Objective: International consensus guidelines suggest that in asthma and chronic obstructive pulmonary disease (COPD), measurements of FEV1 and PEF are equivalent in the assessment of the degree of airflow obstruction when expressed as the per cent of predicted values.
Methodology: In this retrospective study, 2,587 paired measurements of PEF and FEV1 performed by 101 adult patients with asthma (n = 56) and COPD (n = 45) attending an outpatient chest clinic were obtained. The mean differences between FEV1 and PEF measurements when expressed as the percentage of predicted values was determined. The level of agreement between the two measurements in the classification of asthma severity (life-threatening, severe, moderate and mild asthma determined by PEF or FEV1 measurements of <30%, 30-60%, 60-80%, and >80% of the predicted values, respectively) was determined.
Results: There was considerable variability between measurements of FEV1 and PEF when expressed as % predicted values. In both asthma and COPD, the FEV1% predicted was smaller than the PEF % predicted, with the mean difference being -10.9% (95% CI, -12.8% to -8.9%) with limits of agreement of -35.4% to +13.6%. The weighted Kappa statistic for agreement was 0.59 (95% CI, 48-70%) in the classification of the severity of airflow obstruction.
Conclusion: When expressed as percentage of predicted values, PEF and FEV1 values are not equivalent. We recommend that guidelines be modified to state that across the spectrum of the severity of airflow obstruction there is considerable variability between measurements of FEV1 and PEF when expressed as % predicted such that the FEV1 may be as much as 35% lower or up to 15% higher than the PEF for patients with obstructive lung diseases.