NIST micronutrients measurement quality assurance program: characterizing the measurement community's performance over time.

The Micronutrients Measurement Quality Assurance Program (M2QAP) at the National Institute of Standards and Technology was created in 1984 with the goal of improving among-participant measurement comparability for fat-soluble vitamin-related compounds in human serum. We recently described improved tools for evaluating comparison exercise data; we here extend and apply these tools to the evaluation of the measurement community's performance over the entire 15-year history of the M2QAP. We here display measurement performance characteristics for the 14 measurands most commonly reported by the M2QAP community. We confirm that among-participant comparability for total beta-carotene cannot be much improved without improving average long-term within-participant measurement stability. We demonstrate that improved measurand definition and/or identification of interferences may help participants improve comparability for many of the M2QAP's other commonly reported measurands. The reported measurement performance characteristics may be of interest to clinical, nutritional, and epidemiological studies involving any of these measurands. The data analysis techniques utilized may be applicable to other programs.