• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Method Files
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • WTSA2YR - Subsample A weights
  • URXUCR - Creatinine, urine (mg/dL)
  • URX1DC - N-Acel-S-(1,2-dichlorovinl)-L-Cys(ng/mL)
  • URD1DCLC - N-Acel-S-(1,2-dichlorovinl)-L-Cys comt
  • URX2DC - N-Acel-S-(2,2-Dichlorvinyl)-L-Cys(ng/mL)
  • URD2DCLC - N-Acel-S-(2,2-Dichlorvinyl)-L-Cys comt
  • URX2MH - 2-Methylhippuric acid (ng/mL)
  • URD2MHLC - 2-Methylhippuric acid comment code
  • URX34M - 3-Methipurc acd & 4-Methipurc acd(ng/mL)
  • URD34MLC - 3-Methipurc acd & 4-Methipurc acid comt
  • URXAAM - N-Ace-S-(2-carbamoylethyl)-L-Cys(ng/mL)
  • URDAAMLC - N-Ace-S-(2-carbamoylethyl)-L-Cys comt
  • URXAMC - N-Ace-S-(N-methlcarbamoyl)-L-Cys(ng/mL)
  • URDAMCLC - N-Ace-S-(N-methlcarbamoyl)-L-Cys comt
  • URXATC - 2-Amnothiazolne-4-carbxylic acid(ng/mL)
  • URDATCLC - 2-Amnothiazolne-4-carbxylic acid comt
  • URXBMA - N-Acetyl-S-(benzyl)-L-Cysteine(ng/mL)
  • URDBMALC - N-Acetyl-S-(benzyl)-L-Cysteine comt
  • URXBPM - N-Acetyl-S-(n-propyl)-L-Cysteine(ng/mL)
  • URDBPMLC - N-Acetyl-S-(n-propyl)-L-Cysteine comt
  • URXCEM - N-Acetyl-S- (2-Carbxyethyl)-L-Cys(ng/mL)
  • URDCEMLC - N-Acetyl-S-(2-Carbxyethyl)-L-Cys comt
  • URXCYM - N-Acetyl-S-(2-cyanoethyl)-L-Cyst(ng/mL)
  • URDCYMLC - N-Acetyl-S-(2-cyanoethyl)-L-Cyst comt
  • URXDHB - N-Ace-S- (3,4-Dihidxybutl)-L-Cys(ng/mL)
  • URDDHBLC - N-Ace-S- (3,4-Dihidxybutl)-L-Cys comt
  • URXDPM - N-Ace-S-(dimethylphenyl)-L-Cys(ng/mL)
  • URDDPMLC - N-Ace-S-(dimethylphenyl)-L-Cys comt
  • URXGAM - N-Ac-S-(2-carbmo-2-hydxel)-L-Cys(ng/mL)
  • URDGAMLC - N-Ac-S-(2-carbmo-2-hydxel)-L-Cys comt
  • URXHEM - N-Ace-S-(2-Hydroxyethyl)-L-Cys(ng/mL)
  • URDHEMLC - N-Ace-S-(2-Hydroxyethyl)-L-Cys comt
  • URXHP2 - N-Ace-S-(2-hydroxypropyl)-L-Cys(ng/mL)
  • URDHP2LC - N-Ace-S-(2-hydroxypropyl)-L-Cys comt
  • URXHPM - N-Ace-S-(3-Hydroxypropyl)-L-Cys(ng/mL)
  • URDHPMLC - N-Ace-S-(3-Hydroxypropyl)-L-Cys comt
  • URXMAD - Mandelic acid(ng/mL)
  • URDMADLC - Mandelic acid comment code
  • URXMB1 - N-A-S-(1-HydrxMet)-2-Prpn)-L-Cys(ng/mL)
  • URDMB1LC - N-A-S-(1-HydrxMet)-2-Prpn)-L-Cys comt
  • URXMB2 - N-Ac-S-(2-Hydrxy-3-butnyl)-L-Cys(ng/mL)
  • URDMB2LC - N-Ac-S-(2-Hydrxy-3-butnyl)-L-Cys comt
  • URXMB3 - N-A-S-(4-hydrxy-2butn-l-yl)-L-Cys(ng/mL)
  • URDMB3LC - N-A-S-(4-hydrxy-2butn-l-yl)-L-Cys comt
  • URXPHE - N-Ace-S-(phenl-2-hydxyetl)-L-Cys(ng/mL)
  • URDPHELC - N-Ace-S-(phenl-2-hydxyetl)-L-Cys comt
  • URXPHG - Phenylglyoxylic acid(ng/mL)
  • URDPHGLC - Phenylglyoxylic acid comment code
  • URXPMA - N-Acetyl-S-(phenyl)-L-Cysteine(ng/mL)
  • URDPMALC - N-Acetyl-S-(phenyl)-L-Cysteine comt
  • URXPMM - N-A-S-(3-hydrxprpl-1-metl)-L-Cys(ng/mL)
  • URDPMMLC - N-A-S-(3-hydrxprpl-1-metl)-L-Cys comt
  • URXTCV - N-Acetyl-S-(trichlorovinyl)-L-Cys(ng/mL)
  • URDTCVLC - N-Acetyl-S-(trichlorovinyl)-L-Cys comt
  • URXTTC - 2-Thoxothazlidne-4-carbxylic acid(ng/mL)
  • URDTTCLC - 2-thoxothazlidne-4-carbxylic comt

