Table of Contents

Locator Record

Title: Dual Energy X-ray Absorptiometry - Femur (DXXFEM_F )
Contact Number: 1-866-441-NCHS
Years of Content: 2009 - 2010
First Published: November, 2012
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, Dual Energy X-ray Absorptiometry, DXA.
Record Source: NHANES 2009 - 2010
Survey Methodology: NHANES 2009 - 2010 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 2009-2010 dual-energy X-ray absorptiometry femur bone data (DXXFEM_F) 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 (Baran, 1997; Genant, 1996; Heymfield, 1989; Njeh, 1999). DXA scans of the proximal femur were administered in the NHANES mobile examination center (MEC) in 2005 through 2010.

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

 

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:

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 2009-2010 femur scans were acquired with Hologic QDR-4500A fan-beam densitometers (Hologic, Inc., Bedford, Massachusetts) and software version Discovery v12.4. The radiation exposure from DXA for the femur 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. (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 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 2009-10.

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. The most recently released Hologic software, APEX v3.0 (Hologic), which has been shown to have increased precision (Fan, 2008), was used to analyze all femur scans acquired in 2009-10. The Apex algorithms are able to better detect bone edge and provide sharper bone mapping than previous software versions. 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 Hologic Spine (HSP-Q96) and a Hologic Block Phantom 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 2009-2010, 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, 1996). No shifting or drifting of the MEC-specific spine phantom values was found for any of the three MECs during 2009-2010. Therefore no corrections to the participant data were made.

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 assured that scan analysis was accurate and consistent.

 

Data Processing and Editing

During the editing process, data were reviewed for completeness, consistency, and outliers. Edits of the data were performed when errors were identified.

Invalidity Codes
Invalidity codes are included in the data file to indicate the reasons femur regions of interest (ROI) could not be analyzed accurately. Invalidity codes are 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:

Values for invalidity code DXXFMBCC (Total femur BMD)
0 = Valid data
1 = Jewelry or other objects not removed
2 = Non-removable objects such as 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 = 1-6) and all femur data set to missing if data for any femur subregion was invalid. Specifically, 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 DXXFEMUR_F 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 6963 6963
2 Femur scan completed, but all data are invalid 224 7187
3 Femur not scanned, pregnant 81 7268
4 Femur not scanned, weight > 300 lbs 141 7409
5 Femur not scanned, other reason 776 8185
. Missing 0 8185

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 6963 6963
1 Jewelry or other objects not removed 0 6963
2 Non-removable objects such as implants 3 6966
3 Excessive x-ray noise due to obesity 0 6966
4 Body parts out of scan region 0 6966
5 Positioning problem 4 6970
6 Other (includes panniculus, participant motion, unknown artifact) 217 7187
. Missing 998 8185

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
0.326 to 1.719 Range of Values 6963 6963
. Missing 1222 8185

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
4.9 to 71.28 Range of Values 6963 6963
. Missing 1222 8185

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
10.59 to 57.11 Range of Values 6963 6963
. Missing 1222 8185

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
0.334 to 1.626 Range of Values 6963 6963
. Missing 1222 8185

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
0.89 to 9.17 Range of Values 6963 6963
. Missing 1222 8185

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
2.27 to 8.3 Range of Values 6963 6963
. Missing 1222 8185

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
0.214 to 1.382 Range of Values 6963 6963
. Missing 1222 8185

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
0.59 to 21.88 Range of Values 6963 6963
. Missing 1222 8185

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
1.48 to 20.52 Range of Values 6963 6963
. Missing 1222 8185

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
0.394 to 1.968 Range of Values 6963 6963
. Missing 1222 8185

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
3.42 to 46.59 Range of Values 6963 6963
. Missing 1222 8185

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
6.63 to 34.93 Range of Values 6963 6963
. Missing 1222 8185

DXXWDBMD - Wards triangle BMD

Variable Name:
DXXWDBMD
SAS Label:
Wards triangle BMD
English Text:
Wards 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
0.081 to 1.712 Range of Values 6963 6963
. Missing 1222 8185

DXXWDBMC - Wards triangle BMC

Variable Name:
DXXWDBMC
SAS Label:
Wards triangle BMC
English Text:
Wards triangle bone mineral content (gm)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.09 to 2.15 Range of Values 6963 6963
. Missing 1222 8185

DXXWDA - Wards triangle area

Variable Name:
DXXWDA
SAS Label:
Wards triangle area
English Text:
Wards triangle area (cm^2)
Target:
Both males and females 8 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.84 to 1.35 Range of Values 6963 6963
. Missing 1222 8185

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
1.1175 to 1.1557 Range of Values 6963 6963
. Missing 1222 8185

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
40.2213 to 84.4334 Range of Values 6963 6963
. Missing 1222 8185