BACKGROUND: In a previous study to determine the feasibility of common reference intervals in Asia, we found significant differences among populations from 6 cities. In this study, we attempted to define the sources of these differences. METHODS: We enrolled 580 healthy volunteers (279 men, 301 women, 20 -62 years old), after a selection process that was based on the Clinical and Laboratory Standards Institute guidelines, and used a lifestyle questionnaire. All sera were obtained at a basal

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... e and frozen at Ϫ80°C until the collective assay was done. We measured 21 basic chemical analytes and 10 serum proteins. RESULTS: We used 3-level nested ANOVA to separate the variation (SD) into between-city (SD-city), between-sex (SD-sex), between-age (SD-age), and between-individual (SD-indiv) components. SD-indiv corresponds to one-quarter of the "pure" reference interval obtained after removing variations due to city, sex, and age. The SD-sex to SD-indiv ratio was Ͼ0.8 for creatinine, urate, retinol-binding protein, and transthyretin. We observed high SD-city to SD-indiv ratios, ranging from 0.4 to 0.7, for 11 analytes including lactate dehydrogenase (LDH), electrolytes, IgG, and complement components and SD-age to SD-indiv ratios Ͼ0.4 for LDH, alkaline phosphatase, and total cholesterol. Multiple regression analysis demonstrated several other relevant sources of variation, including body mass index, alcohol consumption, and cigarette smoking, although their contributions were generally smaller than those for sex, region, or age. CONCLUSION: We observed unacceptably large regional differences in measured values of some analytes even after adjustment for age, sex, and lifestyle variables. Genetic and environmental factors may account for the residual differences. Two of the stated goals for the preparation of the international serum protein reference material CRM470 7 were to decrease between-manufacturer variance in assays and to determine the feasibility of common reference materials (1-4 ) . In the previous Asian Reference Interval Project, we used a protocol developed by the IFCC Committee on Plasma Proteins (5 ) to explore the feasibility of using common reference intervals (RIs) for these proteins in different geographical regions in Asia. Contrary to our expectation, we found large differences in test results for individuals from 6 Asian cities, not only for major serum proteins but also for other commonly measured biochemical analytes. The analytical results were comparable because all specimens were freshly collected and measured collectively in one laboratory. Unfortunately, we did not obtain detailed demographic information from the donors and were unable to analyze factors possibly accounting for the differences.