• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Method Files
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SAMPLEID - Pool identification number
  • RIAGENDR - Gender
  • RIDRETH3 - Ethnicity - Recode
  • RIDAGGRP - Age at Screening Adjudicated - Recode
  • RIANSMP - Number of samples in a pool
  • WTSMSMPA - Sum of adjusted subsample weights
  • LBCD01 - 1,2,3,7,8-pncdd (fg/g)
  • LBCD02 - 1,2,3,4,7,8-hxcdd (fg/g)
  • LBCD03 - 1,2,3,6,7,8-hxcdd (fg/g)
  • LBCD04 - 1,2,3,7,8,9-hxcdd (fg/g)
  • LBCD05 - 1,2,3,4,6,7,8-hpcdd (fg/g)
  • LBCD07 - 1,2,3,4,6,7,8,9-ocdd (fg/g)
  • LBCF01 - 2,3,7,8-Tetrachlorofuran (tcdf) (fg/g)
  • LBCF02 - 1,2,3,7,8-Pentachlorofuran(pncdf)(fg/g)
  • LBCF03 - 2,3,4,7,8-Pentachlorofuran(pncdf)(fg/g)
  • LBCF04 - 1,2,3,4,7,8-Hexachlorofuran(hcxdf)(fg/g)
  • LBCF05 - 1,2,3,6,7,8-Hexachlorofuran(hxcdf)(fg/g)
  • LBCF06 - 1,2,3,7,8,9-Hexachlorodifuran (fg/g)
  • LBCF07 - 2,3,4,6,7,8-Hexchlorofuran(hxcdf)(fg/g)
  • LBCF08 - 1,2,3,4,6,7,8-Heptachlorodifuran (fg/g)
  • LBCF09 - 1,2,3,4,7,8,9-Heptachlorodifuran (fg/g)
  • LBCF10 - 1,2,3,4,6,7,8,9-Octachlorodifuran (fg/g)
  • LBCHXC - 3,3',4,4',5,5'-hexachlorobiphenyl(fg/g)
  • LBCPCB - 3,3',4,4',5-Pentachlorobiphenyl (fg/g)
  • LBCTC2 - 3,4,4',5-Tetrachlorobiphenyl(tcb)(fg/g)
  • LBCTCD - 2,3,7,8-Tetrachloro-p-dioxn(tcdd)(fg/g)
  • LBCD01LA - 1,2,3,7,8-pncdd lipid adjusted (pg/g)
  • LBCD02LA - 1,2,3,4,7,8-hxcdd lipid adjusted (pg/g)
  • LBCD03LA - 1,2,3,6,7,8-hxcdd lipid adjusted (pg/g)
  • LBCD04LA - 1,2,3,7,8,9-hxcdd lipid adjusted (pg/g)
  • LBCD05LA - 1,2,3,4,6,7,8-hpcdd lipid adjusted(pg/g)
  • LBCD07LA - 1,2,3,4,6,7,8,9-ocdd Lipid Adjust (pg/g)
  • LBCF01LA - 2,3,7,8-tcdf lipid adjusted (pg/g)
  • LBCF02LA - 1,2,3,7,8-pncdf lipid adjusted (pg/g)
  • LBCF03LA - 2,3,4,7,8-pncdf lipid adjusted (pg/g)
  • LBCF04LA - 1,2,3,4,7,8-hcxdf lipid adjusted (pg/g)
  • LBCF05LA - 1,2,3,6,7,8-hxcdf lipid adjusted (pg/g)
  • LBCF06LA - 1,2,3,7,8,9-hxcdf lipid adjusted (pg/g)
  • LBCF07LA - 2,3,4,6,7,8-hxcdf lipid adjusted (pg/g)
  • LBCF08LA - 1,2,3,4,6,7,8-hpcdf lipid adjusted(pg/g)
  • LBCF09LA - 1,2,3,4,7,8,9-hpcdf lipid adjusted(pg/g)
  • LBCF10LA - 1,2,3,4,6,7,8,9-ocdf lipid adjust(pg/g)
  • LBCHXCLA - 3,3',4,4',5,5'-hxcb Lipid adjusted(pg/g)
  • LBCPCBLA - 3,3',4,4',5-pncb lipid adjusted (pg/g)
  • LBCTC2LA - 3,4,4',5-tcb lipid adjusted (pg/g)
  • LBCTCDLA - 2,3,7,8-tcdd lipid adjusted (pg/g)
  • LBDD01LC - 1,2,3,7,8-pncdd comment code
  • LBDD02LC - 1,2,3,4,7,8-hxcdd comment code
  • LBDD03LC - 1,2,3,6,7,8-hxcdd comment code
  • LBDD04LC - 1,2,3,7,8,9-hxcdd comment code
  • LBDD05LC - 1,2,3,4,6,7,8-hpcdd comment code
  • LBDD07LC - 1,2,3,4,6,7,8,9-ocdd comment code
  • LBDF01LC - 2,3,7,8-tcdf comment code
  • LBDF02LC - 1,2,3,7,8-pncdf comment code
  • LBDF03LC - 2,3,4,7,8-pncdf comment code
  • LBDF04LC - 1,2,3,4,7,8-hcxdf comment code
  • LBDF05LC - 1,2,3,6,7,8-hxcdf comment code
  • LBDF06LC - 1,2,3,7,8,9-hxcdf comment code
  • LBDF07LC - 2,3,4,6,7,8-hxcdf comment code
  • LBDF08LC - 1,2,3,4,6,7,8-hpcdf comment code
  • LBDF09LC - 1,2,3,4,7,8,9-hpcdf comment code
  • LBDF10LC - 1,2,3,4,6,7,8,9-ocdf comment code
  • LBDHXCLC - 3,3',4,4',5,5'-hxcb comment code
  • LBDPCBLC - 3,3',4,4',5-pncb comment code
  • LBDTC2LC - 3,4,4',5-tcb comment code
  • LBDTCDLC - 2,3,7,8-tcdd comment code

