Defining airflow limitation and chronic obstructive pulmonary disease: the role of outcome studies.

A guideline to the interpretation of pulmonary function tests, published 20 years ago, made a distinction between the related tasks of “(1) the classification of the derived values with respect to a reference population... and (2) the integration of the spirometric values into the diagnosis, therapy and prognosis for an individual patient.” [1]. The first of these is very straightforward, when appropriate reference data are available, but the latter is difficult in the early stages of disease and will have a degree of uncertainty for any individual. Confusion arises when the distinction between these two aspects of interpretation is blurred.