Organochlorine pesticides are a diverse group of synthetic chemicals that persist in the
environment and tend to bioaccumulate. Most of these chemicals have been banned
from use in the U.S. Assessment of exposure to persistent organochlorine
pesticides in a representative sample of the U.S. population is needed to
determine current prevalence and level of exposure and the potential for human
health effects from exposure to these chemicals.
Examined participants aged 12 years and older from a one-third subsample were eligible.
Description of Laboratory Methodology
Eight organochlorine pesticides and metabolites were measured in serum through the
use of automated liquid/liquid extraction and subsequent sample clean-up. Final
determination of target analytes was performed by isotope dilution gas
chromatography high-resolution mass spectrometry GC/IDHRMS. The analytical
method is described in Jones et al, 2012, which is a modification of the method
described by Sjodin et al., 2004 and Barr et al., 2003.
Refer to the Laboratory Methods 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 2007-2008 cycle. However, the samples were measured in a
pooled fashion for 2007-2008 rather than individual measurements.
Laboratory Method Files
PCBs and Persistent Pesticides
Laboratory Quality Assurance and Monitoring
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 NHANES Laboratory Procedure Manual (LPM).
Vials are stored under appropriate frozen (-30oC) conditions
until they were shipped to the National Center for Environmental Health for
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.
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
NCHS developed and distributed a quality control protocol
for all CDC and contract laboratories, which outlined the use of Westgard rules
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’ 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.
Refer to the 2007-2008 Laboratory Data Overview
for general information on NHANES laboratory data.
Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on the
use of sample weights and analytic issues.
pooled-sample weight created for this file released in July 2013 did not
accurately take into account the new sample design for this NHANES survey cycle
and it was not correctly stratified to the U.S. population total. This new file
contains the corrected sample weight (WTSMSMPA). The corrected sample weight
was created so that analyses using race and Hispanic origin would be comparable
to the three groups used in NHANES 2005-2006 (non-Hispanic white, non-Hispanic
black and Mexican-American). Therefore, analyses of the data for ”Total
Hispanics” is not appropriate. No changes or corrections were made to the lab
analyte data in this new release. Any analyses of the data using the old public
use data file should be repeated using the corrected sample weight on this new
PBDEs and PBB-153
were measured in a one third subsample of persons 12 years and over, and
samples were pooled in groups of 8 samples per pool within 32 demographic
groups. The analysis of NHANES 2007-2008 pooled-sample data must be conducted
with the basic demographic variables provided in this data file. This
pooled-sample data file cannot be linked to other NHANES 2007-2008 data.
Because each sample person does not have an equal probability of
selection, sample weighting is needed to produce correct population estimates
of means, percentiles, and other descriptive statistics.
weights required to produce estimates from these data are included in this data
file. The analysis of pooled-samples from survey data is a relatively new
field of study, and there is currently no established method to produce
variance estimates for these pooled results. These data cannot be
analyzed using software designed for complex surveys because design features
are not available.
Methods used to Create Pooled Results
Centers for Disease Control and Prevention (CDC) provides an ongoing assessment
of the US population's exposure to environmental chemicals by using
biomonitoring in conjunction with CDC's National Health and Nutrition
Examination Survey (NHANES). Characterizing the distributions of concentrations
of environmental compounds or their metabolites in the US population is a
primary objective of CDC's biomonitoring program. Historically, this
characterization has been based on individual measurements of these compounds
in body fluid or tissue from representative samples of the population. Pooling
samples allows for larger sample volumes, which can result in lower limits of
detection and reduces the number of measurements and costs. In NHANES 2007-2008,
a weighted pooled-sample design was implemented to facilitate pooling samples
before making analytical measurements. Table 1 lists the IUPAC
(International Union of Pure and Applied Chemistry (IUPAC) names, common
abbreviation, and NHANES variable name for the analytes included in this
prepared from serum collected from a random one-third subset of the NHANES
2007–2008 participants aged 12 years and older. Samples were pooled based on
gender, race and Hispanic origin, and age. To implement the pooled-sample
design, each participant sample was identified as belonging to one of 32
demographic groups based on race and Hispanic origin (non-Hispanic white: NHW,
non-Hispanic black: NHB, Mexican American: MA,: not non-Hispanic black,
non-Hispanic white or Mexican-American: OTHER), gender (Male, Female), and age
group (12-19, 20-39, 40-59, and 60+ years of age and older The number of pools
created for each of the 32 demographic groups varied depending on the total
number of individual samples available in a demographic group. The one-third
subset of NHANES 2007-2008 represents 2282 individual samples, but because the
pooled-sample design requires that all samples be of sufficient volume and that
there be the same number of samples in each pool, only 2070 samples were
available to create 264 pools with 8 samples per pool. See more details on pool sample formation and
exceptions in Table 2. The variable SAMPLEID denotes the identification number for each pool and ranges from 1 through 260. Please refer to the
Pooled-Sample Technical Support file (POOLTF_E) for detailed information on
individual participants included in each pool.
In order to
incorporate sample weighting into the pooled-sample design it was necessary to
use a different volume of material from each sample contributing to a pool. The
volume chosen for each sample in a pool was based on the ratio of its sampling
weight to the sum of the sampling weights of all samples in the pool. To
physically accomplish the pooling in the laboratory required that the ratio of
the largest to the smallest sampling weight of samples in the same pool be no
larger than about 4 or 5. The individual samples were sorted/stratified by
sampling weight within each of the 32 demographic groups and pools were formed
using samples with sampling weights adjacent to one another in the sorted list.
The number of samples in the one-third subset, number of samples available, the
number of these samples that were usable, and the number of pools formed in
each demographic group are presented in Table 2. Once the pools were created,
summed sampling weights were further adjusted to account for the unused
samples. These adjusted summed sampling weights are represented by the variable
In the dataset,
the whole weight detection limit (ng/g serum) is a variable and dependent on
the available sample size, however, the variation in sample size was low. Three
variables are provided for each of these analytes. The variable name ending in “LC” (ex.,
LBDBHCLC) 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 variable ending in “LA” (ex., LBCBHCLA)
is the lipid adjusted detection limits in ng/g lipid, because the
lipid levels are measured in individual pools/specimens. The third variable prefixed LBC (ex., LBCBHC)
provides the analytic result for that analyte.
analytes with analytic results below the lower limit of detection (ex., LBDBHCLC= 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). The lower limit of
detection (LLOD in ng/g serum and ng/g lipid) for PST:
Table 1. NHANES Variable
name, common abbreviation and International Union of Pure and Applied Chemistry
(IUPAC) name for each analyte reported. Maximum limit of detection (MLOD) expressed
as ng/serum and ng/g lipid).
||Max LOD (ng/g serum)
||Max LOD (ng/g lipid)
Table 2. Number of subjects per demographic group in the
NHANES 2007–2008 one-third subsample, number of individual serum samples
available, number of usable samples, and number of pools formed from usable
Methodological issues for analysis
There are a few pooled-sample methodological publications which address issues related to the following topics (See References.)
- Variance estimation and bias correction when pooling samples from log-normally distributed populations (Caudill, 2010a; Caudill, 2010b; Caudill et al., 2007);
- Models that incorporate measurement error when analyzing pooled-samples from normally or log-normally distributed populations (Caudill, 2010a);
- Incorporation of sample weighting into a pooled-sample design (Caudill, 2012; Caudill, 2010a);
- Estimation of standard errors and confidence limits for point estimates from pooled-samples (Caudill, 2012; Caudill, 2010a).