Reproducibility of spirometric indices
The standard deviation of repeated measurements of the VC and the FEV1 in the same healthy adult is 90-200 mL, on average about 150 mL (ref. 1). The within subject reproducibility expressed as a coefficient of variation varies between 0.3 and 11.4%. Use of the coefficient of variation requires the spread in measurements to be proportional to the average value of the index, which is rarely the case in ventilatory indices. The reproducibility of spirometric measurements performed on different days is better for the FVC and FEV1 than for forced expiratory flows; the above figure illustrates this in healthy adolescents investigated daily for one week.
The colored area represents one standard deviation above and below the mean (white line). Differences in the level of FEV1 and FVC between subjects have been eliminated; hence the figure shows exclusively the variability between subjects. The data derive from adults (figures modified from ref. 2).
If measurements are not performed at the same time of the day, then their reproducibility is affected by within day variations in ventilatory function. The highest values of FEV1 (figure above), FVC (figure below) and peak expiratory flow occur at about noon; the within day variability is greater in smokers and in subjects with respiratory symptoms, and is additionally age dependent (ref. 2).
Short term variability in FEV1 and FVC in patients with lung disease is larger than in healthy subjects. The 95th percentile of a chance change in FEV1 in adults is about 200 mL, in FVC about 340 mL (ref. 3).
|Ref. 1 - Variability in spirometric indices|
|1||Becklake MR. Concepts of normality applied to the measurement of lung function. Am J Med 1986; 80: 1158-1164.|
|2||Becklake MR, Permutt S. Evaluation of tests of lung function for ‘screening’ for early detection of chronic obstructive lung disease. In: PT Macklem & S Permutt (eds), The lung in the transition between health and disease. New York, Dekker, 1979, 345-387.|
|Ref. 2 - Longitudinal behavior of spirometric indices|
|Borsboom GJ, van Pelt W, van Houwelingen HC, van Vianen BG, Schouten JP, Quanjer PhH. Diurnal variation in lung function in subgroups from two Dutch populations: consequences for longitudinal analysis. Am J Respir Crit Care Med 1999; 159: 1163-1171.|
|Ref. 3 - Variability in patients|
|1||Tweeddale PM, Alexander F, McHardy GJR. Short term variability in FEV1 and bronchodilator responsiveness in patients with obstructive ventilatory defects. Thorax 1987; 42: 487-490|
|2||Sourk RL, Nugent KM. Bronchodilator testing: confidence intervals derived from placebo inhalations. Am Rev Respir Dis 1983; 128: 153-157.|
|3||Rozas CJ, Goldman AL. Daily spirometric variability. Normal subjects and subjects with chronic bronchitis with and without airflow obstruction. Arch Intern Med 1982; 142: 1287-1291.|