Intra-session repeatability of FET and FEV6 in the general population

Abstract

Background: The recent American Thoracic Society/European Respiratory Society spirometry guidelines harmonized quality criteria and included assessment of forced expiratory time (FET) in bronchodilation testing when an isolated significant forced vital capacity (FVC) bronchodilation effect is observed. No consensus exists on the criteria of accepted variability of FET. Forced expiratory volume in 6 s (FEV6) has been suggested to replace FVC, especially in primary care. We examined the variability of FET and FEV6 and its determinants in the general population.

Methods: We studied the results of acceptable flow-volume measurements from a general population sample of 603 subjects aged 25-74 years. Intra-class correlation coefficients (ICC), coefficients of variation (CoV), absolute and relative differences of FET, forced expiratory volume in 1 s (FEV1), FEV6 and FVC from the three acceptable curves were calculated.

Results: FET had a mean CoV of 11.3% and ICC of 0.873. The average CoV and ICC for FEV1, FVC and FEV6 were 1.4% and 0.996, respectively. The 95th percentile of FET difference was 2.7 s. Older age was associated with slightly higher variability. Reproducibility of FET (CoV 10.6%) was better, that of both FEV6 (1.5%) and FVC (1.7%) slightly worse, in subjects with airflow limitation (FEV1/FVC < 88% predicted).

Conclusions: Intra-session variability of FET was eightfold compared with other flow-volume variables. Variability of FEV1, FEV6 and FVC were similar. The findings suggest that the upper limit of intra-session repeatability of FET is around 3.0 s. In subjects with airflow limitation FEV6 was slightly but not significantly less variable than FVC.