Interpretation of spirometric test results
Fig. 1 - Circadian and seasonal variation in the level of pulmonary function. Data derived from a normal population, from measurements made at 3 year intervals for up to 12 years .
Lung function tests produce a once-only result. The result does not only reflect the presence or absence of respiratory disease, but is also influenced by the time of the day, daily and seasonal variation, etc. (fig. 1 ). When interpreting test results, do take into account that there is within-subject variability. Thus there is greater uncertainty about the "abnormality" of a finding near the lower limit of normal than if the test result is clearly within the normal range or in a clearly pathological range (fig. 2).
|Fig. 2 - When interpreting test results, do take into account that there is within-subject variability.||Fig. 3 - Relationship between percentile and z-score, and its use in a pictogram to facilitate the interpretation of test results.|
The way in which spirometric test results are usually presented does little to facilitate interpretation and mystifies the inexperienced assessor: observed values of FEV1, FVC, FEV1/FVC together with additional indices, such as pre and post bronchodilator, predicted values, lower limits of normal, percent of predicted, represents an impenetrable array of data that confuses most recipients, whether clinicians, technicians or patients. Conversely, pictograms in which z-scores are depicted relative to a normal range allow interpreting the findings in the wink of an eye (fig. 3 and 4).
Fig. 4 - The large number of data is not conducive to an easy interpretation of lung function measurements. The use of pictograms, which summarise the findings (bottom left), enables interpretation at a glance.
- Borsboom GJJM, van Pelt W, van Houwelingen HC, van Vianen BG, Schouten JP, Quanjer PH. 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. Manuscript
Acknowledgement: Fig. 3 modified and reproduced with permission of the European Respiratory Society. Eur Respir J December 2012 40:1324-1343; published ahead of print June 27, 2012, doi:10.1183/09031936.00080312.