• 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
  • WTFAS2YR - Fatty Acid Subsample 2 Year Weight
  • LBXCAP - Capric acid (C10:0) (umol/L)
  • LBDCAPLC - Capric acid (C10:0) comment code
  • LBXLAR - Lauric acid (C12:0) (umol/L)
  • LBDLARLC - Lauric acid (C12:0) comment code
  • LBXMR1 - Myristic acid (14:0) (umol/L)
  • LBDMR1LC - Myristic acid (14:0) comment code
  • LBXPEN - Pentadecanoic acid (C15:0) (umol/L)
  • LBDPENLC - Pentadecanoic acid (C15:0) comment code
  • LBXPM1 - Palmitic acid (16:0) (umol/L)
  • LBDPM1LC - Palmitic acid (16:0) comment code
  • LBXMRG - Margaric acid (C17:0) (umol/L)
  • LBDMRGLC - Margaric acid (C17:0) comment code
  • LBXST1 - Stearic acid (18:0) (umol/L)
  • LBDST1LC - Stearic acid (18:0) comment code
  • LBXAR1 - Arachidic acid (20:0) (umol/L)
  • LBDAR1LC - Arachidic acid (20:0) comment code
  • LBXDA1 - Docosanoic acid (22:0) (umol/L)
  • LBDDA1LC - Docosanoic acid (22:0) comment code
  • LBXTSA - Tricosanoic acid (C23:0) (umol/L)
  • LBDTSALC - Tricosanoic acid (C23:0) comment code
  • LBXLG1 - Lignoceric acid (24:0) (umol/L)
  • LBDLG1LC - Lignoceric acid (24:0) comment code
  • LBXML1 - Myristoleic acid (14:1n-5) (umol/L)
  • LBDML1LC - Myristoleic acid (14:1n-5) comment code
  • LBXPL1 - Palmitoleic acid (16:1n-7) (umol/L)
  • LBDPL1LC - Palmitoleic acid (16:1n-7) comment code
  • LBXVC1 - cis-Vaccenic acid (18:1n-7) (umol/L)
  • LBDVC1LC - cis-Vaccenic acid (18:1n-7) comment code
  • LBXOL1 - Oleic acid (18:1n-9) (umol/L)
  • LBDOL1LC - Oleic acid (18:1n-9) comment code
  • LBXEN1 - Eicosenoic acid (20:1n-9) (umol/L)
  • LBDEN1LC - Eicosenoic acid (20:1n-9) comment code
  • LBXNR1 - Nervonic acid (24:1n-9) (umol/L)
  • LBDNR1LC - Nervonic acid (24:1n-9) comment code
  • LBXLNA - Linoleic acid (18:2n-6) (umol/L)
  • LBDLNALC - Linoleic acid (18:2n-6) comment code
  • LBXALN - alpha-Linolenic acid (18:3n-3) (umol/L)
  • LBDALNLC - alpha-Linolenic acid (18:3n-3) cmnt code
  • LBXGLA - gamma-Linolenic acid (18:3n-6) (umol/L)
  • LBDGLALC - gamma-Linolenic acid (18:3n-6) cmnt code
  • LBXSD1 - Stearidonic acid (C18:4n-3) (umol/L)
  • LBDSD1LC - Stearidonic acid (C18:4n-3) comment code
  • LBXED1 - Eicosadienoic acid (20:2n-6) (umol/L)
  • LBDED1LC - Eicosadienoic acid (20:2n-6) comnt code
  • LBXHGL - homo-gamma-Linolenic acid(20:3n-6)
  • LBDHGLLC - homo-gamma-Linolenic acd (C20:3n-6)cmnt
  • LBXET1 - Eicosatrienoic acid (C20:3n-9) (umol/L)
  • LBDET1LC - Eicosatrienoic acid (C20:3n-9) comt code
  • LBXARA - Arachidonic acid (20:4n-6) (umol/L)
  • LBDARALC - Arachidonic acid (20:4n-6) comment code
  • LBXEPA - Eicosapentaenoic acid (20:5n-3) (umol/L)
  • LBDEPALC - Eicosapentaenoic acid (20:5n-3) cmt code
  • LBXDTA - Docosatetraenoic acid (22:4n-6) (umol/L)
  • LBDDTALC - Docosatetraenoic acid (22:4n-6) cmt code
  • LBXDP3 - Docosapentaenoic acid (22:5n-3) (umol/L)
  • LBDDP3LC - Docosapentaenoic acid (22:5n-3) cmt code
  • LBXDP6 - Docosapentaenoic acid (22:5n-6) (umol/L)
  • LBDDP6LC - Docosapentaenoic acid (22:5n-6) cmt code
  • LBXDHA - Docosahexaenoic acid (22:6n-3) (umol/L)
  • LBDDHALC - Docosahexaenoic acid (22:6n-3) cmnt code