National Health and Nutrition Examination Survey

2015-2016 Data Documentation, Codebook, and Frequencies

Polychlorinated dibenzo-p-dioxins (PCDDs), Dibenzofurans (PCDFs) & Coplanar Polychlorinated Biphenyls (cPCBs) - Pooled Samples (DOXPOL_I)

Data File: DOXPOL_I.xpt

First Published: July 2024

Last Revised: NA

Component Description

Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls (cPCBs) [coplanar] are similar classes of chlorinated aromatic chemicals that are produced as contaminants or byproducts. They have no known commercial or natural use. PCDDs and PCDFs, referred to as dioxins, are produced primarily during the incineration or burning of waste; the bleaching processes used in pulp and paper mills; and the chemical syntheses of trichlorophenoxyacetic acid, hexachlorophene, vinyl chloride, trichlorophenol, and pentachlorophenol. Both the synthesis and heat-related degradation of PCBs will produce PCDF by-products. PCBs are chlorinated aromatic hydrocarbon chemicals that were once synthesized for use as heat-exchanger, transformer, and hydraulic fluids, and also used as additives to paints, oils, window caulking, and floor tiles.

Production of PCBs peaked in the early 1970s and was banned in the United States after 1979. Together with the PCDDs and PCDFs, cPCBs are often referred to as “dioxin-like” chemicals because they act in the body through a similar mechanism.

In the environment, these dioxin-like chemicals are persistent and usually occur as a mixture of congeners (i.e., compounds that differ by the numbers and positions of chlorine atoms attached to the dibenzo-p-dioxin, dibenzofuran, or biphenyl structures). The general population is exposed to low levels of these dioxin-like chemicals primarily through ingestion of high-fat foods, such as dairy products, eggs, and animal fats, and some fish and wildlife.

Eligible Sample

Examined participants aged 12 years and older from a one-third sample were eligible.

Description of Laboratory Methodology

The seventeen PCDDs/PCDFs, and three cPCBs were measured in serum by gas chromatography/isotope-dilution high-resolution mass spectrometry (GC/ID-HRMS). The analytical method for PCDDs/PCDFs/cPCB is described in Patterson et. al., 1992 and Turner et. al., 1994.

Refer to the Laboratory Method Files 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

Polychlorinated dibenzo-p-dioxins and furans (PCDD/F) and co-planar polychlorinated biphenyls (cPCB) (July 2024)

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 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 Amendments 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 during “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 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’ 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.

