• 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
  • LBXSF1SI - 5-Methyl-tetrahydrofolate (nmol/L)
  • LBDSF1LC - 5-Methyl-tetrahydrofolate cmt
  • LBXSF2SI - Folic acid (nmol/L)
  • LBDSF2LC - Folic acid cmt
  • WTSA2YR - Environmental A two year weights

National Health and Nutrition Examination Survey

2007-2008 Data Documentation, Codebook, and Frequencies

Folate Forms - Individual - Serum (FOLFMS_E)

Data File: FOLFMS_E.xpt

First Published: May 2014

Last Revised: February 2019

Component Description

The objectives of this component are to: 1) provide data for monitoring secular trends in measures of nutritional status in the U.S. population; 2) evaluate the effect of people's habits and behaviors, such as physical activity and the use of alcohol, tobacco, and dietary supplements on nutritional status; and 3) evaluate the effect of changes in nutrition and public health policies including welfare reform legislation, food fortification policy, and child nutrition programs on the nutritional status of the U.S. population.

These data will be used to estimate deficiencies and toxicities of specific nutrients in the population and subgroup, to provide population reference data, and to estimate the contribution of diet, supplements, and other factors to serum levels of nutrients. Data will be used in research to further define nutrient requirements as well as optimal levels for disease prevention and health promotion.

Eligible Sample

Examined participants aged 1 year and older from a one-third sample were eligible.

Description of Laboratory Methodology

Different folate forms (5-methyl-tetrahydrofolate [5-methylTHF], folic acid, 5-formyl-tetrahydrofolate [5-formylTHF], tetrahydrofolate [THF], and 5,10-methenyl-tetrahydrofolate [5,10-methenylTHF]) were measured by the CDC Nutritional Biomarkers Laboratory using an isotope-dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method (Pfeiffer, et al. 2004; Fazili and Pfeiffer, 2004). Serum was combined with buffer and an internal standard mixture and sample clean-up was performed using automated solid phase extraction on phenyl cartridges in the 96-well plate format. Folates were eluted from the cartridge and analyzed by LC-MS/MS. Quantitation was by peak area ratio (analyte to internal standard) and was based on a six-point calibration curve in an aqueous medium.

The method performance was described in the proceedings of the 2010 Roundtable on “NHANES monitoring of biomarkers of folate and vitamin B-12 status” (Yetley, et al. 2011). Results were reported for two folate forms: 5-methylTHF, the major folate vitamer circulating in serum, and folic acid, which may appear in serum if intake of folic acid from fortified food or supplements exceeds 200 micrograms per meal. Results were not reported for the other three minor folate forms (5-formylTHF, THF, and 5,10-methenylTHF). What was measured as 5-formylTHF was in fact the sum of 5-formylTHF and an oxidation product of 5-methylTHF known as MeFox (pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF) (Fazili and Pfeiffer, 2013). More recently, the LC-MS/MS method has been improved to separate and accurately measure 5-formylTHF and MeFox in addition to the other folate forms (Fazili and Pfeiffer, 2013). This improved method has been used to generate data for all folate forms in a full sample of NHANES 2011–2012. Data for 5-methylTHF and folic acid in the current NHANES 2007–2008 one-third sample are comparable to the data in NHANES 2011–2012.

Refer to the Laboratory Method Files section for detailed description on the laboratory methods used.

Laboratory Method Files

Folate Vitamers (December 2018)

Laboratory Quality Assurance and Monitoring

Serum 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 Laboratory Procedure Manual (LPM). Vials are stored under appropriate frozen (-20^oC) 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 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 the 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 2007-2008 Laboratory Data Overview for general information on NHANES laboratory data.

Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on the use of sample weights and other analytic issues.

Subsample Weights

Serum folate forms were measured in a one-third subsample of persons 1 year 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.

NHANES Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2007-2008 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, the time of venipuncture, and the conditions precluding 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., LBDSF1LC) 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 LBX (ex., LBXSF1SI) provides the analytic result for that analyte. For analytes with analytic results below the lower limit of detection (ex., LBDSF1LC=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 nmol/L) for LBXSF1SI and LBXSF2SI are:

No Correction Needed for Serum Folic Acid Results for NHANES 2007–2008

For NHANES samples collected in the 2007-2008 cycle, the CDC Nutritional Biomarkers Laboratory measured serum 5-methyl-tetrahydrofolate and folic acid by isotope-dilution high performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) and the NHANES data were first published in May 2014. In 2015, the CDC laboratory discovered a calibration bias in the folic acid determination that resulted in overestimation of folic acid concentrations by about 25% as a result of solubility issues with the calibrator (see Analytic Notes for “Folate Forms – Serum” in NHANES 2011–2012). The laboratory corrected the assay and conducted a crossover study to assess whether folic acid data from NHANES 2007–2008, which were analyzed a few years prior to the NHANES 2011–2012, needed to be adjusted.