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Volatile Organic Compound (VOC) Metabolites - Urine (UVOC_G)

Data File: UVOC_G.xpt

First Published: December 2013

Last Revised: November 2021

Component Description

Exposure to volatile organic compounds (VOCs) is ubiquitous. Chronic exposure to extremely high levels of some VOCs can lead to cancer and neurocognitive dysfunction. Urinary metabolites of VOCs can be detectable in urine for a longer period of time than the parent VOCs can be detected in blood.

Nearly 200 air toxics have been associated with adverse health effects in occupational studies or laboratory studies, but have not been monitored in general population groups. Information on levels of exposure to these compounds, as measured by their metabolite levels in urine, is essential to determine the need for regulatory mechanisms to reduce the levels of hazardous air pollutants to which the general population is exposed.

Eligible Sample

Examined participants aged 6 years and older from a one-third subsample were eligible.

Description of Laboratory Methodology

This method is a quantitative procedure for the measurement of VOC metabolites in human urine using ultra performance liquid chromatography coupled with electrospray tandem mass spectrometry (UPLC-ESI/MSMS) as described by Alwis et al., (2012). Chromatographic separation is achieved using an Acquity UPLC® HSS T3 (Part no. 186003540, 1.8 µm x 2.1 mm x 150 mm, Waters Inc.) column with 15 mM ammonium acetate and acetonitrile as the mobile phases. The eluent from the column is ionized using an electrospray interface to generate and transmit negative ions into the mass spectrometer. Comparison of relative response factors (ratio of native analyte to stable isotope labeled internal standard) with known standard concentrations yields individual analyte concentrations.

Refer to NHANES 2011-2012 Lab Methods for Volatile Organic Compounds (VOCs) and Metabolites - Urine for detailed description of the laboratory method used.

Laboratory Method Files

Volatile Organic Compounds (VOCs) and Metabolites - Urine (Updated July 2017)

Laboratory Quality Assurance and Monitoring

Urine specimens 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 2011-2012 Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–20°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 LPM.

Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured quality assurance evaluation during unscheduled visits to evaluate both the quality of the laboratory work and the QC procedures. Each laboratory staff person is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff. 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 CDC and contract laboratories. In the MEC, these methods include performing second examinations on previously examined participants and blind split samples collected on “dry run” sessions.

NCHS developed and distributed a QC protocol for all the contract laboratories, which outlined the Westgard rules used 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 Laboratory Sciences' QA/QC 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 2011-2012 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. For example, in 2011-2012, approximately 79% 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.

Subsample Weights

VOC metabolites in urine were assessed in a subsample of participants aged 6 years and older. Use the special one-third weights included in this data file when analyzing data. 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 with the key survey design and basic demographic variables. The NHANES 2011-2012 Demographic 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.

This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier SEQN.

Bias Adjustments Applied to VOC Metabolites in Urine Assay in NHANES 2011–2012 cycle 

Several systematic biases were discovered in measurements of certain urinary VOC metabolites in the 2005-2006 and 2011-2012 NHANES cycles. Upon further investigation, the following adjustments have been applied to the previously published data and included in the present dataset.

