A method to derive lower limit of normal for the FEV1/forced expiratory volume at 6 s of exhalation ratio from FEV1/FVC data.

Background: The FEV(1)/forced expiratory volume at 6 s of exhalation (FEV(6)) ratio has been suggested as a surrogate for the FEV(1)/FVC ratio to detect airway obstruction. Current guidelines require that lower limit of normal (LLN) values be implemented to detect an abnormality. In most populations, LLN equations are available for the FEV(1)/FVC ratio but not for the FEV(1)/FEV(6) ratio. We propose a simplified statistical method to approximate reasonably the FEV(1)/FEV(6) LLN in a population for which FEV(1)/FVC LLN values are already available.

Methods: Spirometric data were collected from 8,273 European patients aged 20 to 85 years. We computed by receiver operator characteristics analysis the best-fit cutoff FEV(1)/FEV(6) ratio distributions in function of age and sex for obstruction as diagnosed from FEV(1)/FVC LLN values obtained from the relevant reference equations for subjects aged 20 to 70 and 65 to 85 years. We compared the diagnosis of obstruction obtained from these surrogate equations against the reference diagnosis made by FEV(1)/FVC LLN.

Results: Misdiagnoses from the surrogate equations (FEV(1)/FEV(6) = 75.58 - 0.11 x age for men, and 77.70 - 0.09 x age for women aged 20 to 70 years) were all within 2.3 +/- 2.0% of the reference LLN. Similar results were found in the group aged 65 to 85 years.

Conclusions: The study confirms the feasibility of computing a surrogate LLN equation for the FEV(1)/FEV(6) ratio in a population for which the accepted FEV(1)/FVC LLN already exists. Surrogate equations for FEV(1)/FEV(6) ratio may extend its use for screening and case finding when simplified spirometry is needed.