Spirometric test results should be interpreted in conjunction with predicted values and their lower limits of normal, clinical data, and when available the response to bronchodilator drugs. There are many publications on predicted values. The ‘lower limit of normal’ is commonly derived in either of three ways:
It matters which set of reference values for spirometry is adopted, and how the ‘lower limit of normal’ is determined, in particular in the case of borderline values. This can be shown by the following case of a Caucasian male, standing height 1.78 m, age 55 yr, observed FEV1 3.06 L. Predicted values and lower limits according to some published equations are as follows:
As shown in the table, both the sensitivity and the specificity of spirometric test results is influenced by the selection of reference values as well as by the strategy adopted to compute the ‘lower limit of normal’:
See also: Reference values for children, adolescents and adults