either the spine or femur phantom data during 2007-2008, and therefore no corrections to the participant data were needed. Comparison of data for the phantoms that circulated among the MECs indicated no significant differences so that data from the three MECs could be combined.

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.

 

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.

Invalidity Codes
Invalidity codes were included in the data file to indicate the reasons femur regions of interest (ROI) could not be analyzed accurately. Invalidity codes were applicable to completed scans only (DXAFMRST=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 code
DXXFMBCC = total femur BMD

Values for DXXFMBCC
0 = Valid data
1 = Jewelry or other objects not removed
2 = Non-removable objects (includes prostheses, implants)
3 = Excessive x-ray “noise” due to obesity
4 = Body parts out of scan region
5 = Positioning problem
6 = Other (includes panniculus, participant motion, unknown artifacts)

The total femur BMD was coded as invalid (DXXFMBCC = 1-7) and all femur data set to missing if data for any femur subregion was invalid. In specific, validity codes were provided for the femoral neck and trochanter by the quality control center because these are the femur regions that have been used the most in clinical situations. However, if either of these regions was invalid, the intertrochanter and Ward's triangle were also considered invalid. The 2007 Official Positions of the International Society for Clinical Densitometry (ISCD) state that only the femoral neck or total femur regions of interest should be used for diagnosis of osteoporosis at the femur (2007 ISCD Official Positions).

 

Analytic Notes

The NHANES examination sample weights should be used for any analyses using the DXXFEM_E data. Please refer to the Analytic Guidelines on the NHANES website for further details on the use of sample weights and other analytic issues

 

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 - 150 YEARS

DXAFMRST - Femur scan status

Variable Name:
DXAFMRST
SAS Label:
Femur scan status
English Text:
Femur scan status
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1 Femur scan completed, all data are valid 6517 6517
2 Femur scan completed, all data are invalid 238 6755
3 Femur not scanned, pregnant 57 6812
4 Femur not scanned, weight > 300 lbs 98 6910
5 Femur not scanned, other reason 824 7734
. Missing 0 7734

DXXFMBCC - Total femur BMD invalidity code

Variable Name:
DXXFMBCC
SAS Label:
Total femur BMD invalidity code
English Text:
Total femur BMD invalidity code
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 Valid data 6518 6518
1 Jewelry or other objects not removed 1 6519
2 Non-removable objects (includes prostheses, implants) 2 6521
3 Excessive x-ray "noise" due to obesity 0 6521
4 Body parts out of scan region 6 6527
5 Positioning problem 0 6527
6 Other (includes panniculus, participant motion, unknown artifacts) 228 6755
. Missing 979 7734

DXXOFBMD - Total femur BMD

Variable Name:
DXXOFBMD
SAS Label:
Total femur BMD
English Text:
Total femur bone mineral density (gm/cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.419 to 1.579 Range of Values 6517 6517
. Missing 1217 7734

DXXOFBMC - Total femur BMC

Variable Name:
DXXOFBMC
SAS Label:
Total femur BMC
English Text:
Total femur bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
5.88 to 74.27 Range of Values 6517 6517
. Missing 1217 7734

DXXOFA - Total femur area

Variable Name:
DXXOFA
SAS Label:
Total femur area
English Text:
Total femur area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
12.2 to 59.6 Range of Values 6517 6517
. Missing 1217 7734

DXXNKBMD - Femoral neck BMD

Variable Name:
DXXNKBMD
SAS Label:
Femoral neck BMD
English Text:
Femoral neck bone mineral density (gm/cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.34 to 1.487 Range of Values 6517 6517
. Missing 1217 7734

DXXNKBMC - Femoral neck BMC

Variable Name:
DXXNKBMC
SAS Label:
Femoral neck BMC
English Text:
Femoral neck bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1.46 to 8.59 Range of Values 6517 6517
. Missing 1217 7734

DXXNKA - Femoral neck area

Variable Name:
DXXNKA
SAS Label:
Femoral neck area
English Text:
Femoral neck area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
2.85 to 7.38 Range of Values 6517 6517
. Missing 1217 7734

DXXTRBMD - Trochanter BMD

Variable Name:
DXXTRBMD
SAS Label:
Trochanter BMD
English Text:
Trochanter bone mineral density (gm/cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.303 to 1.25 Range of Values 6517 6517
. Missing 1217 7734

DXXTRBMC - Trochanter BMC

Variable Name:
DXXTRBMC
SAS Label:
Trochanter BMC
English Text:
Trochanter bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.86 to 20.57 Range of Values 6517 6517
. Missing 1217 7734

DXXTRA - Trochanter area

Variable Name:
DXXTRA
SAS Label:
Trochanter area
English Text:
Trochanter area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
2.15 to 21.47 Range of Values 6517 6517
. Missing 1217 7734

DXXINBMD - Intertrochanter BMD

Variable Name:
DXXINBMD
SAS Label:
Intertrochanter BMD
English Text:
Intertrochanter bone mineral density (gm/cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.487 to 1.875 Range of Values 6517 6517
. Missing 1217 7734

DXXINBMC - Intertrochanter BMC

Variable Name:
DXXINBMC
SAS Label:
Intertrochanter BMC
English Text:
Intertrochanter bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
3.45 to 51.46 Range of Values 6517 6517
. Missing 1217 7734

DXXINA - Intertrochanter area

Variable Name:
DXXINA
SAS Label:
Intertrochanter area
English Text:
Intertrochanter area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
6.76 to 35.69 Range of Values 6517 6517
. Missing 1217 7734

DXXWDBMD - Ward's triangle BMD

Variable Name:
DXXWDBMD
SAS Label:
Ward's triangle BMD
English Text:
Ward's triangle bone mineral density (gm/cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.11 to 1.786 Range of Values 6517 6517
. Missing 1217 7734

DXXWDBMC - Ward's triangle BMC

Variable Name:
DXXWDBMC
SAS Label:
Ward's triangle BMC
English Text:
Ward's triangle bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.13 to 1.85 Range of Values 6517 6517
. Missing 1217 7734

DXXWDA - Ward's triangle area

Variable Name:
DXXWDA
SAS Label:
Ward's triangle area
English Text:
Ward's triangle area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1.04 to 1.31 Range of Values 6517 6517
. Missing 1217 7734

DXAFMRK - Calculated K for femur

Variable Name:
DXAFMRK
SAS Label:
Calculated K for femur
English Text:
Calculated K for femur
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1.1139 to 1.1544 Range of Values 6752 6752
. Missing 982 7734

DXAFMRD0 - Calculated DO for femur

Variable Name:
DXAFMRD0
SAS Label:
Calculated DO for femur
English Text:
Calculated DO for femur
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
40.504 to 63.0186 Range of Values 6752 6752
. Missing 982 7734