Should forced expiratory volume in six seconds replace forced vital capacity to detect airway obstruction?

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Abstract
It has been suggested that forced expiratory volume in six seconds (FEV6) should be substituted for forced vital capacity (FVC) to measure fractions of timed expired volume for airflow obstruction detection. The present authors hypothesised that this recommendation might be questionable because flow after 6 s of forced expiration from more diseased lung units with the longest time constants was most meaningful and should not be ignored. Furthermore, previous studies comparing FEV6and FVC included few subjects with mild or no disease.The present study used spirometric data from the USA Third National Health and Nutrition Evaluation Survey with prior published ethnicity- and sex-specific equations for FEV1/FEV6, FEV1/FVC and FEV3/FVC, and new equations for FEV3/FEV6, all derived from ∼4,000 adult never-smokers aged 20–80 yrs.At 95% confidence intervals, 21.3% of 3,515 smokers and 41.3% of smokers aged >51 yrs had airway obstruction; when comparing FEV1/FEV6with FEV1/FVC, 13.5% were concurrently abnormal, 1.5% were false positives and 4.1% were false negatives; and when comparing FEV3/FEV6with FEV3/FVC, 11.6% were concurrently abnormal, 3.3% were false positives and 5.7% were false negatives.Substituting forced expiratory volume in six seconds for forced vital capacity to determine the fractional rates of exhaled volumes reduces the sensitivity of spirometry to detect airflow obstruction, especially in older individuals and those with lesser obstruction.