Background: Early detection of airflow obstruction is particularly important among young adults because they are more likely to benefit from intervention. Using the FEV1/FVC<70% fixed ratio, airflow obstruction may be under-diagnosed. The lower limit of normal (LLN) which is statistically defined by the lower 5th percentile of a reference population, is physiologically appropriate but it still needs a clinical validation.
Methods: To evaluate the characteristics and longitudinal outcomes of subjects misidentified as normal by the fixed ratio with respect to the LLN, 6,249 participants (aged 20-44 years) in the European Community Respiratory Health Survey (ECRHS) were examined and classified into 3 groups (absence of airflow obstruction by the LLN and the fixed ratio; presence of airflow obstruction only by the LLN; presence of airflow obstruction by the two criteria) in 1991-93. LLN equations were obtained from the normal non-smoking participants. A set of clinical and functional outcomes was evaluated in 1999-2002.
Results: The misidentified subjects were 318 (5.1%); only 45.6% of the subjects with airflow obstruction by the LLN were also identified by the fixed cut-off. At baseline, FEV1 (107%, 97%, 85%) progressively decreased and bronchial hyperresponsiveness (slope 7.84, 6.32, 5.57) progressively increased across the 3 groups. During the follow-up, misidentified subjects had a significantly higher risk of developing COPD and a significantly higher use of health resources (medicines, ED visits/hospital admissions) because of breathing problems than the subjects without airflow obstruction (p<0.001).
Conclusions: Our findings show the importance of using statistically derived spirometric criteria to identify airflow obstruction.