The relationship between FEV1 and peak expiratory flow in patients with airways obstruction is poor

Study objectives: To evaluate the correlation between FEV1 and peak expiratory flow (PEF) values expressed as a percentage of their predicted value, and to assess factors influencing differences between the two measurements. Design: Cross-sectional. Setting: Pulmonary function laboratory at a tertiary-level teaching hospital in northern India. Participants: A total of 6,167 adult patients showing obstructive pattern on spirometry over a 6-year period. Interventions: None. Measurements and results: There was considerable variability between percentage of predicted FEV1 (FEV1%) and percentage of predicted PEF (PEF%). Locally weighted least-square modeling revealed that PEF% overestimated FEV1% in patients with less severe obstruction and underestimated it in those with more severe obstruction. Using Bland-Altman analysis, PEF% underestimated FEV1% by a mean of only 0.7%; however, limits of agreement were wide (-27.4 to + 28.8%), indicating that these two measurements cannot be used interchangeably. PEF% and FEV1% were > 5% apart in approximately three fourths and differed by > 10% in approximately one half of the patients. On multivariate analysis, discordance > 5% was significantly influenced by female gender (odds ratio, 1.26; 95% confidence interval [CI], 1.01 to 1.58) and increasing FEV1% (odds ratio, 1.09 for every 10% increase; 95% CI, 1.04 to 1.14) but not by height or age. Conclusions: FEV1% and PEF% are not equivalent in many patients, especially women and those with less severe airflow limitation. Assumptions of parity between PEF% and FEV1% must be avoided.