How to evaluate "spirometric" lung age--what method is approvable?

The method predicting lung age was originally proposed by Morris and Temple (1985). In their method, lung age was estimated by counting back the regression formula predicting normal value of FEV1. Since the normal value of FEV1 at a given age is not unique and exists within a certain range defined as 95% confidence interval, the backward value of lung age calculated with the original method includes statistical and physiological problems. Analyzing the problems related to the original method, we have developed a novel method with revising age-elicited variation in FEV1. When the original method is applied, the lung age of a person with measured FEV1 beyond upper-limit-of-normal (ULN) results in being remarkably young (sometimes, below zero), while that of a person with FEV1 below lower-limit-of-normal (LLN) is estimated as being very elderly (sometimes, over 100). On the other hand, the novel method, in which age-related variation in FEV1 is reliably corrected, predicts a wide range of lung age even when measured FEV1 is significantly above ULN or below LLN.