Are reference equations for spirometry an appropriate criterion for diagnosing disease and predicting prognosis?

In the last few years, there has been considerable debate on the use of threshold criteria for the diagnosis of obstructive lung disease based on FEV(1) and FEV(1)/FVC ratio. It has been argued that a fixed ratio and fixed percentage criterion result in misclassification. The author argues that this critique is based on a false presumption about the validity of reference equations as a criterion for normality. The flaw lies in the methods used to derive reference equations, which involve arbitrary and circular criteria for exclusion of some members of the population, use potentially non-representative reference populations and include predictive variables that are really risk factors for disease or for adverse outcomes of disease. The author argues for a new interpretative approach for the use of lung function data in clinical practice based on prognostic equations analogous to the Framingham cardiovascular risk factor equations. These interpretative equations should be based on data from cohort studies and randomised controlled trials, rather than cross-sectional studies, and if properly formulated, will prove to be valuable aids to clinical decision making.