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.
Mercury
Mercury is a non-essential element that becomes toxic to various organ systems in the body; especially the central nervous system, including the brain and the renal system, including the kidneys. Three main sources of mercury exposure in the U.S. are consumption of fish, dental repairs, and occupational exposure. The general population may be exposed to three forms of mercury: elemental, inorganic, or organic (primarily methyl mercury). The concentration of total mercury in urine is a measure of exposure primarily to elemental and inorganic mercury, although some mercury in urine comes from de-methylation of methyl mercury in blood (Abe et al., 1995).
Health effects related to low exposure in the general population are not well defined.
All examined participants aged 3 to 5 years and a one-third subsample of examined participants aged 6 years and older in the NHANES 2017-March 2020 pre-pandemic sample were eligible.
This method directly measures the mercury and iodine content of urine specimens using inductively coupled plasma mass spectrometry (ICP-MS) after a simple dilution sample preparation step. Liquid samples are reduced to small droplets in an argon aerosol via a nebulizer, and then the droplets enter the ICP where the thermal energy atomizes the sample and then ionizes the atoms. The ions are pulled into the mass spectrometer, where they are focused through the dynamic reaction cell (DRC), the quadrupole mass filter, and finally are selectively counted in rapid sequence at the detector according to their mass-to-charge (m/z) ratio. When detecting mercury, the DRC is filled with oxygen gas to eliminate polyatomic interferences. Quantification of mercury is accomplished by comparing the blank-subtracted counting rate of mercury from a sample, ratioed to an internal standard, to the counting rate, ratioed to an internal standard, of blank-subtracted, matrix-matched, external calibrators, tested within the same analytical run.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
Iodine and Mercury, Urine Lab Procedure Manual (October 2020)
Iodine and Mercury, Urine Lab Procedure Manual (November 2021)
Urine specimens 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 (LPMs). 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 on “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 running 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/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.
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 demographic data 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 documents 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. 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 additional re-weighting for item non-response is necessary.
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.
Subsample Weights
The analytes included in this dataset were measured in all examined participants aged 3-5 years, and in a one-third subsample of participants 6 years and older. Special sample weights are required to analyze these data properly. Variable (WTSAPRP) encoding of the specific sample weights for this subsample is included in this data file and should be used when analyzing these data. These special sample weights were created to account for the subsample selection probability, as well as the additional nonresponse to these lab tests. Therefore, if participants were eligible for the subsample, but did not provide a urine specimen, they would have the sample weight value assigned as “0” in their records.
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.
This laboratory data file can be linked to the other NHANES data files using
the unique survey participant identifier (i.e., SEQN).
The variable URXUCR (urine creatinine) will not be reported in this file. URXUCR can be found in the data file titled Albumin & Creatinine – Urine.
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 named ending in “LC” (ex., URDUHGLC) 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. The other variable prefixed URX (ex., URXUHG) provides the analytic result for the analyte. For analytes with analytic results below the lower limit of detection (ex., URDUHGLC=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 lower limit of detection (LLOD, in ug/L) for urinary mercury is:
| Variable Name | Analyte Description | LLOD |
|---|---|---|
| URXUHG | Urinary Mercury (ug/L) | 0.13 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 2395.195359 to 955677.30961 | Range of Values | 4727 | 4727 | |
| 0 | No Lab Specimen | 163 | 4890 | |
| . | Missing | 0 | 4890 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.09 to 30.1 | Range of Values | 4600 | 4600 | |
| . | Missing | 290 | 4890 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0 | At or above the detection limit | 1947 | 1947 | |
| 1 | Below lower detection limit | 2653 | 4600 | |
| . | Missing | 290 | 4890 |