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 were eligible.
This method directly measures the mercury content of urine specimens using mass spectrometry after a simple dilution sample preparation step. Liquid samples are introduced into the mass spectrometer through the inductively coupled plasma (ICP) ionization source, reduced to small droplets in an argon aerosol via a nebulizer, and then the droplets enter the ICP. The ions first pass through a focusing region, followed by the dynamic reaction cell (DRC), the quadrupole mass filter, and finally are selectively counted in rapid sequence at the detector, allowing individual isotopes of an element to be determined.
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.
There were no changes to the lab method, lab equipment, or lab site for this component in the 2017-2018 cycle.
Iodine and Mercury, Urine Lab Procedure Manual (October 2020)
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 NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to the National Center for Environmental Health for testing.
The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.
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 the 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.
Refer to the 2017-2018 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. 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 (WTSA2YR) 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 basic demographic variables. The NHANES 2017-2018 Demographics File contains demographic data, health indicators, and other related information collected during the household interviews as well as the sample design variable. 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 |
|---|---|---|---|---|
| 3829.845402 to 1502431.3423 | Range of Values | 2871 | 2871 | |
| 0 | No Lab Specimen | 108 | 2979 | |
| . | Missing | 0 | 2979 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.09 to 30.1 | Range of Values | 2812 | 2812 | |
| . | Missing | 167 | 2979 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0 | At or above the detection limit | 1143 | 1143 | |
| 1 | Below lower detection limit | 1669 | 2812 | |
| . | Missing | 167 | 2979 |