Component Description
Trace metals have been associated with adverse health effects in occupational
studies or laboratory studies, but have not been monitored in general population
groups.
This method is used to achieve rapid and accurate quantifications of multiple
elements of toxicological and nutritional interest. The method is sensitive and
rapid enough to analyze urine specimens from subjects suspected of being exposed
to a number of important toxic elements, or to evaluate environmental or other
non-occupational exposure to these same elements.
Eligible Sample
All examined participants aged 3 to 5 years were eligible and participants
aged 6 years and older from a one-third subsample were eligible.
Description of Laboratory
Methodology
This method directly measures multiple metals in 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 NHANES 2015-2016 cycle.
Laboratory Method Files
Urinary Metals and Total Arsenic Laboratory Procedure Manual
(June 2018)
Laboratory Quality
Assurance and Monitoring
Urine samples are 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 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
quality-control 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 quality control protocol for all CDC and
contract laboratories, which outlined the use of Westgard rules (Westgard, et
al. 1981) used 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’ quality control
and quality assurance performance criteria for accuracy and precision, similar
to the Westgard rules (Caudill et al., 2008).
Data Processing and Editing
The data were reviewed. Incomplete data or improbable values were sent to the
performing laboratory for confirmation.
Analytic Notes
Refer to the 2015-2016
Laboratory Data Overview for general information on NHANES laboratory
data.
Subsample Weights
Urinary metals were measured in a full
sample of participants ages 3-5 and a one-third subsample of participants 6
years and older. Special sample weights are required to analyze these data
properly. Specific sample weights for this subsample are included in this data
file and should be used when analyzing these data.
Demographic and Other Related
Variables
The analysis of NHANES laboratory data must be
conducted using the appropriate survey design and demographic variables. The
NHANES 2015-2016
Demographic Data File contains demographic and 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.
Starting in the 2015-2016 NHANES cycle, the variable URXUCR (urine
creatinine) will not be reported in this file. URXUCR can be found in the data
file titled Albumin & Creatinine - Urine.
This laboratory data file can be linked to the other NHANES data files using
the unique survey participant identifier 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., URDUBALC) 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., URDUBALC=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 URX (ex.,
URXUBA) provides the analytic result for that analyte.
The lower limit of detection (LLOD, in µg/L) for the urinary metals in
the data set is:
| Variable Name |
SAS Label |
LLOD |
| URXUBA |
Urinary Barium |
0.060 |
| URXUCD |
Urinary Cadmium |
0.036 |
| URXUCS |
Urinary Cesium |
0.086 |
| URXUCO |
Urinary Cobalt |
0.023 |
| URXUMN |
Urinary Manganese |
0.130 |
| URXUMO |
Urinary Molybdenum |
0.800 |
| URXUPB |
Urinary Lead |
0.030 |
| URXUSB |
Urinary Antimony |
0.022 |
| URXUSR |
Urinary Strontium |
2.340 |
| URXUTL |
Urinary Thallium |
0.018 |
| URXUSN |
Urinary Tin |
0.090 |
| URXUTU |
Urinary Tungsten |
0.018 |
| URXUUR |
Urinary Uranium |
0.002 |
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.