National Health and Nutrition Examination Survey

Methylmalonic acid and Homocysteine

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.

Methylmalonic acid and Homocysteine

Participants aged 3 year and older who do not meet any of the exclusion criteria are eligible.

Methylmalonic acid

Methylmalonic acid (MMA) is extracted from plasma along with an added internal standard using an anion exchange resin. The extracted acid is then derivatized with cyclohexanol to form a dicyclohexyl ester. The derivatized samples are injected onto a gas chromatograph for separation from other constituents. The effluent from the gas chromatograph is monitored with a mass selective detector using selected ion monitoring. Results are quantitated by internal calibration using peak area ratios of MMA and the internal standard (d3MMA).

Increased concentrations of methylmalonic acid in plasma or serum and excessive urinary excretion of MMA are believed to be direct measures of tissue stores of cobalamin (vitamin B12) and to be the first indication of cobalamin deficiency. The concentration of MMA in plasma or serum was found to be a useful indicator of cobalamin deficiency, especially in patients with few or no hematological abnormalities, normal results for the Schilling test, or normal or only slightly depressed serum cobalamin concentrations. In folate deficiency, methylmalonic acid is normal. Methylmalonic acid may be elevated due to inborn errors of metabolism.

The range of methylmalonic acid in plasma from “healthy adults” is 0.05 to 0.26 umol/L.
An international round robin performed in 1999 demonstrated that this method is fully equivalent to the original method of Rasmussen, but also to the method of Marcell et al..

Homocysteine

The method is a fluorescence polarization immunoassay (FPIA) from Abbott Diagnostics. Total homocysteine (tHcy) in plasma is measured by the Abbott Homocysteine assay on the Abbott AxSym analyzer, a fully automated FPIA method. DTT reduces homocysteine bound to albumin and to other small molecules, homocysteine, and mixed disulfides, to free thiol. S-adenosylhomocysteine (SAH) hydrolase catalyzes conversion of homocysteine to SAH in the presence of added adenosine. The specific monoclonal antibody and the fluoresceinated SAH analog tracer constitute the FPIA detection system. Plasma total homocysteine concentrations are calculated by the Abbott AxSym Immunoassay Analyzer using a machine-stored calibration curve.

As part of ongoing methods comparisons studies, an international round robin was conducted in 1998. Results obtained using the FPIA method described earlier were compared to results obtained using high performance liquid chromatography (HPLC) with fluorometric detection at 385 nm excitation and 515 nm emission. The international round robin demonstrated that the FPIA method was fully equivalent to other frequently used methods (i.e., HPLC-FD, HPLC-ED, and GC/MS). Thus, the Abbott Homocysteine assay was used as the primary method for determination of plasma total homocysteine in NHANES 2003–2004.

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.

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.

Homocysteine method change:

The Homocsteine (uMol/L) method changed in 2002 from an Abbott IMX to an Abbott AxSym method. A crossover study was performed and revealed the following Deming regression (n=361, r**2 = 0.9817):

AxSym = 10**(0.983*log10(IMX) + 0.0418)