Lung function testing: the dilemma of predicted values in relation to the individual variability

Quantitated lung function parameters are usually interpreted in relation to so-called "normal ranges' obtained from healthy study groups. The aim of this paper is the critical review of formulas and the evaluation of intraindividual variation in modern lung function testing. To which extent is the total variation of lung function parameters in cross-sectional studies (usually serving as basis for the normal range) attributed to the intraindividual variation between repeated measurements? This question raises a further question: are lung function values in the normal range really normal? To assess spirometric and body plethysmographic parameters 26 healthy subjects from three medical centers underwent 30-72 measurements over a period of 2 months for the determination of variations due to (1) intraindividual variation over time and (2) interindividual variation. For each subject, predicted values of different lung function parameters published by Quanjer et al. [Eur Respir J 1993; 6:5-40.1], of intrathoracic gas volume by Ulmer et al. [Die Lungenfunktion; Stuttgart, Thieme, 1991] and of total airway resistance by Ruehle and Matthys [Pneumologie 1976;153:223] were applied. When converted into percent predicted and adjusted for differences in medical centers, the intraindividual standard deviation was estimated to be about half of the interindividual standard deviation. We conclude that the normal range of lung function parameters derived from the standard deviation within populations is too wide for the assessment of individual values. Interpretation of individual lung function measurements should primarily be based on the "individual normal range' derived from former lung function measurements of the individual and only secondly on the "predicted value'.