The FEV1/forced expiratory volume at 6 s of exhalation (FEV6) ratio has been suggested as a surrogate for the FEV1/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 FEV1/FVC ratio but not for the FEV1/FEV6 ratio. We propose a simplified statistical method to approximate reasonably the FEV1/FEV6 LLN in a population for which FEV1/FVC LLN values are already available.
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 FEV1/FEV6 ratio distributions in function of age and sex for obstruction as diagnosed from FEV1/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 FEV1/FVC LLN.
Misdiagnoses from the surrogate equations (FEV1/FEV6 = 75.58 − 0.11 × age for men, and 77.70 − 0.09 × 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.
The study confirms the feasibility of computing a surrogate LLN equation for the FEV1/FEV6 ratio in a population for which the accepted FEV1/FVC LLN already exists. Surrogate equations for FEV1/FEV6 ratio may extend its use for screening and case finding when simplified spirometry is needed.