A. Adjustment to correct calibrator errors of the CDC lab prepared calibration materials

During the 2005–2006 and 2011–2012 NHANES cycles, calibrators used for tests on several VOC metabolites were prepared from a multi-analyte stock solution formulated from neat materials prepared by the testing lab at CDC. Beginning with the NHANES 2015–2016 cycle, calibrators and the multi-analyte stock solution were prepared externally. A systematic bias was observed between the cycles that used in-house calibration materials and the cycle using externally prepared calibration materials.  The systematic nature of these biases permitted retrospective adjustment through a statistical approach based on linear regression modeling where adjusted measurements are predicted from unadjusted measurements using suitable data. The “statistical recalibrations” implemented here have the general form of a univariate linear regression equation (accounting for dilution factors routinely used during measurement):

 

where exp[·] and ln(·) are the natural exponential and natural logarithm functions, respectively. Natural log-transformation of both the unadjusted and adjusted measurements moderated adverse influence on model fit from extreme measurements. The estimated regression slope and intercept are m and b, respectively, which are tabulated below for each analyte. The dilution factor was accounted for only once for each analyte that underwent more than one statistical recalibration (e.g., URXMB3). Statistical recalibration for the mass fraction of salt (URXHP2, URXGAM) only required a regression slope (i.e., b=0).

The explanatory power of the linear regressions was excellent (R²≥0.95), and residuals exhibited distributions with good approximation to normality, as well as stable variance over the observed range of measurements. These favorable diagnostics demonstrate the efficacy of statistical recalibration for the retrospective amelioration of systematic bias.

Correction for Variation in Preparation of Neat Native Metabolites Standards

Validations of the externally prepared calibrators suggested that the calibrators previously prepared by the CDC testing lab were inaccurate due to special requirements for handling and preparing accurate solutions from neat materials, some of which are highly hygroscopic. Below is the list of analytes affected and the corresponding adjustment factors used in correction equation. 

Analyte	m used in correction	b used in correction
URXAMC	0.99687	0.12843
URXCEM	1.01462	-0.13642
URXDHB	0.99654	0.04687
URXHEM	1.00947	0.30195
URXHPM	0.98949	-0.23802
URXMAD	1.00736	-0.18993
URXTTC	1.03057	-0.0746
URXMB3	0.97491	-0.43202
URX2DC	1.00155	-0.32374

Correction for Mass Fraction of Salt in Neat Compound Standard

Validations conducted by the CDC testing lab confirmed that the large mass fraction of salt in the neat materials was not accounted for during the formulation of calibration materials. This discrepancy between the calculated concentration of the calibrators and the actual concentration led to a systematic bias in results for URXHP2 and URXGAM. See below for the adjustment factors used in correction measures.

Analyte	m used in correction
URXHP2	0.54711990
URXGAM	0.57755045

Correction for Stereoisomerism in Urine Samples

UXMB3 occurs in human urine predominantly in the cis isomer, but quantitation in the NHANES 2005–2006 and NHANES 2011–2012 cycles used standards containing the trans isomer. This discrepancy led to a systematic bias in the URXMB3 results. Neat material that is purely cis isomer cannot be readily obtained, but a predictive regression equation can account for these quantitative differences.

Analyte	m used in correction	b used in correction
URXMB3	0.97741	-0.18775

B. Adjustment to correct calibrator errors of calibration materials prepared by an external vendor

Correction of N-Acetyl-S-(3-Hydroxypropyl-1-Methyl)-L-Cysteine (URXPMM)

In the 2005-2006 and 2011-2016 NHANES cycles, calibrators used to test N-Acetyl-S-(3-hydroxypropyl-1-methyl)-L-cysteine (URXPMM) were externally prepared, by a vendor, from a multi-analyte stock solution formulated from neat materials. In 2021, accuracy solutions from a new vendor indicated a systematic bias. Validations conducted by the CDC testing lab confirmed that the large mass fraction of salt in the neat materials was not accounted for during formulation of the externally prepared calibration materials from the previous vendor. This discrepancy between the calculated concentration of the calibrators and the actual concentration led to a systematic bias in results for URXPMM. An algebraic correction factor of 0.565 were applied to the original URXPMM values and released in the present file as below:

URXPMM = URXPMM_Original × 0.565

Detection limits

The detection limits were constant for the analytes in the data set. Two variables are provided for each of these analytes. The variable named ending in “LC” (ex., URDAAMLC) 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., URDAAMLC=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., URXAAM) provides the analytic result for the analyte.

