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.
Lead is a known environmental toxin that has been shown to affect deleteriously the nervous, hematopoietic, endocrine, renal, and reproductive systems. In young children, lead exposure is a particular hazard because children more readily absorb lead than adults, and children’s developing nervous systems also make them more susceptible to the effects of lead. The primary sources of exposure for children are lead-laden paint chips and dust as a result of deteriorating lead-based paint. The risk for lead exposure is disproportionately higher for children who are poor, non-Hispanic black, living in large metropolitan areas, or living in older housing. Among adults, the most common high exposure sources are occupational. Blood lead levels measured in previous NHANES cycles have been the cornerstone of lead exposure surveillance in the U.S. The data have been used to document the burden and dramatic decline of elevated blood lead levels, to promote the reduction of lead use, and to help to redefine national lead poisoning prevention guidelines, standards, and abatement activities.
Examined participants aged 1 year and older in the NHANES 2017-March 2020 pre-pandemic sample were eligible. This file only includes data from examined participants aged 1 to 5 years with limited access. See Analytic Notes for information on data from participants aged 6 years and older.
This method directly measures lead (Pb), cadmium (Cd), total mercury (Hg), manganese (Mn), and selenium (Se) content of whole blood specimens using mass spectrometry after a simple dilution sample preparation step.
During the sample dilution step, a small volume of whole blood is extracted from a larger whole blood patient specimen after the entire specimen is mixed (vortexed) to create a uniform distribution of cellular components. This mixing step is important because some metals (e.g., Pb) are known to be associated mostly with the red blood cells in the specimen and a uniform distribution of this cellular material must be produced before a small volume extracted from the larger specimen will accurately reflect the average metal concentration of all fractions of the larger specimen. Coagulation is the process in which blood forms solid clots from its cellular components. If steps are not taken to prevent this process from occurring, i.e., addition of anti-coagulant reagents such as EDTA in the blood collection tube prior to blood collection, blood will immediately begin to form clots once leaving the body and entering the tube. These clots prevent the uniform distribution of cellular material in the blood specimen even after rigorous mixing, making a representative sub-sample of the larger specimen unattainable. It is important that prior to or during sample preparation the analyst identify any sample having clots or micro-clots (small clots). Clotted samples are not analyzed by this method due to the inhomogeneity concerns (i.e., all results for the sample are processed as “not reportable”).
Liquid samples are introduced into the mass spectrometer through the inductively coupled plasma (ICP) ionization source. The liquid diluted blood sample is forced through a nebulizer with argon gas, which converts the bulk liquid into an aerosol of small droplets. The smaller droplets in the aerosol are selectively passed through the spray chamber by a flowing argon stream into the 6000-8000K plasma of the ICP. The high energy of the plasma results in the ionization of the atoms of lead, cadmium, mercury, manganese, and selenium from the droplets from the sample. The ions enter the mass spectrometer through an interface that separates the atmospheric pressure, ~760 torr, of the ICP from the vacuum region, ~10-5 torr, within the mass spectrometer. After the interface, the ions pass through a focusing region, then the first quadrupole mass filter (Q1), the collision-reaction cell (or octopole reaction system, ORS), a second quadrupole mass filter (Q2), and finally are selectively counted in rapid sequence of mass-to-charge ratios at the detector.
Electrical signals associated with each mass-to-charge ratio monitored are summed by a computer, and the magnitude of the signals detected while aspirating an unknown sample is correlated to an elemental concentration through comparison of the blank-subtracted analyte / internal standard signal ratio of the unknown sample with the signal ratio obtained when aspirating calibrators. This method was originally based on the method by Jones (Jones, et al., 2017).
Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used. There was a change to the method between the 2017-2018 and the 2019-March 2020 cycles.
Cadmium, Lead, Manganese, Mercury, and Selenium Lab Procedure Manual (June 2020)
Cadmium, Lead, Manganese, Mercury, and Selenium Lab Procedure Manual (November 2021)
Whole blood specimens were processed, stored, and shipped to the 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 the 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 Environmental
Health 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.
One additional variable was created for this data file. The variables were created using the following formulas:
LBDBPBSI: The analyte lead value in µg/dL (LBXBPB) was converted to µmol/L (LBDBPBSI) by multiplying LBXBPB by 0.0483 (Round 3 decimal points).
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. 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 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.
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 change in methods between the 2017-2018 and
2019-March 2020 cycles resulted in lower limits of detection for the 2019-March
2020 survey cycle. In order to make the
merged dataset compatible the detection limits were made 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., LBDBCDLC) 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., LBXBCD)
provides the analytic result for the analyte. For analytes with analytic
results below the lower limit of detection (ex. LBDBCDLC=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/dL) for lead:
| Variable Name | Analyte Description | LLOD |
|---|---|---|
| LBXBPB | Lead, blood | 0.070 |
Data Access
The public released data file “Lead, Cadmium, Total Mercury, Selenium, & Manganese – Blood (P_PBCD)” includes blood lead data for participants aged 6 years and older.
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.11 to 20.84 | Range of Values | 995 | 995 | |
| . | Missing | 579 | 1574 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0.005 to 1.007 | Range of Values | 995 | 995 | |
| . | Missing | 579 | 1574 |
| Code or Value | Value Description | Count | Cumulative | Skip to Item |
|---|---|---|---|---|
| 0 | At or above the detection limit | 995 | 995 | |
| 1 | Below lower detection limit | 0 | 995 | |
| . | Missing | 579 | 1574 |