National Health and Nutrition Examination Survey

2005 - 2006 Data Documentation, Codebook, and Frequencies

Dual Energy X-ray Absorptiometry - Spine (DXXSPN_D)

Data File: DXXSPN_D.xpt

First Published: October, 2010

Last Revised: NA

Locator Record

Title: Dual Energy X-ray Absorptiometry - Spine (DXXSPN_D )
Contact Number: 1-866-441-NCHS
Years of Content: 2005 - 2006
First Published: October, 2010
Revised: NA
Access Constraints: None
Use Constraints: None
Geographic Coverage: National
Subject:Personal examination data on spine bone mineral content, bone area, and bone mineral density.
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 spine bone data (DXXSPN_D) are encouraged to read the documentation before accessing the data file.

Note: all data in the DXXSPN_D file were analyzed with Hologic APEX v3.0 software.

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, Heymsfield, 1989, Njeh, 1999). Beginning in 2005, DXA scans of the lumbar spine have been administered in the NHANES mobile examination center (MEC).

The spine scans provide bone measurements for the total spine and vertebrae L1 – L4. Measurements include:

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

 

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).
• 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 variable DXASPNST indicates the examination status for the spine scan. The codes for DXASPNST are as follows:

DXASPNST – spine scan examination status variable
1 = Spine scan completed, all vertebrae are valid 
2 = Spine scan completed, but one or more vertebrae are invalid
3 = Spine not scanned, pregnancy
4 = Spine not scanned, weight > 300 lbs
5 = Spine not scanned, other reason

The main reasons for completed, but invalid, spine scans were an insufficient scan area or partial scan, degenerative disease/severe scoliosis, and sclerotic spine/spinal fusion/laminectomy. 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 spine scans were acquired with Hologic QDR-4500A fan-beam densitometers (Hologic, Inc., Bedford, Massachusetts). Hologic software version 8.26:a3* was used to acquire all scans through mid 2005. In 2005, the acquisition software was updated to Hologic Discovery v12.4. The radiation exposure from DXA for the 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 (/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 was 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. Ongoing 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 2005-06.

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 the NHANES. The most recently released Hologic software, APEX v3.0, which has been shown to have increased precision (Fan, 2008), was used to analyze all spine scans acquired in 2005-06. 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 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. 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 2005-2006, longitudinal monitoring was conducted through the daily spine phantom scans as required by the manufacturer 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). There were no break points for any of the three MECs in the spine phantom data during 2005-2006, and therefore no corrections to the participant spine data were needed. Comparison of data for the phantoms that circulated among the MECs indicated no statistically 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 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.

Invalidity Codes
Invalidity codes were included in the data file to indicate the reasons and spine regions of interest (ROI) could not be analyzed accurately. Invalidity codes were applicable to completed scans only (DXASPNST = 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
DXXOSBCC = total spine BMD
DXXL1BCC = L1 vertebra BMD
DXXL2BCC = L2 vertebra BMD
DXXL3BCC = L3 vertebra BMD
DXXL4BCC = L4 vertebra BMD

Values for DXXL1BCC, DXXL2BCC, DXXL3BCC, DXXL4BCC

0 = Valid data
1 = Objects not removed
2 = Non-removable objects such as implants
3 = Excessive x-ray “noise” due to obesity
4 = Insufficient scan area
5 = Movement
6 = Other (degenerative diseases, spinal fusion, fractures) 

Values for DXXOSBCC

0 = valid data
1 = invalid data

If one or more spine vertebrae were coded as invalid, total spine BMD was coded as invalid (DXXOSBCC = 1) and all spine data were set to missing.

 

Analytic Notes

The NHANES examination sample weights should be used for any analyses using the DXXBONE_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 (/nchs/nhanes.htm).

All spine scans acquired in 2005-2006 and 2007-2008 were first analyzed with Hologic Discovery v12.4, then reanalyzed with Hologic APEX v3.0 software. Comparisons of total spine BMD from each software version were made for the total sample, and by age group, race/ethnicity, and gender.

Mean differences in BMD for the years 2005-2006 and 2007-2008 combined ranged from 0.00045 for the Other race/ethnic category to 0.00518 for participants 60 years of age and older. Simple kappa statistics for BMD ranged from 0.9622 for participants 50-59 years of age to 0.9899 for participants 8-49 years of age. There were no differences between the two software versions in the percentages of participants in the total sample with BMD values +/- 3 standard deviations (SD). Differences were observed by subgroup, but percentages were not consistent across the two software versions, meaning for some subgroups the Discovery version produced higher percentages of +/- 3 SD and for other subgroups the APEX software produced higher percentages.

All data in the DXXSPN_D file were analyzed with Hologic APEX v3.0 software.

 

References

• Baran DT, Faulkner KG, Genant HK, Miller PD, Pacifici R. Diagnosis and management of osteoporosis: guidelines for the utilization of bone densitometry.Calcif Tissue Int 1997:62:433-440.
• Fan B, Lewiecki EM, Sherman M, Lu Y, Miller PD, Genant HK, Shepherd JA. Improved precision with Hologic Apex software. Osteoporos Int. 2008;19:1597-602.
• Genant HK, Engelke K, Fuerst T, Güer C-C, Grampp S, Harris ST, Jergas M, Lang T, Lu Y, Majumdar S, Mathur A, Takada M. Noninvasive assessment of bone mineral and structure: state of the art. J Bone Miner Res 1996;11:707-30.
• Heymsfield SB, Wang J, Heshka S, Kehayias JJ, Pierson RN Jr. Dual-photon absorptiometry: comparison of bone mineral and soft tissue measurements in vivo with established methods. Am J Clin Nutr 1989;49:1283-9.
• Hololgic Product Support: Discovery. http://www.hologic.com/en/product-support/bone-densitometry/discovery/ [Accessed on: 06/24/2010]
• Lu Y, Mathur AK, Blunt BA, et al. Dual X-ray absorptiometry quality control: comparison of visual examination and process-control charts. JBMR 1996;11:626-37.
• Njeh CF, Fuerst T, Hans D, Blake GM, Genant HK. Radiation exposure in bone mineral density assessment. Appl Radiat Isot 1999;50:215-36.

Codebook and Frequencies