National Health and Nutrition Examination Survey

The NHANES 2001-2002 Second Day Dual-Energy X-ray Absorptiometry Exam was conducted for quality assurance and research purposes in the NHANES mobile examination center (MEC) in 2000 through 2002. The procedures for the second day exam were the same as those for the primary exam and can be found in the Body Composition Procedures Manual located on the NHANES website. Second day DXA data for 2000 can be accessed only through the National Center for Health Statistics Research Data Center.

No statistical sampling was applied for selection of second day exam participants; the sample was self-selected and nonrandom. For the second day exam, the MEC staff recruited participants aged 16-69 years, approximating percentages in demographic categories for the primary exam. Participants were given the option to decline the second exam. In general, about 11% of those who completed the first exam also completed the second exam. Seven hundred fifty (750) participants completed the second day DXA exam.

Participants were asked to come for the second day exam at least 8 days after their first exam. The second day exams were conducted during the same time period as the primary exams for a particular survey stand or location. The same radiology technologist who conducted the second day exam was not always the same as the technologist who conducted the first exam. Participants were remunerated for the second day exam.

Missing and invalid DXA data from the second day exam were not multiply imputed as were data from the primary exam. The second day DXA data file (DXX_2_B) contains only 1 record for each survey participant. Users of the second day DXA data (DXX_2_B) are encouraged to read this documentation before accessing the data file.

The second day DXA scans provide bone and soft tissue measurements for the total body, for both arms and both legs, the trunk, and head. Bone measurements also were obtained for the pelvis, left and right ribs, thoracic spine, and lumbar spine. Values for the total body and regions include:

• Total mass (gm) 
• Bone mineral content (BMC) (gm) 
• Bone area (cm2) 
• Bone mineral density (BMD) (gm/cm2) 
• Fat mass (gm) 
• Lean mass excluding BMC (gm) 
• Lean mass including BMC (gm) 
• Percent body fat (%)

The second day exam data file also contains variables on the number of days between the primary and second day exams and whether the same technologist conducted both primary and second day exams or whether different technologists conducted the two exams.

The second day DXA scans were administered to eligible survey participants aged 16-69 years in 2000-2002 who had completed the primary exam. Participants excluded from the primary DXA examination for reasons other than pregnancy, such as self-reported history of radiographic contrast material (barium) use in past 7 days, weight over 300 pounds, or height over 6’5 (limitations of the DXA table), were considered to be eligible nonrespondents. Pregnant females were not eligible for the DXA exam.

DXAEXSTS – examination status variable
1 = Scan completed
2 = Scan completed, but invalid

Whole body DXA scans were taken 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 densitometer scanned participants with an x-ray source using fan-beam scan geometry in three passes (1 minute per pass). The participants were positioned supine on the tabletop with their feet in a neutral position and hands flat by their side. A Velcro strap was used to keep the feet stationary and together. 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.

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 2001-2002 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 scan was reviewed and analyzed by the UCSF using standard radiologic techniques and study-specific protocols developed for the NHANES. Hologic Discovery software, version 12.1, was used to analyze the scans. Expert review was conducted by the UCSF on 100% of analyzed participant scans to verify the accuracy and consistency of the results. Invalidity codes were applied by the UCSF to regions of the body that could not be analyzed accurately. The invalidity codes are provided in the data file (see Analytic Notes for a description of the invalidity codes).

The percentage of eligible survey participants in 2001-2002 with 100% valid data (no invalid regions) decreased 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 pacemakers, stents, and hip replacements and higher rates of obesity resulting in excessive x-ray “noise” due to truncal adiposity, which interfered with the ability to obtain valid measurements. The percentage of participants with 100% valid data also decreased with increasing BMI due to truncal adiposity. For these reasons, as well as the exclusion of participants over 300 pounds from the DXA exam, users should be aware that the second day DXA data are not representative of all survey participants. Users should also be aware that regions of the body that may have been invalid in the primary exam may be valid in the second-day exam and vice versa. For example, jewelry may not have been removed or a positioning problem may have occurred in one scan and not the other.

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 were scanned at the start of operations at each survey site.

Air scans, phantom-less scans using the whole body scan mode, were used to describe and monitor the systems’ radiographic uniformity across the entire scan field. The complete phantom scanning schedule is described in the Body Composition Procedures Manual located on the NHANES website.

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. The correction factors were judged to be small and adjustment of the participant data not required.

The NHANES lean soft tissue mass and fat mass for the total body and regions were adjusted based on the results of an analysis of QDR- 4500A DXA data from seven research laboratories indicating that the QDR-4500A algorithm underestimated fat mass and overestimated lean mass (Schoeller, 2005). Based on criterion measurements of body composition from total body water by dilution, underwater weighing, and four-compartment analysis, the QDR-4500A was determined to overestimate lean mass (p < 0.05) in the cohort of 1198 subjects. The NHANES DXA lean mass was decreased by 5% and an equivalent kilogram weight added to the fat mass so the total mass did not change.

No other adjustments were made to the second day DXA data.

Because the second day sample was self-selected and nonrandom, no survey sample weights are associated with the second day DXA data. NCHS recommends that survey design variables such as sample weight not be linked with the second exam data, since these variables were created for use with the primary sample. The second day DXA data were intended to be used only for quality control and research purposes such as assessing intra-individual variability.

The second day exam data can be linked to the primary exam data and household interview data using the unique participant identifier (SEQN). Caution should be used in the analysis of the second day exam data. Analyses should include an investigation of the potential selection bias of the second day exam sample and evaluation of differences in characteristics such as age, gender, and race/ethnicity between the first and second day exam samples.

Because the second day exam data were not multiply imputed, the numbers and types of variables included in the second day exam data file will differ from those in the data file for the first exam. The variable nomenclature also differs: the first exam variable names have an X or D in the third position, while the second day exam variables have a 2 in the third position. For example, DXXTOPF becomes DX2TOPF.

Invalidity Codes
Invalidity codes were applied only to completed and valid scans (DXAEXSTS=1). Codes 1-7 indicate the reasons regions could not be analyzed accurately.

Invalidity codes
DXADISP = entire scan
DXATOBCC = total bone mineral density
DXATOPFC = total percent body fat
DXASTBCC = subtotal (excludes head) bone mineral density
DXASTPFC = subtotal (excludes head) percent body fat
DXAHEBV = head bone
DXAHETV = head soft tissue
DXALABV = left arm bone
DXALATV = left arm soft tissue
DXALLBV = left leg bone
DXALLTV = left leg soft tissue
DXARABV = right arm bone
DXARATV = right arm soft tissue
DXARLBV = right leg bone
DXARLTV = right leg soft tissue
DXATRBV = trunk bone, includes invalid thoracic or lumbar spine, left or right ribs, or pelvis
DXATRTV = trunk soft tissue

Values for invalidity codes
1 = Jewelry and other objects not removed
2 = Non-removable objects (includes prostheses, implants, casts)
3 = Excessive x-ray “noise” due to truncal adiposity, 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/leg overlap
5 = Body parts out of scan region
6 = Positioning problem (head, arms/hands or feet turned)
7 = Other (includes participant motion, unknown artifacts, deformities)