National Health and Nutrition Examination Survey

2011-2012 Data Documentation, Codebook, and Frequencies

Fatty Acids - Serum (FAS_G)

Data File: FAS_G.xpt

First Published: January 2019

Last Revised: February 2020

Component Description

The analysis of individual plasma or serum fatty acids is important in the recognition of essential fatty acid deficiency (Siguel E. 1998) and in the differential diagnosis of inborn errors of metabolism, such as mitochondrial fatty acid oxidation disorders (Costa, et al. 1998; Jones, et al. 2000; Moser, et al. 1998). Long-chain polyunsaturated fatty acids are essential for normal development (Hamosh, et al. 1998). The dietary content of saturated, monounsaturated, and polyunsaturated fatty acids influence the concentration of cholesterol in low-density and high-density lipoproteins, and consequently the development of atherosclerosis (O’Keefe, et al. 1995). Regular consumption of or supplementation with omega-3 polyunsaturated fatty acids can have beneficial effects on long-term cardiovascular health due to anti-inflammatory and possibly antiarrhythmic effects (Kang, et al. 1996). The goal of this method is to obtain US reference ranges for most circulating fatty acids. Public health recommendations advise increasing or decreasing the intake of various classes of fatty acids (saturated, monounsaturated, polyunsaturated) but relatively little fatty acid biomarker data exist to support these recommendations and reference range data are scarce.

Eligible Sample

All examined participants aged 3 to 11 and participants aged 12 and over who were examined in the morning session were eligible.  

Description of Laboratory Methodology

Esterified fatty acids are hydrolyzed primarily from triglycerides, phospholipids and cholesteryl esters using sequential treatment with mineral acid and base in the presence of heat. Using a modification of (Lagerstedt, et al. 2001), total fatty acids are hexane-extracted from the matrix (100uL serum or plasma) along with an internal standard solution containing eighteen stable isotopically-labeled fatty acids to account for recovery.  The extract is derivatized with pentafluorobenzyl bromide (PFBBr) in the presence of triethylamine to form pentafluorobenzyl esters.  The reaction mixture is injected onto a capillary gas chromatograph column to resolve individual fatty acids of interest from other matrix constituents.  Fatty acids are detected using electron capture negative-ion mass spectrometry within 34 minutes.  Thirteen saturated, seven monounsaturated, and fifteen polyunsaturated fatty acids (thirty-five fatty acids in total) are measured using selected ion monitoring.  Quantitation is accomplished by comparing the peak area of the analyte in the unknown with the peak area of a known amount in a calibrator solution.  Calculations are corrected based on the peak area of the internal standard in the unknown compared with the peak area of the internal standard in the calibrator solution. 

Refer to the Laboratory Method 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 2011-2012 cycle.

Laboratory Method Files

Fatty Acids - Serum (January 2019)

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 testing. 

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials were stored under appropriate frozen (-30°C) conditions until they were 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 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.”

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 quality control protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard, 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.

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

The appropriate sample weights are provided in the variable WTFAS2YR in this data file for all participants and should be used when analyzing these data.

Serum fatty acids were measured for all examined participants aged 3 to 11 years, and a fasting subsample of participants 12 years and older. For participants aged 3 to 11 years, their WTFAS2YR are equivalent to their MEC exam sample weights. These participants have completed at least one physical exam component in the MEC; therefore, they all have an exam sample weight larger than “0,” regardless of their lab test results. For participants 12 years and older, special sample weights were created for the subsample. These special weights accounted for the additional probability of selection into the subsample, as well as the additional nonresponse to these lab tests. Therefore, if participants 12 years and older were selected as part of the fasting subsample but did not provide a blood specimen, they would have the subsample weight value assigned as “0” in their records.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2011-2012 Demographics 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.

The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast, and the time of venipuncture.

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

Detection Limits

The detection limits were constant for all of the analytes in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., LBDCAPLC) 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 other variable prefixed URX (ex., LBXCAP) provides the analytic result for that analyte. For analytes with analytic results below the lower limit of detection (ex., LBDCAPLC =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 lower limit of detection (LLOD in µmol/L) for serum fatty acids: 

