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.
Soluble transferrin receptor (sTfR) is a measure of iron deficiency and is particularly useful in people with inflammation, infection, or chronic disease, where ferritin levels do not correlate with true iron levels. Low storage of iron can lead to iron deficiency anemia. High levels of iron storage, also called iron overload, occurs when excess iron is accumulated in the body, primarily the liver.
The objectives of this component are: 1) to provide data for monitoring secular trends in measures of nutritional status in the U.S. population; 2) to evaluate the effects of people's habits and behaviors, such as physical activity and the use of alcohol, tobacco, and dietary supplements on nutritional status; and 3) to evaluate the effect of changes in nutrition and public health policies, including welfare reform legislation, food fortification policy, and child nutrition programs on the nutritional status of the U.S. population.
These data will be used to estimate deficiencies and toxicities of specific nutrients in the population and subgroups, to provide population reference data, and to estimate the contribution of diet, supplements, and other factors to serum levels of nutrients. Data will be used for research to further define nutrient requirements as well as optimal levels for disease prevention and health promotion.
Examined participants aged 1 to 5 years and female participants aged 12 to 49 years, in the NHANES 2017-March 2020 pre-pandemic sample, were eligible.
Transferrin receptor was measured among examined participants aged 1 to 5 years and 12 years and older in NHANES 2017-2018. In the NHANES 2019-2020 data collection cycle, the target group for serum transferrin receptor test was changed to only include examined participants aged 1 to 5 years and female participants aged 12 to 49 years. Therefore, only data from child participants aged 1 to 5 years and female participants aged 12 to 49 years are available for both data collection cycles and included in the present dataset.
The method principle for measurement of sTfR is a particle enhanced immunoturbidimetric assay that uses Roche kits on the Cobas® c501 clinical analyzer. Latex particles coated with anti-sTfR antibodies react with the antigen in the sample to form an antigen/antibody complex. Following agglutination, the precipitate is determined photometrically.
Refer to the Laboratory Method Files
section for a detailed description of the laboratory methods used.
Transferrin Receptor (February 2021)
Transferrin Receptor (July 2022)
Serum samples were processed, stored, and shipped to the Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA for analysis.
Detailed instructions on specimen collection and processing are discussed in the 2017-2018 and 2019-2020 NHANES Laboratory Procedures Manuals (LPM). Vials are stored under appropriate frozen (-30°C) conditions until they are shipped to National Center for Environmental Health for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Amendments mandates. Detailed QA/QC instructions are discussed in the NHANES LPMs.
Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the QC procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.
Analytical Laboratories
NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split samples collected during “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.
NCHS developed and distributed a QC protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard, et al. 1981) when testing NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.
All QC procedures recommended by the manufacturers were followed. Reported results for all assays meet the Division of Laboratory Sciences’ QA and QC performance criteria for accuracy and precision, similar to the Westgard rules (Caudill, et al. 2008).
The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.
One variable was created in this data file. The variable LBDTFRSI was created using the following:
LBDTFRSI: The transferrin receptor value in mg/L (LBXTFR) was converted to nmol/L (LBDTFRSI) by multiplying LBXTFR by 11.8.
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. Data collection was cancelled for the remaining 12 locations. 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. A special weighting process was applied to the 2017-March 2020 pre-pandemic data file. The resulting sample weights in the present file should be used to calculate estimates from the combined cycles. These 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.
Refer to the 2017-2018 and 2019-2020 Laboratory Data Overview for general information on NHANES laboratory data.
There are over 800 laboratory tests performed on NHANES participants. However, not all participants provided biospecimens or enough volume for all the tests to be performed. The specimen availability can also vary by age or other population characteristics. For example, in 2017-March 2020, approximately 76% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 95% of examined adults age 18 and older provided a blood specimen. Analysts should evaluate the extent of missing data in the dataset related to the outcome of interest as well as any predictor variables used in the analyses to determine whether the data are useable without additional re-weighting for item non-response.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on the use of sample weights and other analytic issues.
Demographic and Other Related Variables
The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-March 2020 Pre-Pandemic Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.
The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast, and the time of venipuncture.
This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier (i.e., SEQN).
Detection Limits
The detection limits were constant for all of the analytes in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., LBDTFRLC) indicates whether the result was below the limit of detection: the value “0” means that the result was at or above the limit of detection, “1” indicates that the result was below the limit of detection. For analytes with analytic results below the lower limit of detection (ex., LBDTFRLC =1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by the square root of 2 (LLOD/sqrt[2]). The other variable prefixed LBX (ex., LBXTFR, provides the analytic result for that analyte.
The lower limit of detection (LLOD in mg/L) for LBXTFR:
|
Variable Name |
SAS Label |
LLOD |
|
LBXTFR |
Transferrin receptor |
0.5 mg/L |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 1.24 to 35.1 | Range of Values | 3622 | 3622 | |
| . | Missing | 946 | 4568 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 14.6 to 414 | Range of Values | 3622 | 3622 | |
| . | Missing | 946 | 4568 |