The CDC laboratory randomly selected approximately 10% of the NHANES 2007–2008 serum samples stratified by 4 time periods based on date of analysis (n = 300) and reanalyzed 25% of samples from each time period by random selection based on available sample volume (n = 75; concentration range 0.34–14.1 nmol/L). The Pearson correlation for new vs. original folic acid results was r = 0.989 and the weighted Deming regression equation did not show a significant slope or intercept (95% confidence interval for slope included 1 and for intercept included 0) (nmol/L):

New folic acid = 0.9570 * Original folic acid – 0.01359; 95% CI of slope (0.85 to 1.06) and intercept (-0.1016 to 0.07445).

Therefore, the NHANES 2007–2008 serum folic acid results did not have to be adjusted.

Comparability of Serum 5-methylTHF and Folic Acid Concentrations in NHANES 2007-2008 versus NHANES 1999-2002

Serum 5-methylTHF and folic acid concentrations were measured by Tufts University using HPLC with electrochemical detection in American seniors (≥60 y) in NHANES 1999–2002 as part of a surplus specimen project to assess the relationship to anemia, macrocytosis and cognitive test performance (Bailey, et al. 2010; Morris, et al. 2010). To assess method differences between Tufts and CDC, a method comparison study was performed in 2013 on 311 surplus serum specimens from American seniors (≥60 y) in NHANES 2001–2002. Results for serum 5-methylTHF compared well between the two methods: mean (95% CI, nmol/L): 47.7 (44.5–50.9) [CDC] and 48.6 (45.8–51.4) [Tufts]; median (95% CI, nmol/L): 42.9 (38.2–45.4) [CDC] and 45.0 (41.3–48.0) [Tufts]; Pearson r: 0.89; no bias based on Deming regression (using log-transformed data) and Bland Altman relative difference analysis. The CDC folic acid results in this method comparison study had to be corrected for the above mentioned calibration bias which occurred after 2011 and resulted in overestimation of folic acid concentrations by about 25% due to solubility issues with the calibrator (see analytic note for “Folate Forms – Serum ” in NHANES 2011–2012). The equation used to correct the CDC folic acid concentrations in this method study was: New folic acid = 0.7586 * Original folic acid – 0.00016. The CDC method detected folic acid in all samples (LOD = 0.09 nmol/L), while the detection rate of the Tufts method (LOD = 0.18 nmol/L) was ~35% in the full sample set (n = 1330; See the 2001-2002 Surplus Sera Folate data). Overall, the correlation was low in the surplus sample set (Pearson r: 0.64, n = 311). In the subset of samples that had detectable folic acid results by the Tufts method (n = 163) and that did not include extreme outliers (n = 156; difference between the two results was <20 nmol/L), the CDC folic acid results were on average comparable to the Tufts results: mean (95% CI, nmol/L): 3.71 (2.40–5.02) [CDC] and 3.11 (2.19–4.02) [Tufts]; median (95% CI, nmol/L): 1.10 (0.99–1.37) [CDC] and 1.20 (0.88–1.62) [Tufts]; Pearson r: 0.92; Deming slope using log-transformed data (95%CI): 0.89 (0.76 to 1.02); Deming intercept using log-transformed data (95% CI): 0.06 (0.00 to 0.12); Bland Altman relative difference (95% CI): 10.3% (-1.9% to 22.5%). Samples with sufficient sample volume (n = 17) were repeated by the CDC method; the repeat results confirmed the original results and were on average (SD) 5% (7%) higher for UMFA and 2% (5%) higher for 5-methylTHF. The findings from this method comparison study suggest that the NHANES 1999–2002 Tufts surplus results can be compared for 5-methylTHF to the NHANES 2007–2008 CDC results but the comparability for folic acid is limited due to the different detection rates for these two methods.  

References

• Bailey RL, Mills JL, Yetley EA, Gahche JJ, Pfeiffer CM, Dwyer JT, Dodd KW, et al. Unmetabolized serum folic acid and its relation to folic acid intake from diet and supplements in a nationally representative sample of adults aged ≥60 y in the United States. Am J Clin Nutr. 2010;92:383–9.
• Fazili Z and Pfeiffer CM. Accounting for an isobaric interference allows correct determination of folate vitamers in serum by isotope dilution-liquid chromatography-tandem mass spectrometry. J Nutr. 2013;143:108-113.
• Fazili Z and Pfeiffer CM. Measurement of folates in serum and conventionally prepared whole blood lysates: Application of an automated 96-well plate isotope-dilution tandem mass spectrometry method. Clin Chem 2004; 50: 2378-81.
• Morris MS, Jacques PF, Rosenbert IH, Selhub J. Circulating unmetabolized folic acid and 5-methyltetrahydrofolate in relation to anemia, macrocytosis, and cognitive test performance in American seniors. Am J Clin Nutr. 2010;91:1733–44.
• Pfeiffer CM, Fazili Z, McCoy L, Zhang M, and Gunter EW. Determination of folate vitamers in human serum by stable-isotope dilution tandem mass spectrometry and comparison to radioassay and microbiologic assay. Clin Chem 2004; 50: 423-32.
• Yetley EA, Pfeiffer CM, Phinney KW, Fazili Z, Lacher DA, Bailey RL, Blackmore S, et al. Biomarkers of folate status in the National Health and Nutrition Examination Survey (NHANES): a roundtable summary. Am J Clin Nutr. 2011; 94:303S-12S.

Codebook and Frequencies