Correct interpretation of pulmonary function tests requires the use of appropriate reference values with which the patient's results are compared [1-3]. Unlike many physiologic parameters, for which normal values do not vary with the characteristics of the particular patient, predicted values of pulmonary function depend upon age, height, gender, and race. Therefore, interpretation of pulmonary function tests performed for the first time must take these and other factors into consideration. In practice, this is usually done by a computer using linear regression equations (reference equations) for calculation of "predicted values," as determined by published studies of large numbers of healthy individuals.
Even when anthropometric factors are taken into consideration, the "normal" range for measurements of pulmonary function remains wide, often 80 to 120 percent of the predicted value. This means that large changes with disease progression or therapy can easily occur while the patient's values remain within the normal range. Therefore, follow-up pulmonary function tests in adults should be compared with the patient's previous (baseline) values, not the predicted values.
With children, the normal growth of lung function is fast when compared to the changes observed with disease or therapy. As a result, the use of growth charts (looking for trends across percentile lines) most accurately reflects changes that cannot be attributed to growth (figure 1A-B).
The American Thoracic Society (ATS) statement on the standardization of spirometry, as well as other ATS guidelines, can be accessed through the ATS web site at www.thoracic.org/statements.
EFFECTS OF AGE AND GROWTH
During childhood, the lungs grow in proportion to the increase in height. This growth results in an exponential increase in lung volumes (and maximal flows) with each year from preschool age through adolescence . On average, girls reach both their maximal height and their maximal lung volume earlier than boys, but boys achieve larger lung volumes. Additional factors may affect maximal lung function: