Use of cholesterol reference materials in a nation wide study of the cholesterol content of eggs

Summary National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1563-2, Cholesterol and Fat-Soluble Vitamins in Coconut Oil, and SRM 1845, Cholesterol in Whole Egg Powder, were used to monitor the accuracy and precision of analyses in a recent nationwide study of cholesterol in eggs. A two-phase study was conducted between June, 1988 and May, 1989 by USDA in collaboration with the egg industry to update the USDA nutrient data for eggs. Determination of cholesterol levels received special emphasis since dietary cholesterol is monitored as part of the assessment of the relationship between diet and coronary disease. During each phase of the study eggs were collected from 122 of the top 200 US egg handlers in six regions of the country, representing 67% of the nation's monthly egg production. During Phase I egg yolks were composited into 24 analytical samples. As a result of low variance in cholesterol values for Phase I, egg yolks in Phase II were composited into 6 samples. The selection of the analytical contractor was based on results of analyses of SRM 1563-2, a USDA frozen egg material, and a whole egg powder material (SRM 1845, candidate status) submitted by several laboratories. In all cases the results by the selected contractor were within 1% of the certified or recommended values. The contractor used SRM 1563-2 as well as an in-house cholesterol-spiked oil research material to validate the modified AOAC method which incorporated ethanolic potassium hydroxide saponification followed by gas chromatography. During the analysis of nationwide samples the contractor analyzed SRMs 1563-2 and 1845, as well as the cholesterol-spiked oil and the USDA frozen egg material to monitor accuracy and precision. Over the course of the study coefficients of variation for all materials were less than 5%. As a result of rigorous quality control the cholesterol results for eggs have been regarded with confidence and used to update various public and private data bases used to monitor dietary cholesterol levels.