• Component Description
• Eligible Sample
• Protocol and Procedure
• Quality Assurance & Quality Control
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • DXAFMRST - Femur scan status
  • DXXFMBCC - Total femur BMD invalidity code
  • DXXOFBMD - Total femur BMD
  • DXXOFBMC - Total femur BMC
  • DXXOFA - Total femur area
  • DXXNKBMD - Femoral neck BMD
  • DXXNKBMC - Femoral neck BMC
  • DXXNKA - Femoral neck area
  • DXXTRBMD - Trochanter BMD
  • DXXTRBMC - Trochanter BMC
  • DXXTRA - Trochanter area
  • DXXINBMD - Intertrochanter BMD
  • DXXINBMC - Intertrochanter BMC
  • DXXINA - Intertrochanter area
  • DXXWDBMD - Wards triangle BMD
  • DXXWDBMC - Wards triangle BMC
  • DXXWDA - Wards triangle area
  • DXAFMRK - Calculated K for femur
  • DXAFMRD0 - Calculated DO for femur

National Health and Nutrition Examination Survey

2017-March 2020 Data Documentation, Codebook, and Frequencies

Dual-Energy X-ray Absorptiometry - Femur (P_DXXFEM)

Data File: P_DXXFEM.xpt

First Published: September 2021

Last Revised: NA

Component Description

The NHANES program suspended field operations in March 2020 due to the coronavirus disease 2019 (COVID-19) pandemic. As a result, data collection for the NHANES 2019-2020 cycle was not completed and the collected data are not nationally representative. Therefore, data collected from 2019 to March 2020 were combined with data from the NHANES 2017-2018 cycle to form a nationally representative sample of NHANES 2017-March 2020 pre-pandemic data. These data are available to the public. Please refer to the Analytic Notes section for more details on the use of the data.

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) from 2005-2010, 2013-14 and 2017-March 2020.

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)

Eligible Sample

All participants aged 50 years and over, in the NHANES 2017-March 2020 pre-pandemic sample were eligible. 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, such as dyes or barium, in the past 7 days.
• Measured weight over 450 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
4 = Femur not scanned, weight > 450 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.

Protocol and Procedure

In 2017-18, the femur scans were acquired on Hologic Discovery model A densitometers (Hologic, Inc., Bedford, Massachusetts), using software version Apex 3.2. In 2019-March 2020, the femur scans were acquired on Hologic Horizon model A densitometers (Hologic, Inc., Bedford, Massachusetts), using software version Apex version 5.6.0.5. The radiation exposure from DXA for the femur scan is extremely low at less than 20 uSv. All scans in the P_DXXFEM file were analyzed with Hologic APEX version 4.0 software.

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 Shepherd Research Lab at the University of California, San Francisco (UCSF), Department of Radiology (2017) and at the University of Hawaii (UH) Cancer Center (2018-2020) 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 Shepherd Research Lab when problems were noted during review of the scans. Constant communication was maintained throughout the year among the Shepherd Research Lab, 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 quality control center when the scanners were serviced or repaired so any changes in measurement as a result of the work could be assessed.

In 2019, three new DXA densitometer systems (Hologic Horizon Model A) were installed in the MECs. A cross calibration study was conducted to assure accuracy of NHANES longitudinal assessment. We followed International Society for Clinical Densitometry (ISCD recommendations in replacing a bone densitometer unit within the same model and included both phantoms and human subjects in the study (ISCD DXA Machine Cross Calibration Tool). The spine bone mineral density values in the new instrument were within 1% of the prior instrument.

Scan Analysis
Each participant and phantom scan was reviewed and analyzed by the Shepherd Lab using standard radiologic techniques and study-specific protocols developed for the NHANES. The Hologic software, APEX v4.0 (Hologic) was used to analyze all femur scans acquired in 2017-Mar 2020. Expert review was conducted by the Shepherd Research Lab on 100% of analyzed participant scans to verify the accuracy and consistency of the results.

Invalidity Codes
Invalidity codes were applied by the Shepherd Research Lab 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 2017-Mar 2020, 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 Shepherd Research Lab 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 2017-Mar 2020. 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 = Removable or non-removable objects
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 sub region 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 COVID-19 pandemic required suspension of NHANES 2019-2020 field operations in March 2020 after data were collected in 18 of the 30 survey locations in the 2019-2020 sample. Because the collected data from 18 locations were not nationally representative, these data were combined with data from the previous cycle (2017-2018) to create a 2017-March 2020 pre-pandemic data file. The resulting exam sample weights in the demographic file should be used to calculate estimates from the combined cycles. These exam sample weights are not appropriate for independent analyses of the 2019-2020 data and will not yield nationally representative results for either the 2017-2018 data alone or the 2019-March 2020 data alone. Please refer to the NHANES website for additional information for the NHANES 2017-March 2020 pre-pandemic data, and for the previous 2017-2018 public use data file with specific weights for that 2-year cycle.

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.

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.
• 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 https://www.hologic.com/
• ISCD DXA Machine Cross Calibration Tool. April 2021. https://iscd.org/learn/resources/calculators/
• ISCD Official Positions 2007. October 2007 http://www.iscd.org/wp-content/themes/iscd/pdfs/official-positions/ISCD2007OfficialPositions-Adult.pdf.
• 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