Reference materials to evaluate measurement systems for the nutrient composition of foods: results from USDA's National Food and Nutrient Analysis Program (NFNAP).

Over a 6.5-year period a total of 2554 values were reported by nine laboratories for 259 certified or reference nutrient concentrations in 26 certified reference materials (CRM) submitted to contract laboratories, blinded, as part of the qualifying process for analytical contracts and in the routine sample stream as part of the National Food and Nutrient Analysis Program. Each value was converted to a Z'-score, reflecting the difference from the assigned value related to the combined expected analytical uncertainty plus the uncertainty in the CRM value. Z'-scores >/3.0/ were considered unacceptable. For some nutrients (Na, folate, dietary fiber, pantothenic acid, thiamin, tocopherols, carotenoids, monounsaturated, and polyunsaturated fatty acids), >20% of Z'-scores were >/3.0/. For total fat, vitamin C, and niacin >25% of Z'-scores were >/2.0/. Components for which CRM data were best (more than 90% of Z'-scores </2.0/) were Mg, P, Mn, Se, and vitamin B12. In some cases deviations from assigned values were not uniform across laboratories and materials. For Na almost all high Z'-scores were for low-Na matrices, suggesting analytical problems related to concentration. Figure Z'-scores for vitamins in certified reference materials.