Per cent of predicted as the limit of normal in pulmonary function testing: a statistically valid approach.

Abstract

It has been traditional in pulmonary function testing to set the limits of normal as the predicted .+-. 20%. Since most pulmonary function tests, and all spirometric tests, are abnormal only when below predicted, the rule of thumb for the limit of normal becomes 80% of predicted. How this factor came into being is unknown to this writer. It is a misfortune that it did since it has neither statistical nor physiological validity. It implies that all functions in pulmonary physiology have a variance around the predicted, which is a fixed per cent of predicted. Despite the fact that there is no validity to the 80% rule or to the use of percentage of predicted to indicate severity, both practices are fully ingrained by usage. If they could be made statistically valid they would continue to provide the physician with a method with which he is comfortable. There is a simple means of rendering the per cent of predicted and therefore the 80% of predicted not only statistically valid but having the same implication of all subjects. The method can be applied to all tests of pulmonary function where the predicted values is defined simply by a mean or a regression equation. The method means "normalizing" all subjects to a single value for each function.