Within- and Between-Subject Variation in Commonly Measured Anthropometric and Biochemical Variables (General Clinical Chemistry‪)‬

Physiological and metabolic studies commonly aim to describe the metabolic and biochemical status of individual subjects and to relate such variables to others, or to examine the effect of interventions. In many cases, it is assumed that the metabolic status of an individual may be represented by measurements taken on a single day, with allowance for confounding variables such as obesity where appropriate. Such studies often pay little attention to how such measurements fluctuate from day to day in healthy individuals. The importance of biological variation, however, is well recognized in the clinical chemistry literature (1) and may have several sources. These include assay variation (2, 3), variation in technique (e.g., blood sampling), day-to-day variation in the conditions under which the measurement is taken (2, 3), and underlying "biological" variation (4, 5), which reflects a multitude of exogenous and endogenous perturbations on biological systems and the degree to which they are homeostatically controlled. The present study examines the between-subject variation, within-subject day-to-day biological variation, and analytical variation of several metabolic variables commonly measured in biochemical and physiological studies in a group of resting, healthy individuals after an overnight fast, the condition in which most metabolic studies are performed. For several of these analytes, systematic studies of within-subject day-to-day variation have been performed (6), and we present our data for comparison as well as presenting new data for the others. We present other useful statistics derived from these measures of variation, including reference change values, indices of individuality ([I.sub.ind]) [4] and numbers of determinations required to estimate underlying within-subject values.