Concepts of normality applied to the measurement of lung function.

Lung function testing has become an integral part of the clinical assessment of pulmonary disease. However, increasing application of such testing in research into the origins of disease as well as in the assessment of health raises the issue of what is meant by normal respiratory function. In essence, this depends on the sources of variation in lung function measurements that are of interest (the signal) and those that are not (the noise). For instance, clinicians are primarily interested in variation due to disease, all other sources of variation being considered noise. Physiologists are concerned with sources of variation other than disease, and the interest of epidemiologists (and their definition of normality) varies according to the specific objectives of each particular study, report or program. Consideration of the sources of variation in lung function (within a subject, between subjects, and between populations) not only is useful in clarifying the concepts of normality for clinical application, physiologic studies, and epidemiologic purposes, but also is mandatory for an understanding of lung function in the transition between health and disease, a major thrust in all three areas of endeavour.