Mapping the Zone of Uncertainty in Pulmonary Function Test Interpretation.

Background: European Respiratory Society and American Thoracic Society (ERS/ATS) guidelines for pulmonary function test (PFT) interpretation posit the existence of a "zone of uncertainty." However, as reference equations have been defined only for healthy lung function, the precise borders of the zone of uncertainty remain unspecified. To address this limitation, we sought to define distributions of both healthy and diseased lung function and to use these distributions to map the zone of uncertainty. Methods: We used a latent class model to define distributions of healthy and diseased FEV Results: Pre-bronchodilator spirometry data were collected from 14,075 NHANES participants-of whom 6,063 were without respiratory symptoms or a history of tobacco use-and 41,437 UPHS patients. Healthy lung function was represented by a Box-Cox Cole Green distribution with a median of 0.81, a coefficient of variation of 0.08, and skewness of 1.70. Diseased lung function was best represented by a Box-Cox power exponential distribution with a median of 0.56, a coefficient of variation of 0.30, skewness of 0.91, and kurtosis of 2.36. In the latent class model, the prior probability that an FEV Conclusion: This exploratory study presents evidence that in a clinical cohort, most FEV