Variable Name	SAS Label	LLOD (µmol/L)
LBXCAP	Capric acid (C10:0) (µmol/L)	1.59
LBXLAR	Lauric acid (C12:0) (µmol/L)	2.33
LBXMR1	Myristic acid (14:0) (µmol/L)	4.90
LBXPEN	Pentadecanoic acid (C15:0) (µmol/L)	0.75
LBXPM1	Palmitic acid (16:0) (µmol/L)	78.1
LBXMRG	Margaric acid (C17:0) (µmol/L)	3.36
LBXST1	Stearic acid (18:0) (µmol/L)	39.1
LBXAR1	Arachidic acid (20:0) (µmol/L)	0.82
LBXDA1	Docosanoic acid (22:0) (µmol/L)	0.68
LBXTSA	Tricosanoic acid (C23:0) (µmol/L)	0.90
LBXLG1	Lignoceric acid (24:0) (µmol/L)	1.09
LBXML1	Myristoleic acid (14:1n-5) (µmol/L)	0.29
LBXPL1	Palmitoleic acid (16:1n-7) (µmol/L)	6.56
LBXVC1	cis-Vaccenic acid (18:1n-7) (µmol/L)	2.31
LBXOL1	Oleic acid (18:1n-9) (µmol/L)	17.7
LBXEN1	Eicosenoic acid (20:1n-9) (µmol/L)	0.87
LBXNR1	Nervonic acid (24:1n-9) (µmol/L)	0.69
LBXLNA	Linoleic acid (18:2n-6) (µmol/L)	22.6
LBXALN	alpha-Linolenic acid (18:3n-3) (µmol/L)	1.54
LBXGLA	gamma-Linolenic acid (18:3n-6) (µmol/L)	0.42
LBXSD1	Stearidonic acid (C18:4n-3) (µmol/L)	0.24
LBXED1	Eicosadienoic acid (20:2n-6) (µmol/L)	0.31
LBXHGL	homo-gamma-Linolenic acid(20:3n-6) (µmol/L)	1.14
LBXET1	Eicosatrienoic acid (C20:3n-9) (µmol/L)	0.39
LBXARA	Arachidonic acid (20:4n-6) (µmol/L)	7.34
LBXEPA	Eicosapentaenoic acid (20:5n-3) (µmol/L)	0.79
LBXDTA	Docosatetraenoic acid (22:4n-6) (µmol/L)	0.31
LBXDP3	Docosapentaenoic acid (22:5n-3) (µmol/L)	0.55
LBXDP6	Docosapentaenoic acid (22:5n-6) (µmol/L)	0.24
LBXDHA	Docosahexaenoic acid (22:6n-3) (µmol/L)	1.84

 

Previous survey cycles provided limited information on selected fatty acids. Plasma fatty acids were measured in NHANES 2003-2004 as part of a surplus specimen project. Please see the NHANES web site for descriptions of the 2003-2004 surplus specimen fatty acid data.

References

• Caudill SP, Schleicher RL, Pirkle JL. 2008. Multi-rule quality control for the age-related eye disease study. Stat Med 27:4094-4106.
  
• Costa CG, Dorland L, Holwerda U, de Almeida IT, Poll-The BT, Jakobs C, Duran M. Simultaneous analysis of plasma free fatty acids and their 3-hydroxy analogs in fatty acid β-oxidation disorders. Clin Chem 1998; 44(4):463-471.
• Hamosh M, Salem N Jr. Long chain polyunsaturated fatty acids. Biol Neonate 1998;74:106-120.
• Jones PM, Quinn R, Fennessy PV, Tjoa S, Goodman SI, Fiore S, Burlina AB, Rinaldo P, Boriack RL, Bennett MJ. Improved stable isotope dilution gas chromatography mass spectrometry method for serum or plasma free 3-hydroxy-fatty acids and its utility for the study of disorders of mitochondrial fatty acid β-oxidation. Clin Chem 2000; 46:149-155.
• Kang JX, Leaf A. The cardiac antiarrhythmic effects of polyunsaturated fatty acids. Lipids 1996; 31: S41-S44.
• Lagerstedt S, Hinrichs D, Batt S, Magera M, Rinaldo P, McConnell J. Quantitative Determination of Plasma C8-C26 Total Fatty Acids for the Biochemical Diagnosis of Nutritional and Metabolic Disorders. Mol Genet & Metab 2001; 73: 38-45. 
• Moser AB, Kreiter N, Bezman L, Lu S, Raymond CV, Naidu S, Moser HW. Plasma very long chain fatty acid assay in 3,000 peroxisome disease patients and 29,000 controls. Ann Neurol 1998; 45:100-110.
• O’Keefe JH, Nguyen T, Nelson J, O’Keefe JO, Miles JM. Potential beneficial effects of monounsaturated and polyunsaturated fats in elderly patients with or at high risk of coronary artery disease. Cardiol Elderly 1995;3: 5-10.
• Siguel E. Diagnosing essential fatty acid deficiency. Circ 1998; 97:2580-2583.
• Westgard J.O, Barry P.L., Hunt, M.R., Groth R. A Multi-rule quality control for the age-related eye disease study. Statis. Med (2008) 27(20):4094-40106.

Codebook and Frequencies