• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Data Processing and Editing
• Laboratory Quality Assurance and Monitoring
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • LBXVIC - Vitamin C (mg/dL)
  • LBDVICSI - Vitamin C (umol/L)

National Health and Nutrition Examination Survey

2003-2004 Data Documentation, Codebook, and Frequencies

Vitamin C (L06VIT_C)

Data File: L06VIT_C.xpt

First Published: April 2006

Last Revised: October 2009

Component Description

The objectives of this component are: 1) to provide data for monitoring secular trends in measures of nutritional status in the U.S. population; 2) to evaluate the effect of people's habits and behaviors such as physical activity and the use of alcohol, tobacco, and dietary supplements on people's nutritional status; and 3) to 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 subgroups, 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 for research to further define nutrient requirements as well as optimal levels for disease prevention and health promotion.

Eligible Sample

Participants aged 6 years and older who do not meet any of the exclusion criteria are eligible. 

Description of Laboratory Methodology

Vitamin C (ascorbic acid) in serum is measured by isocratic HPLC with electrochemical detection at 650 mV. One part serum is mixed with four parts of 6% MPA to acidify the serum and stabilize the ascorbate. The specimen is frozen at –70°C until analysis. After the specimen is thawed at room temperature and centrifuged at 2500 rpm, the supernatant is decanted. This supernatant is mixed with a solution containing trisodium phosphate and dithiothreitol (to reduce dehydroascorbate to ascorbate) and then re-acidified with 40% MPA to stabilize the ascorbate. The sample is filtered to remove insoluble material. An aliquot is injected onto a C-18 reversed-phase column and eluted with a mobile phase containing 0.15 mol/L monochloroacetic acid, 2 mmol/L disodium ethylenediamine tetraacetate, and 0.13 mmol/L octylsulfonic acid, adjusted to pH 3.00 ± 0.05 with 10 N sodium hydroxide. Quantitation is by peak height and is based on a standard curve generated by using three different concentrations of an external standard (0.005, 0.030, and 0.100 mg/dL).A detailed description of the laboratory method used can be found on the NHANES website.

Data Processing and Editing

Serum specimens are processed, stored, and shipped to the Division of Laboratory Sciences, National Center for Environmental Health, and Centers for Disease Control and Prevention for analysis.

Detailed specimen collection and processing instructions are discussed in the NHANES LPM. Vials are stored under appropriate frozen (–20°C) conditions until they are shipped to National Center for Environmental Health for testing.

This file contains no top coding; however, 11 results were set to missing because of possible contamination.

One derived variable was created in this data file. The formula for its derivation is as follows:
The vitamin C in mg/dL was converted to µmol/L by multiplying by 56.78.Testing for vitamin C began in 2003.  Detailed instructions on specimen collection and processing can be found on the NHANES website.

Laboratory Quality Assurance and Monitoring

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 Laboratory/Medical Technologists Procedures Manual (LPM). Read the LABDOC file for detailed QA/QC protocols.

A detailed description of the quality assurance and quality control procedures can be found on the NHANES website.

Analytic Notes

Comparing Vitamin C in NHANES 2003-2004 to NHANES III (1988-1994)

An isocratic reverse-phase HPLC method with electrochemical detection of vitamin C (ascorbic acid) was used for NHANES 2003–2004 (McCoy et al, 2005) and a similar method was used for NHANES III during 1988-1994 (Gunter et al, 1996). The methods differed in several ways. For 2003–2004, but not for NHANES III, calibrators were treated as samples in the assay, i.e., subjected to the same sample preparation steps. Also, an internal standard was used in all sample preparations to correct results for recovery for NHANES 2003–2004 but not for NHANES III. Quality assurance data were compared to assess differences in serum vitamin C assay precision with the use of the two assay methods. NHANES III bench quality control (QC) data for 3 pools, ranging in concentration from 21.0 to 88.6 umol/L, had coefficients of variation (CV) between 5% and 8%. During NHANES 2003–2004, 3 bench QC pools, ranging from 13.6 to 122.7 umol/L, had CVs from 5% to 9%. Blind QC pools used in NHANES III ranged in concentration from 5.1 to 48.9 umol/L and had CVs from 51% to 74%. In NHANES 2003–2004, the blind QC pool concentrations ranged from 13.1 to 92.1 umol/L and had CVs of 5–9%. Split sample data (n=151) collected during NHANES III showed an average CV of 35%. Split sample data was not collected during NHANES 2003-2004. A comparison of the accuracy of methods was made with the use of NIST standard reference materials SRM 970 (Margolis et al, 1996). With the use of the NHANES III assay method, values were 92–93% of target values compared with 100–102% of target values with the NHANES 2003–2004 method (McCoy et al, 2005). With the use of 308 convenience specimens, the average bias between assay methods was 2.6% with the NHANES 2003–2004 method, giving higher values than the NHANES III method, however, the bias was concentration dependent with less bias at lower concentrations. For comparison with NHANES 2003– 2004, NHANES III data can be adjusted with the use of a Deming regression equation, where the adjusted values:
NHANES 2003-2004 Vitamin C = 1.0566 (NHANES III Vitamin C) -1.9345 using Vitamin C units of umol/L (McCoy et al, 2005).  See (Schleicher et al, 2009) for further details.
 
General Note on the Use of NHANES 2003-2004 Data:

The analysis of NHANES 2003–2004 laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES 2003–2004 Household Questionnaire Data Files contain demographic data, health indicators, and other related information collected during household interviews. The Household Questionnaire Data Files also contain all survey design variables and sample weights required to analyze these data. The Phlebotomy Examination file includes auxiliary information on duration of fasting, the time of day of the venipuncture, and the conditions precluding venipuncture. The Household Questionnaire and Phlebotomy Exam files may be linked to the laboratory data file using the unique survey participant identifier SEQN.

References

• Gunter EW, Lewis BG, Koncikowski SM. Laboratory procedures used for the Third National Health and Nutrition Examination Survey
  (NHANES III), 1988–1994. 1996. Available from: http://www.cdc.
  gov/nchs/data/nhanes/nhanes3/cdrom/nchs/manuals/labman.pdf (cited 8June 2009).
  
• Margolis SA, Duewer DL. Measurement of ascorbic acid in human
  plasma and serum: stability, intralaboratory repeatability, and interlaboratory reproducibility. Clin Chem 1996;42:1257–62.
  
• McCoy LF, Bowen MB, Xu M, Chen H, Schleicher RL. Improved
  HPLC assay for measuring serum vitamin C with 1-methyluric acid used as an electrochemically active internal standard. Clin Chem 2005;51:1062–4.
  
• Schleicher RL, Carroll MD, Ford ES, Lacher DA. Serum vitamin C and the prevalence of vitamin C deficiency in the United States: 2003-2004 National Health and Nutrition Examination Survey (NHANES). In press 2009, Am J Clin Nutr 2009;90(5):1252-63.

Codebook and Frequencies