• Component Description
• Eligible Sample
• Protocol and Procedure
• Quality Assurance & Quality Control
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • DXDAACST - Abdominal Aortic Calcification Status
  • DXXAACTV - AAC total invalidity code
  • DXXAL1CC - AAC L1 invalidity code
  • DXXAL2CC - AAC L2 invalidity code
  • DXXAL3CC - AAC L3 invalidity code
  • DXXAL4CC - AAC L4 invalidity code
  • DXXAL1 - AAC L1 Posterior score
  • DXXAL1AN - AAC L1 Anterior score
  • DXXAL2 - AAC L2 Posterior score
  • DXXAL2AN - AAC L2 Anterior score
  • DXXAL3 - AAC L3 Posterior score
  • DXXAL3AN - AAC L3 Anterior score
  • DXXAL4 - AAC L4 Posterior score
  • DXXAL4AN - AAC L4 Anterior score
  • DXXAL4MD - AAC L4 mid score
  • DXXAC8AN - AAC Anterior 8 score
  • DXXAC8PO - AAC Posterior 8 score
  • DXXAAC8 - AAC Total 8 Score
  • DXXAAC24 - AAC Total 24 Score

National Health and Nutrition Examination Survey

2013-2014 Data Documentation, Codebook, and Frequencies

Dual-Energy X-ray Absorptiometry - Abdominal Aortic Calcification (DXXAAC_H)

Data File: DXXAAC_H.xpt

First Published: September 2018

Last Revised: NA

Component Description

Users of the 2013-2014 dual-energy X-ray absorptiometry abdominal aortic calcification data (DXXAAC_H) are encouraged to read the documentation before accessing the data file.

Abdominal aortic calcification (AAC) can be accurately recognized on lateral spine images intended for Vertebral Fracture Assessment (VFA) that are obtained with dual-energy X-ray absorptiometry (DXA).Several studies have found that lateral spine images obtained with DXA to detect prevalence of VFA can detect AAC with reasonably good sensitivity and specificity (Schousboe 2006, Schousboe 2017). The image resolution of lateral spine scans obtained with DXA is close to the image resolution of a standard radiograph, but the DXA scan produces much lower radiation exposure (Schousboe 2006, Schousboe 2007).

Abdominal Aortic Calcification is significantly associated with both cardiovascular heart disease and stroke even after adjustment for the traditional risk factors of age, cigarette use, diabetes mellitus, high systolic blood pressure, left ventricular hypertrophy, body mass index, and cholesterol (Szulc 2016, Schousboe 2017).

In 2013-2014, lateral DXA scans of the thoraco-lumbar spine were administered in the NHANES mobile examination center (MEC).

The Instant Vertebral Assessment (IVA) lateral spine scans provide vertebral fracture information for 13 individual vertebrae from T4-L4 and AAC measurement for vertebrae L1–L4.

Eligible Sample

DXA scans were administered to eligible survey participants 40 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 “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 weight over 450 pounds (DXA table limitation).

Participants were excluded from all spine scans if they reported a Harrington Rod in the spine for scoliosis.

The variable DXDAACST indicates the examination status for abdominal aortic calcification using IVA lateral spine scan. The codes for DXDAACST are as follows:

DXDAACST- Abdominal aortic calcification status
1 = IVA lateral spine scan completed, L1-L4 are valid
2 = IVA lateral spine scan completed, among L1-L4 one or more vertebrae is invalid
3 = IVA lateral spine not scanned, pregnancy
4 = IVA lateral spine not scanned, weight > 450 lbs
5 = IVA lateral spine not scanned, other reason

The main reasons for completed, but invalid, IVA lateral spine scans (code 2) were an insufficient scan area or partial scan, degenerative disease/severe scoliosis, sclerotic spine/spinal fusion/laminectomy and poor image quality due to morbid obesity. The “Not scanned, other reason” (code 5) 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 IVA lateral spine scans were acquired on Hologic Discovery model A densitometers (Hologic, Inc., Marlborough, Massachusetts), using software version Apex 3.2. The radiation exposure from DXA for the IVA lateral spine scan is extremely low at less than 20 uSv. All scans in the DXXAAC_H file were viewed using Optasia SpinAnalyzer software both AAC-24 and AAC-8 scoring semi-quantitative techniques (Kauppila 1997, Schousboe 2007) were used for the evaluation.

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

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).

