National Health and Nutrition Examination Survey

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.

Participants aged 20 year and older who do not meet any of the exclusion criteria are eligible.

Total homocysteine (tHcy) in plasma is measured by the “Abbott Homocysteine (HCY) assay”, a fully automated fluorescence polarization immunoassay (FPIA) from Abbott Diagnostics (1,2). In brief, dithiothreitol (DTT) reduces homocysteine bound to albumin and to other small molecules, homocysteine, and mixed disulfides, to free thiol. S-adenosyl-homocysteine (SAH) hydrolase catalyzes conversion of homocysteine to SAH in the presence of added adenosine. In the subsequent steps, the specific monoclonal antibody and the fluoresceinated SAH analog tracer constitute the FPIA detection system (2). Plasma total homocysteine concentrations were calculated by the Abbott Axsym® using a machine-stored calibration curve (3,4).

An international round robin performed in 1998 (5) demonstrated that this method is fully equivalent to other most frequently used methods in this field (i.e., HPLC-FD, HPLC-ED, GC/MS). Thus, the Abbott Homocysteine (HCY) assay will be used as primary method for the determination of plasma total homocysteine in NHANES 2005-2006. The HPLC assay will be used as a reference method and will be performed on a subset of NHANES 1999+ for continuing method comparison and on smaller studies.

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

Detailed specimen collection and processing instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (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.

The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed quality control and quality assurance instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM). Read the LABDOC file for detailed QA/QC protocols.

Adjustment of Homocysteine Values:

During 2006, the manufacturer of the Homocysteine reagents (Abbott) distributed, without prior notice, a reformulated reagent kit that increased the Homocysteine values (on average about 13%). Crossover studies were performed on NHANES specimens to compare pre- and post-reformulation of the kits to develop regression equations to adjust post-kit reformulation values. The regression equations adjusted the post-kit reformulation values appropriately and were checked on sample person distributions before and after kit reformulation.

General Comments:

The analysis of NHANES 2005–2006 laboratory data must be conducted with the key survey design and basic demographic variables.

The NHANES 2005–2006 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.