The lower limit of detection (LLOD, in µg/L) for urinary VOC metabolites:

VARIABLE	Analyte	LLOD
URXAAM	Urinary N-Acetyl-S-(2-carbamoylethyl)-L-cysteine	2.20
URXAMC	Urinary N-Acetyl-S-(N-methylcarbamoyl)-L-cysteine	6.26
URXATC	Urinary 2-Aminothiazoline-4-carboxylic acid	15.0
URXBMA	Urinary N-Acetyl-S-(benzyl)-L-cysteine	0.500
URXBPM	Urinary N-Acetyl-S-(n-propyl)-L-cysteine	1.20
URXCEM	Urinary N-Acetyl-S-(2-carboxyethyl)-L-cysteine	6.96
URXCYM	Urinary N-Acetyl-S-(2-cyanoethyl)-L-cysteine	0.500
URX1DC	Urinary N-Acetyl-S-(1,2-dichlorovinyl)-L-cysteine	12.6
URX2DC	Urinary N-Acetyl-S-(2,2-dichlorovinyl)-L-cysteine	4.70
URXDHB	Urinary N-Acetyl-S-(3,4-dihydroxybutyl)-L-cysteine	5.25
URXDPM	Urinary N-Acetyl-S-(dimethylphenyl)-L-cysteine	0.500
URXGAM	Urinary N-Acetyl-S-(2-carbamoyl-2-hydroxyethyl)-L-cysteine	9.40
URXHEM	Urinary N-Acetyl-S-(2-hydroxyethyl)-L-cysteine	0.791
URXHPM	Urinary N-Acetyl-S-(3-hydroxypropyl)-L-cysteine	13.0
URXHP2	Urinary N-Acetyl-S-(2-hydroxypropyl)-L-cysteine	5.30
URXPMM	Urinary N-Acetyl-S-(3-hydroxypropyl-1-methyl)-L-cysteine	1.13
URXMAD	Urinary Mandelic acid	12.0
URX2MH	Urinary 2-Methylhippuric acid	5.00
URX34M	Urinary 3- and 4-Methylhippuric acid	8.00
URXMB1	Urinary N-Acetyl-S-(1-hydroxymethyl-2-propenyl)-L-cysteine	0.700
URXMB2	Urinary N-Acetyl-S-(2-hydroxy-3-butenyl)-L-cysteine	0.700
URXMB3	Urinary N-Acetyl-S-(4-hydroxy-2-butenyl)-L-cysteine	0.600
URXPHE	Urinary N-Acetyl-S-(phenyl-2-hydroxyethyl)-L-cysteine	0.700
URXPHG	Urinary Phenylglyoxylic acid	12.0
URXPMA	Urinary N-Acetyl-S-(phenyl)-L-cysteine	0.600
URXTCV	Urinary N-Acetyl-S-(trichlorovinyl)-L-cysteine	3.00
URXTTC	Urinary 2-Thioxothiazolidine-4-carboxylic acid	11.2

Withdrawn of t,t-Muconic acid (URXMUCA) Data

In November 2016, the t,t-Muconic acid (URXMUCA) data were withdrawn from the NHANES 2011-2012 cycle report due to an observed issue with the internal standard transition used for quantitation. During sample measurements, the absolute response of the internal standard decreased over time resulting in inconsistent intra-day t,t-Muconic acid (MUCA) concentrations. However, the change in absolute response in the unlabeled standards was small enough that the correlation coefficients of the calibration curves passed CDC testing lab’s quality control requirements. Further investigation by the CDC lab determined that the internal standard quantitation scheme used was not consistent during the 2011-2012 cycle, and the data could not be recalculated even after the issue was identified and corrected.

 

References

• Alwis KU, Blount BC, Britt AS, Patel D, Ashley DL. 2012. Simultaneous analysis of 28 urinary VOC metabolites using ultra high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (UPLC-ESI/MSMS). Anal Chim Acta 750:152-160.
• Caudill SP, Schleicher RL, Pirkle JL. Multi-rule quality control for the age-related eye disease study. Stat Med 2008;27:4094-106.
• Westgard J.O., Barry P.L., Hunt M.R., Groth T. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem (1981) 27:493-501.

Codebook and Frequencies