Table of Contents

Component Description

Vitamin C (ascorbic acid) is a water-soluble vitamin, which has very low toxicity.  It is recommended for the prevention and treatment of scurvy, which is uncommon in the United States (U.S). In excess, vitamin C has shown to cause diarrhea and kidney stones. 

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

Examined participants aged 6 years and older were eligible.

Description of Laboratory Methodology

Vitamin C (ascorbic acid) in serum is measured using isocratic ultra‐high performance liquid

chromatography (UPLC) with electrochemical detection at 450 mV (range 200 nA). One-part serum is mixed with four parts 6% metaphosphoric acid (MPA) to acidify the serum and stabilize ascorbate. The specimen is frozen at ‐70 °C until analysis. After the specimen is thawed at room temperature and centrifuged at 3,000 rpm, the supernatant is decanted. This supernatant is mixed with a solution containing trisodium phosphate and dithiothreitol (to reduce dehydroascorbate to ascorbate) and an internal standard (1‐methyl uric acid) to reduce dehydroascorbate to ascorbate. It is re‐acidified with 40% MPA to stabilize the ascorbate. The sample is filtered to remove insoluble material. A 4 μL aliquot is injected onto a C‐18 reversed‐phase column and eluted with a mobile phase containing 14.1 g/L monochloroacetic acid, 0.76 g/L disodium ethylenediamine tetraacetate, 1% (by volume) 10 N sodium hydroxide, and 1.5% (by volume) methanol, adjusted to pH 3.00 ± 0.03 with 10 N sodium hydroxide.  Quantitation is accomplished by comparing the peak area of vitamin C 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.

This component was last included in the NHANES 2005-2006 survey cycle.

Laboratory Method Files

Vitamin C Laboratory Procedure Manual (June 2020)

Laboratory Quality Assurance and Monitoring

Serum specimens 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 analysis.

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 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 Act 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 QC 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 QC 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’ QA/QC 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.

One variable was created in this data file. The variable LBDVICSI was created using the formula:

LBDVICSI: The vitamin C value in mg/dL (LBXVIC) was converted to umol/L (LBDVICSI) by multiplying LBXVIC by 56.78 (rounded to 3 significant figures).

Analytic Notes

Refer to the 2017-2018 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 2017-2018, approximately 80% 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 details on the use of sample weights and other analytic issues.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-2018 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 “LC” (ex., LBDVICLC) 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., LBXVIC) provides the analytic result for that analyte.  For analytes with analytic results below the lower limit of detection (ex., LBDVICLC=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 mg/dL) for LBXVIC: 

Variable Name

SAS Label

LLOD

LBXVIC

Vitamin C

0.03 mg/dL

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 6 YEARS - 150 YEARS

LBXVIC - Vitamin C (mg/dL)

Variable Name:
LBXVIC
SAS Label:
Vitamin C (mg/dL)
English Text:
Vitamin C (mg/dL)
Target:
Both males and females 6 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.0212 to 14.6 Range of Values 6740 6740
. Missing 695 7435

LBDVICSI - Vitamin C (umol/L)

Variable Name:
LBDVICSI
SAS Label:
Vitamin C (umol/L)
English Text:
Vitamin C (umol/L)
Target:
Both males and females 6 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
1.2 to 829 Range of Values 6740 6740
. Missing 695 7435

LBDVICLC - Vitamin C comment code

Variable Name:
LBDVICLC
SAS Label:
Vitamin C comment code
English Text:
Vitamin C comment code
Target:
Both males and females 6 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above detection limit 6738 6738
1 Below lower detection limit 2 6740
. Missing 695 7435