• 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
  • WTSSBH2Y - Surplus specimen B 13-14 2 year weights
  • SSNPFOA - Linear perfluorooctanoate (ug/L)
  • SDNPFOAL - Linear perfluorooctanoate Comment Code
  • SSBPFOA - Br. iso of perfluorooctanoate (ug/L)
  • SDBPFOAL - Br. iso perfluorooctanoate Comment Code
  • SSNPFOS - Linear perfluorooctane sulfonate (ug/L)
  • SDNPFOSL - Lin. perfluorooctane sulfonate Cmt. Code
  • SSMPFOS - Monomethyl branched iso of PFOS (ug/L)
  • SDMPFOSL - Monomethyl br. iso of PFOS Comment Code

National Health and Nutrition Examination Survey

2013-2014 Data Documentation, Codebook, and Frequencies

Perfluoroalkyl and Polyfluoroalkyl Substances - Linear and Branched PFOS and PFOA Isomers (Surplus) (SSPFAS_H)

Data File: SSPFAS_H.xpt

First Published: July 2016

Last Revised: April 2022

Component Description

NOTE: These four perfluoroalkyl and polyfluoroalkyl substances (PFAS) isomers were analyzed using surplus serum specimens from NHANES 2013-2014. This dataset should be used in conjunction with the Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS_H) dataset on this website. Starting with NHANES 2015-2016, these isomers will be included with the perfluoroalkyl and polyfluoroalkyl substances (PFAS) dataset.

You may also use the link below to access the dataset for PFAS_H:

https://wwwn.cdc.gov/nchs/nhanes/2013-2014/pfas_h.xpt

Synthesis of perfluoroalkyl and polyfluoroalkyl substances (PFAS) employed electrochemical fluorination (ECF) or fluorotelomerization. ECF, used from the 1950s until the early 2000s, yielded branched and linear isomers. By contrast, fluorotelomerization produces almost exclusively linear compounds (Vyas et al. 2007). The structural isomer patterns of perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) in humans may be useful for understanding routes and sources of exposure (Benskin et al. 2010). Therefore, concentrations of linear PFOA (n-PFOA), sum of branched isomers of PFOA (Sb-PFOA, branched PFOA isomers), linear PFOS (n-PFOS), and sum of perfluoromethylheptane sulfonate isomers (Sm-PFOS, monomethyl branched PFOS isomers) were measured in serum collected from eligible participants aged 12 years and older from NHANES 2013-2014.

The calculated sum of isomers in the PFAS (formerly PFC) dataset for the 2013-2014 cycle is comparable to the total levels reported in previous cycles of NHANES.

Eligible Sample

Participants aged 12+ years from NHANES 2013-2014 with stored serum.

Description of Laboratory Methodology

n-PFOA, Sb-PFOA, n-PFOS, and Sm-PFOS were quantified with on-line solid phase extraction coupled to isotope dilution-high performance liquid chromatography tandem mass spectrometry. The PFOS isomers known to be included in Sm-PFOS are perfluoro-3-methylheptane sulfonate, perfluoro-4-methylheptane sulfonate, perfluoro-5-methylheptane sulfonate, and perfluoro-6-methylheptane sulfonate. The PFOA isomers known to be included in Sb-PFOA are perfluoro-3-methylheptanoic acid, perfluoro-4-methyheptanoic acid, perfluoro-5-methyheptanoic acid, perfluoro-6-methyheptanoic acid, perfluoro-4,4-dimethylhexanoic acid, perfluoro-5,5-dimethylhexanoic acid, perfluoro-3,5-dimethylhexanoic acid, and perfluoro-4,5-dimethylhexanoic acid.

To estimate the total (sum of the isomers measured) concentrations of PFOS and PFOA for each participant, sum the concentrations of n-PFOA and Sb-PFOA for PFOA, and those of n-PFOS and Sm-PFOS for PFOS. If the value of one isomer is less than the limit of detection (LOD), then the imputed value is used. The imputed value is calculated as the LOD divided by the square root of 2 (Hornung et al. 1990) and imputed values are provided in the NHANES public release file. If the values of all isomers are reported as less than the LOD, then the sum will be the sum of each imputed value.

Laboratory Method Files

Perfluoroalkyl and Polyfluoroalkyl Substances (July 2016)

Laboratory Quality Assurance and Monitoring

The analytical measurements were conducted following strict quality control/quality assurance CLIA guidelines. Along with the study samples, each analytical run included high- and low-concentration quality control materials (QCs) and serum blanks to assure the accuracy and reliability of the data. The concentrations of the high-concentration QCs and the low-concentration QCs, averaged to obtain one measurement of high-concentration QC and low-concentration QC for each run, were evaluated using standard statistical probability rules (Caudill et al. 2008).

Data Processing and Editing

Data were received after all analyses were complete. The data were not edited.

Data Access: All data are publicly available.

Analytic Notes

Refer to the 2013-2014 Laboratory Data Overview for general information on NHANES laboratory 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 2013-2014 Demographic Data 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.

This laboratory data file can be linked to other NHANES data files using the unique survey participant identifier (i.e., SEQN).

Detection Limits

Detection limits were constant for all of the analytes in the data set. Two variables are provided for each analyte.

The variable name ending in “L” (ex., SDNPFOAL) 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 results below the lower limit of detection (ex., SDNPFOAL=1), an imputed fill value was placed in the results field. This value is the lower limit of detection divided by square root of 2 (LLOD/sqrt2).

The other variable prefixed SS (ex., SSNPFOA) provides the analytic result for each analyte.

The lower limit of detection (LLOD, in ng/mL) for the four PFAS isomers are:

Variable Name	SAS Label	LLOD
SSNPFOA	Linear perfluorooctanoate (ng/mL)	0.1
SSBPFOA	Branched isomers of perfluorooctanoate (ng/mL)	0.1
SSNPFOS	Linear perfluorooctane sulfonate (PFOS) (ng/mL)	0.1
SSMPFOS	Monomethyl branched isomers of PFOS (ng/mL)	0.1

Subsample Weights

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. Previous versions of this data file included a sample weight variable (WTSB2YR) but when observations were removed from this data file, a new sample weight was created, and a new sample weight variable was added to this data file (WTSSBH2Y). 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.

References

• Benskin JP, De Silva AO, Martin JW. 2010. Isomer Profiling of Perfluorinated Substances as a Tool for Source Tracking: A Review of Early Findings and Future Applications. Rev Environ Contam Toxicol:111-160
• Caudill SP, Schleicher RL, Pirkle JL. 2008. Multi-rule quality control for the age-related eye disease study. Statist Med 27: 4094-4106.
• Hornung RW, Reed LD. 1990. Estimation of average concentration in the presence of nondetectable values. Appl Occup Environ Hyg 5(1):46-51.
• Vyas SM, Kania-Korwel I, Lehmler HJ. 2007. Differences in the isomer composition of perfluoroctanesulfonyl (PFOS) derivatives. J Environ Sci Health A Tox Hazard Subst Environ Eng 42:249-255.

Codebook and Frequencies