Intraindividual variation and effect of learning in lung function examinations. A population study.

The aims of this study were to analyse the influence of co-operation and instruction on inter-and intraindividual variations of normal lung function and to provide reference values for intraindividual variation. A sample of 125 non-smokers (age 30-70 yr) representative of the population of Copenhagen were studied with spirometry, complete and partial flow-volume curves, nitrogen washout volume, volume of trapped gas and transfer factor. The examination was repeated 0.5-6 months later for 112 of the individuals. Much of the variance observed at the first examination, i.e. interindividual variation, could be accounted for by the sex, age, height and weight of the individuals, whereas differences in individual motivation, social status, the ability of the technicians to instruct during the examination and interactions induced by the subject and the technician being of the same or of opposite sex were negligible. The intraindividual difference between the second and the first examinations were very small. Factors such as the subjects' sex, age, height, weight, social status, motivation, whether the examination was conducted by the same or a different technician at the two examinations, and whether the interval of time between the two examinations was short or long, were insignificant. It is concluded that while much of the interindividual variance of a lung function test is influenced by a variety of confounding factors, the intraindividual variation is not. Reference values for intraindividual variation are given.