Other data set: Additional PCB congeners are reported under data file name PCBPOL_I. Among these PCB congeners PCB 105, 114, 118, 156, 157, 167 and 189 has a toxic equivalence factor (TEF) and may be used with this data set for calculating a total TCDD equivalence.

Subsample Weights

PCDDs, PCDFs, and cPCBs were measured in a one-third subsample of participants 12 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.

Samples were pooled in groups of 8 samples per pool within 48 demographic groups. The analysis of NHANES 2015-2016 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 2015-2016 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.

The pooled-sample 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 design effect adjusted variance estimates for these pooled results. These data cannot be analyzed using software designed for complex surveys because design features are not available.

Rationale and Methods Used to Create Pooled Results

The 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 2015-2016, a weighted pooled-sample design was implemented to facilitate pooling samples before making analytical measurements. Table 1 lists the International Union of Pure and Applied Chemistry (IUPAC) names, common abbreviation, and NHANES variable name for the analytes included in this data file.

Pools were prepared using serum collected from the eligible sample. 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 48 demographic groups based on race and Hispanic origin (Mexican American: MA, Other Hispanic: OH, non-Hispanic white: NHW, non-Hispanic black: NHB, non-Hispanic Asian: NHA, Other race including multiracial: OTHER), gender (Male, Female), and age group (12-19, 20-39, 40-59, and 60+ years of age and older). Eight (8) samples were included in each pool. The number of pools created for each of the 48 demographic groups varied depending on the total number of individual samples available in a demographic group.The one-third subset of NHANES 2015-2016 represents 2,133 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, 1,989 samples were available to create 265 pools with 8 samples per pool. For NHANES 2015-2016, due to limited sample size, no pooled sample was formed for females 60 years and older in the “OTHER” race/Hispanic origin group. Data user needs to be cautious when making national inference including this population subgroup. 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 265. Please refer to the Pooled-Sample Technical Support file (POOLTF_I) 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 48 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 named WTSMSMPA.

Detection Limits

In the dataset, the whole-weight detection limit (fg/g serum) is a variable and dependent on the available sample size; however, the variation in sample size was low. Lipid adjusted detection limits are variable (pg/g lipid) due to differences of the lipid levels measured in individual pools/specimens. In cases where the result was below the limit of detection, the value for that variable is the detection limit divided by the square root of 2.

Table 1. NHANES Variable name, common abbreviation and International Union of Pure and Applied Chemistry (IUPAC) name for each analyte reported and maximum limit of detection (MLOD)

Variable Name	Common Abbrev.	IUPAC Name	Max LODa (fg/g serum)	Max LODa (pg/g lipid)
LBCTCD	2378-TeCDD	2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)	3.0	0.62
LBCD01	12378-PeCDD	1,2,3,7,8-Pentachlorodibenzo-p-dioxin	4.8	0.99
LBCD02	123478-HxCDD	1,2,3,4,7,8-Hexachlorodibenzo-p-dioxin	5.9	1.3
LBCD03	123678-HxCDD	1,2,3,6,7,8-Hexachlorodibenzo-p-dioxin	21	4.3
LBCD04	123789-HxCDD	1,2,3,7,8,9-Hexachlorodibenzo-p-dioxin	3.9	0.78
LBCD05	1234678-HpCDD	1,2,3,4,6,7,8-Heptachlorodibenzo-p-dioxin	29	7.1
LBCD07	OCDD	Octachlorodibenzo-p-dioxin	150	37
LBCF01	2378-TeCDF	2,3,7,8-Tetrachlorodibenzofuran	3.0	0.62
LBCF02	12378-PeCDF	1,2,3,7,8-Pentachlorodibenzofuran	6.9	1.4
LBCF03	23478-PeCDF	2,3,4,7,8-Pentachlorodibenzofuran	3.3	0.78
LBCF04	123478-HxCDF	1,2,3,4,7,8-Hexachlorodibenzofuran	9.9	2.1
LBCF05	123678-HxCDF	1,2,3,6,7,8-Hexachlorodibenzofuran	7.8	1.6
LBCF06	123789-HxCDF	1,2,3,7,8,9-Hexachlorodibenzofuran	11	2.2
LBCF07	234678-HxCDF	2,3,4,6,7,8-Hexchlorodibenzofuran	24	5
LBCF08	1234678-HpCDF	1,2,3,4,6,7,8-Heptachlorodibenzofuran	30	6.3
LBCF09	1234789-HpCDF	1,2,3,4,7,8,9-Heptachlorodibenzofuran	3.0	0.62
LBCF10	OCDF	Octachlorodibenzofuran	8.5	1.7
LBCTC2	PCB81	3,4,4´,5-Tetrachlorobiphenyl	20	4.2
LBCPCB	PCB126	3,3´,4,4´,5-Pentachlorobiphenyl	25	5.2
LBCHXC	PCB169	3,3´,4,4´,5,5´-Hexachlorobiphenyl	20	4.2

