NIST NCI MICRONUTRIENTS MEASUREMENT QUALITY ASSURANCE PROGRAM - MEASUREMENT REPEATABILITIES AND REPRODUCIBILITIES FOR FAT-SOLUBLE VITAMIN-RELATED COMPOUNDS IN HUMAN SERA/

The NIST/NCI Micronutrient Measurement Quality Assurance Program has c onducted 33 interlaboratory comparison exercises for fat-soluble vitam in-related compounds in human sera over the past 12 years, Periodic re analysis of lyophilized serum samples prepared from more than 70 diffe rent sera has enabled estimation of the short- and long-term measureme nt characteristics. Median- and interquartile-range-based statistics a dequately estimate the distribution of results from laboratories that are in analytical control from total distributions that include a sign ificant minority of outlier data, Short-term interlaboratory reproduci bility standard deviations (SDs) are predictable functions of analyte concentration, with an asymptotic limit at low analyte concentration a nd a linear relationship at high concentrations, Long-term trends in t he interlaboratory reproducibility can be estimated by standardizing t he short-term SD at the observed analyte concentration to an expected SD at a given physiologically significant analyte concentration. The ' 'average'' laboratory's same-day analytical repeatability SD is about one-third of the estimated interlaboratory reproducibility; repeatabil ity for longer periods between analyses is, on average, no better than the reproducibility. While a few exceptional laboratories have mainta ined excellent repeatability over the entire decade, long-term study m easurements generated within a single laboratory are not generally mor e internally consistent than results from multiple laboratories, Enhan ced and more consistently implemented intralaboratory quality control and quality assurance methods are required to further improve and main tain interlaboratory measurement comparability.