Inherent biological variation and reference values

There is a need for revisiting theoretical concepts and practical applications of conventional populationbased reference values to make for better clinical use of laboratory data. Knowledge of the underlying biological variation of quantities examined in medical laboratories is vital to understanding the proper generation and application of traditional populationbased reference values. Appreciation of the biological changes that occur over the span of life is a necessary prerequisite to deciding whether stratification of reference values according to age is likely to be necessary. Knowledge of the detail of predictable biological cyclical rhythms is required for correct clinical interpretation of laboratory data and appropriate collection of specimens at times relevant to the clinical purpose. Quantitative data on inherent within and betweensubject biological components of variation have shown the marked individuality of most quantities of interest in laboratory medicine. This individuality casts light on why examinations are not generally very successfully applied in population screening or casefinding. Consideration of individuality demonstrates why stratification of reference values is often very advantageous. Individuality provides an indisputable argument for better use of individual specific reference values.