^aThe values given in the tables are the maximum limit of detection (MLOD) divided by the square root of 2 and are expressed as fg/g serum and pg/g lipid.

Table 2. Number of subjects in the one-third subsample, number of individual serum samples available, number of usable samples, and number of pools formed from NHANES 2015-2016 participants per demographic group

Race/Ethnicity	Gender	Number of subjects in the one-third subsample/ Number of Samples Available/Number of Usable Samples (Number of Pools)
		12–19 years (RIDAGEYR=1)1	20–39 years (RIDAGEYR=2)	40–59 years (RIDAGEYR=3)	60+ years (RIDAGEYR=4)
Mexican American (RIDRETH3=1) 1	Male (RIAGENDR=1) 1	49/49/471 (6)	57/56/56 (7)	31/31/312 (4)	44/43/40 (5)
	Female (RIAGENDR=2)	44/43/373 (5)	48/48/444 (6)	60/60/605 (8)	54/54/536 (7)
Other Hispanics (RIDRETH3=2)	Male (RIAGENDR=1)	23/23/227 (3)	36/36/32 (4)	35/35/348 (5)	31/31/299 (4)
	Female (RIAGENDR=2)	27/27/24 (3)	38/38/3710 (5)	39/39/3511 (5)	48/48/4712 (6)
Non-Hispanic White (RIDRETH3=3)	Male (RIAGENDR=1)	60/59/56 (7)	83/83/7713 (10)	81/80/7914 (10)	113/112/10815 (14)
	Female (RIAGENDR=2)	51/50/40 (5)	89/89/8416 (11)	75/75/72 (9)	104/104/10117 (13)
Non-Hispanic Black (RIDRETH3=4)	Male (RIAGENDR=1)	48/48/3918 (5)	54/53/5019 (7)	59/59/5520 (7)	57/57/5121 (7)
	Female (RIAGENDR=2)	38/38/2922 (4)	79/79/72 (9)	97/96/9223 (12)	54/54/5124 (7)
Non-Hispanic Asian     (RIDRETH3=6)	Male (RIAGENDR=1)	24/24/2025 (3)	41/41/3826 (5)	34/33/32 (4)	20/20/2027 (3)
	Female (RIAGENDR=2)	22/22/1928 (3)	34/34/32 (4)	40/40/3729 (5)	23/23/2130 (3)
Other Race  including  Multiracial (RIDRETH3=7)	Male (RIAGENDR=1)	15/15/1131 (2)	16/16/1632 (3)	16/16/1433 (2)	7/7/534 (1)
	Female (RIAGENDR=2)	9/9/8 (1)	12/12/1135 (2)	11/11/1136 (2)	13/13/1037 (2)

