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The FEV1 (the amount that can be forcefully exhaled in the first second from a full inspiration) expressed as a percentage of the FVC (the total amount that can be exhaled) is an indicator of airflow obstruction. If obstruction is present the air will not come out as fast and the percentage will be lower.
The FEV1/FVC% changes with age. For this reason, using a single % to define the Lower Limit of Normal (LLN) for all subjects is not recommended.
Some studies give a reference equation for FEV1/FVC% and its LLN (for example the Knudson study used here or the Hankinson JL, et al reference values: Hankinson jl, Odencrantz JR, Fedan KB. Spirometric reference values from a sample of the general U.S. population. AM J RESPIR CRIT CARE MED 1999;159:179-187).