Submitted to: Analytical and Bioanalytical Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/17/2007
Publication Date: 6/22/2007
Citation: Phillips, K., Wolf, W.R., Sharpless, K., Holden, J.M. 2007. Reference materials for determination of the nutrient composition of foods: Results from USDA's National Food and Nutrient Analysis Program (NFNAP). Analytical and Bioanalytical Chemistry. Available: doi:10.2007/s00216-007-1366-0. Interpretive Summary: The National Food and Nutrient Analysis Program (NFNAP) at the USDA has been designed to determine the nutrient content in a wide range of foods. Most of the analyses of these foods for the program have been performed by major U.S. commercial laboratories. Food matrix certified reference materials (CRMs) with a known nutrient content have been sent to the laboratories to determine if the analyses are performed accurately. Over a 6.5 year period, a total of 2,554 values reported by nine laboratories for 259 certified concentrations in 26 CRMs were obtained. From these data it is possible to evaluate proficiency of the overall commercial laboratory measurement system for a specific nutrient. There were some nutrients that were done well by essentially all the laboratories such as magnesium and vitamin B12. Vitamin C and total fat were done moderately well, but there appeared to be some analytical problems with some laboratories measuring unsaturated fatty acids, thiamin in meat, and sodium when the amount in the sample is very low. Further work will help to identify the various reasons for the difficulties in making the measurements. This information will be used to make recommendations for improvement of the public and private sector measurement systems for nutrients in foods to be used in generating accurate and precise analytical food composition data to be included in the USDA National Nutrient Database for Standard Reference and other applications.
Technical Abstract: Certified reference materials (CRMs) play a critical role in validating the accuracy of nutrient data for food samples. A number of available food CRMs of differing matrix composition have assigned concentrations for various nutrients, along with associated uncertainty intervals (UIs) for those values. These CRMs have been used extensively in the USDA's ongoing National Food and Nutrient Analysis Program (NFNAP) to monitor the accuracy of nutrient assays in a wide range of foods. Samples of the CRMs were submitted to contract laboratories, blinded, as part of the qualifying process for analytical contracts, and in the routine sample stream. Over a 6.5-year period (1999-2006) a total of 2,554 values, for over 100 different food components, were reported by nine laboratories for 259 certified or reference nutrient concentrations in 26 CRMs. Each reported nutrient value was converted to a Z-score, which is its difference from the CRMs certified or reference value related to the combined expected analytical uncertainty plus the uncertainty in the CRM assigned value. A low Z score (< 2.0) reflects good agreement with the assigned value. Z-scores from |2.0| to |3.0| were considered questionable, and those greater than |3.0| unacceptable. For some nutrients [sodium, folate, dietary fiber, pantothenic acid, thiamin, tocopherols, carotenoids, monounsaturated and polyunsaturated fatty acids], more than 20% of Z-scores exceeded |3.0|. For total fat, vitamin C, and niacin, greater than 25% of Z-scores were greater than |2.0|. Components for which CRM data were best (greater than 90% of Z-scores less than|2.0|) were magnesium, phosphorous, manganese, selenium, and vitamin B12. In some cases deviations from assigned CRM values were not uniform across laboratories and materials. In the case of sodium, almost all high Z-scores were for low-sodium matrices, suggesting analytical problems related to concentration. Further work will explore these and additional control sample data from the NFNAP, to understand the implications of various factors (matrix, laboratory, method, etc.) in generating accurate and precise analytical food composition data to be included in the USDA National Nutrient Database for Standard Reference and other applications.