^1: 47 usable samples, one 6 sample pool, and one 8 sample pool; ^2: 31 usable samples, one 7 sample pool, and three 8 sample pool ; ^3: 37 usable samples, one 5 sample pool, and four 8 sample pool; ^4: 44 usable samples, one 4 sample pool, and five 8 sample pool ; ^5: 60 usable samples, one 4 sample pool, and seven 8 sample pool ; ^6: 53 usable samples, one 5 sample pool, and six 8 sample pool; ^7: 40 usable samples, one 6 sample pool, and two 8 sample pool ; ^8: 34 usable samples, one 2 sample pool, and four 8 sample pool; ^9: 29 usable samples, one 5 sample pool, and three 8 sample pool; ^10: 53 usable samples, one 5 sample pool, and four 8 sample pool; ^11: 35 usable samples, one 3 sample pool, and four 8 sample pool; ^12: 47 usable samples, one 7 sample pool, and five 8 sample pool; ^13: 77 usable samples, one 5 sample pool, and nine 8 sample pool; ^14: 79 usable samples, one 7 sample pool, and nine 8 sample pool; ¹⁵: 108 usable samples, one 4 sample pool, and thirteen 8 sample pool; ^16: 84 usable samples, one 4 sample pool, and ten 8 sample pool; ^17: 101 usable samples, one 5 sample pool, and twelve 8 sample pool; ^18: 39 usable samples, one 7 sample pool, and four 8 sample pool; ^19: 50 usable samples, one 2 sample pool, and six sample pool; ^20: 55 usable samples, one 7 sample, and six 8 sample pool; ^21: 51 usable samples, one 3 sample pool, and six 8 sample pool; ^22: 29 usable samples, one 5 sample pool, and three 8 sample pool; ^23: 92 usable samples, one 4 sample pool, and eleven 8 sample pool; ^24: 51 usable samples, one 3 sample pool, and six 8 sample pool; ^25: 20 usable samples, one 4 sample pool, and two 8 sample pool; ^26: 38 usable samples, one 6 sample pool, and four 8 sample pool; ^27: 20 usable samples, one 4 sample pool, and two 8 sample pool; ^28: 29 usable samples, one 3 sample pool, and two 8 sample pool; ^29: 37 usable samples, one 5 sample pool, and four 8 sample pool; ^30: 21 usable samples, one 5 sample pool, and two 8 sample pool; ^31: 11 usable samples, one 3 sample pool, and one 8 sample pool; ^32: 16 usable samples, one 2 sample pool, one 6 sample pool, and one 8 sample pool; ^33: 14 usable samples, one 6 sample pool, and one 8 sample pool; ^34: 5 usable samples, one 5 sample pool; ^35: 11 usable samples, one 3 sample pool, and one 8 sample pool; ^36: 11 usable samples, one 3 sample pool, and one 8 sample pool; ^37: 10 usable samples, one 2 sample pool, and one 8 sample pool.

Methodological approaches for analysis

There are a few pooled-sample methodological publications which address approaches related to the following topics:

    • Variance estimation and bias correction when pooling samples from log-normally distributed populations (Caudill, 2010a; Caudill, 2010b; Caudill et. al., 2007; Li et. al., 2014);

    • Models that incorporate measurement error when analyzing pooled-samples from normally or log-normally distributed populations (Caudill, 2010a; Caudill, 2015);

    • 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).

References

• Caudill SP, Characterizing populations of individuals using pooled samples. J Expo Sci Environ Epidemiol 2010a;20(1):29-37.
• Caudill SP, Confidence interval estimation for pooled-sample biomonitoring from a complex survey design. Environ Int 2015;85:40-45.
• Caudill SP, Important issues related to using pooled samples for environmental chemical biomonitoring. Stat Med 2010b;30(5):515-21.
• Caudill SP, Turner WE, and Patterson Jr. DG. Geometric mean estimation from pooled samples. Chemosphere 2007;69:371-80.
• Caudill SP, Use of pooled samples from the National Health and Nutrition Examination Survey. Stat Med 2012;31(27):3269-77.
• Caudill, S.P., Schleicher, R.L., Pirkle, J.L. Multi-rule quality control for the age-related eye disease study. Statist. Med. (2008) 27(20):4094-40106.
• Patterson DG Jr, Alexander LR, Turner WE, Isaacs SG, Needham LL. The development and application of a high resolution mass spectrometry method for measuring polychlorinated dibenzo p dioxins and dibenzofurans in serum. Chapter 6. In: Clement RE, Sui KM, Hill HH Jr, eds, Instrumentation for Trace Organic Monitoring. Chelsea, MI: Lewis Publishers, Inc. 1992:119-54.
• Turner WE, DiPietro ES, Cash TP, McClure PC, Patterson, DG Jr, Shirkhan H. An improved SPE extraction and automated sample cleanup method for serum PCDDs, PCDFs, and coplanar PCBs. Organohalogen Compounds 1994;19:31-5. http://www.dioxin20xx.org/pdfs/1994/94-213.pdf
• Westgard JO, Barry PL, Hunt MR, Groth T. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem 1981;27(3):493-501.

Codebook and Frequencies