Table of Contents

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

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.

Subsample weights

n-PFOA, Sb-PFOA, n-PFOS, and Sm-PFOS were measured in a one third subsample of persons 12 years and older. Special sample weights, included in this data file, are required to analyze these data properly.

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

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

Codebook and Frequencies

SEQN - Respondent sequence number

Variable Name:
SEQN
SAS Label:
Respondent sequence number
English Text:
Respondent sequence number.
Target:
Both males and females 12 YEARS - 150 YEARS

WTSB2YR - Subsample B Weights

Variable Name:
WTSB2YR
SAS Label:
Subsample B Weights
English Text:
Subsample B Weights
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
17149.00472 to 482310.7883 Range of Values 2165 2165
0 No Lab Result 0 2165
. Missing 0 2165

SSNPFOA - Linear perfluorooctanoate (ug/L)

Variable Name:
SSNPFOA
SAS Label:
Linear perfluorooctanoate (ug/L)
English Text:
Linear perfluorooctanoate (ug/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.07 to 85.2 Range of Values 2165 2165
. Missing 0 2165

SDNPFOAL - Linear perfluorooctanoate Comment Code

Variable Name:
SDNPFOAL
SAS Label:
Linear perfluorooctanoate Comment Code
English Text:
Linear perfluorooctanoate Comment Code
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2148 2148
1 Below lower detection limit 17 2165
. Missing 0 2165

SSBPFOA - Br. iso of perfluorooctanoate (ug/L)

Variable Name:
SSBPFOA
SAS Label:
Br. iso of perfluorooctanoate (ug/L)
English Text:
Br. iso of perfluorooctanoate (ug/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.07 to 3 Range of Values 2165 2165
. Missing 0 2165

SDBPFOAL - Br. iso perfluorooctanoate Comment Code

Variable Name:
SDBPFOAL
SAS Label:
Br. iso perfluorooctanoate Comment Code
English Text:
Br. iso perfluorooctanoate Comment Code
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 408 408
1 Below lower detection limit 1757 2165
. Missing 0 2165

SSNPFOS - Linear perfluorooctane sulfonate (ug/L)

Variable Name:
SSNPFOS
SAS Label:
Linear perfluorooctane sulfonate (ug/L)
English Text:
Linear perfluorooctane sulfonate (ug/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.07 to 1270 Range of Values 2165 2165
. Missing 0 2165

SDNPFOSL - Lin. perfluorooctane sulfonate Cmt. Code

Variable Name:
SDNPFOSL
SAS Label:
Lin. perfluorooctane sulfonate Cmt. Code
English Text:
Lin. perfluorooctane sulfonate Cmt. Code
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2146 2146
1 Below lower detection limit 19 2165
. Missing 0 2165

SSMPFOS - Monomethyl branched iso of PFOS (ug/L)

Variable Name:
SSMPFOS
SAS Label:
Monomethyl branched iso of PFOS (ug/L)
English Text:
Monomethyl branched iso of PFOS (ug/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.07 to 133 Range of Values 2165 2165
. Missing 0 2165

SDMPFOSL - Monomethyl br. iso of PFOS Comment Code

Variable Name:
SDMPFOSL
SAS Label:
Monomethyl br. iso of PFOS Comment Code
English Text:
Monomethyl br. iso of PFOS Comment Code
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above the detection limit 2131 2131
1 Below lower detection limit 34 2165
. Missing 0 2165