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 IVA lateral spine images were viewed using Optasia Spinalizer software. Both AAC-24 and AAC-8 scoring semi-quantitative (SQ) techniques were used for the abdominal aortic calcification evaluation (Kauppila 1997, Schousboe 2007).

In the scoring method for AAC-24, the anterior and posterior aortic walls are divided into four segments, corresponding to the areas in front of the lumbar vertebrae L1-L4. Within each of these 8 segments, aortic calcification was recognized visually as either a diffused white stippling of the aorta, extending out to the anterior and/or posterior aortic walls, or as white linear calcification of the anterior and/or posterior walls. Aortic calcification was scored as “0” if there was no calcification; “1” if one-third or less of the aortic wall in that segment was calcified; “2” if more than one-third but less than two-thirds was calcified; or “3” if more than two-thirds was calcified. The scores were obtained separately for the anterior and posterior aortic wall, resulting in a range from “0” to “6” for each vertebral level and “0” to “24” for the total score.

The AAC-8 scale estimates the total length of calcification of the anterior and posterior aortic walls in front of the vertebrae L1 to L4 as “0” if no calcification is seen; “1” if aggregate length of calcification is equal to the height of one vertebrae or less; “2” if aggregate length of calcification is more than one but less than or equal to the height of two vertebra; “3” if aggregate length of calcification is more than two but less than or equal to the heights of three vertebra; and “4” if aggregate length of calcification is more than the height of three vertebra.

VFA images were read by a single reader at the UCSF quality control center, who was trained by Dr. John Schousboe.

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 Hologic Spine (HSP-Q96) Phantom and a Hologic Block Phantom were circulated among the MECs, and were scanned at the start of operations of each MEC.

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

In 2013-2014, longitudinal monitoring was conducted through 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, as well as to monitoring the vertebrae dimension measurement.

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.

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 why spine regions of interest (ROI) could not be analyzed accurately. Invalidity codes were applicable to completed scans only (DXDAACST = 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:

DXXAACTV = AAC total invalidity code
DXXAL1CC = AAC L1 invalidity code
DXXAL2CC = AAC L2 invalidity code
DXXAL3CC = AAC L3 invalidity code
DXXAL4CC = AAC L4 invalidity code

Values for invalidity codes DXXAL1CC, DXXAL2CC, DXXAL3CC and DXXAL4CC

• 0 = Valid data
• 1 = Removable or non-removable objects
• 3 = Excessive x-ray noise due to morbid obesity
• 4 = Insufficient scan area
• 5 = Movement
• 6 = Other (degenerative diseases, spinal fusion, fractures)

 

Values for DXXAACTV

• 0 = Valid data
• 1= Invalid data

If one or more spine vertebrae were coded as invalid, total AAC invalidity code was coded as invalid (DXXAACTV=1) and AAC total 8 score and AAC total 24 score were set to missing.

Analytic Notes

The NHANES examination sample weights should be used for any analyses using the DXXAAC_H 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.

References

• Hologic Product Support http://www.hologic.com
• Kauppila LI, Polak JF, Cupples LA, et al. New indices to classify location, severity and progression of calcific lesions in the abdominal aorta: a 25-year follow-up study. Atherosclerosis 1997; 132:245-250.
• Schousboe JT, Debold CR. Reliability and accuracy of vertebral fracture assessment with densitometry compared to radiography in clinical practice. Osteoporos Int 2006; 17:281-289
• Schousboe JT, Lewis JR, Kiel DP. Abdominal aortic calcification (AAC) can be accurately recognized on lateral spine images intended for vertebral fracture assessment that are obtained with dual-energy X-ray absorptiometry (DXA). Bone. 2017; 104:91-100.
• Schousboe JT, Vokes T, Broy SB, Ferrar L, McKiernan F, Roux C, Binkley N. Vertebral Fracture Assessment: the 2007 ISCD Official Positions. J Clin Densitom. 2008; 11: 92-108.
• Schousboe JT, Wilson KE and Hangartner TN. Detection of Aortic Calcification during Vertebral Fracture Assessment (VFA) compared to Digital Radiography. PloS ONE 2007; 2(8): e715.
• Schousboe JT, Wilson KE and Kiel DP. Detection of abdominal aortic calcification with lateral spine imaging using DXA. J Clin Densitom 2006; 9:302-308.
• Szulc P. Abdominal aortic calcification: A reappraisal of epidemiological and pathophysiological data. Bone 2016; 84:25-37.

Codebook and Frequencies