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Effect of ethnicity on reference intervals.

The clinical chemist desiring to set up appropriate reference intervals must consider race or ethnicity as a possible factor on which to subdivide an interval. The effect of ethnicity on various analyte values has not been systematically studied (1)(2). The advent of National Health and Nutrition Examination Survey (NHANES) data (3) has made it possible to determine the effect of ethnicity on reference interval estimation and whether separate reference intervals should be derived for each ethnic group.

The Third National Health and Nutrition Examination Survey (NHANES III) contains data for 33 994 persons ages 2 months and older who participated in the survey. Clinical chemistry measurements were made on several analytes, and health status was determined by a physician. In this report, we will use only individuals with the “Excellent”rating of health status as recommended by the IFCC (4) and NCCLS (5).

The issue of whether to use separate reference intervals has been addressed by Harris and Boyd (6). Their recommendation, which is used by the NCCLS (5), is that two groups should be combined into a single group unless their means and/or SDs exceed appropriate predetermined thresholds. By treating the known healthy individuals as a “gold standard”, the effect of the addition of ethnicity on these estimators will be examined. We used a previously described outlier detection scheme (7) to reduce the effect of atypical observations on these estimators.

The NHANES population was obtained from NHANES 3x Ver. 1.21 (3). The data are the result of a complex survey design involving stratification and clustering that yielded individual weights for each observation. However, for our purposes in this report, we will treat the individuals as coming from a random sample, i.e., individual weights …