Which criterion is essential to diagnose asthma via pulmonary function testing?

To diagnose asthma using pulmonary function testing, the presence of significant improvement in FEV1 after the administration of a bronchodilator is critical. This criterion reflects the reversible nature of airway obstruction commonly associated with asthma. A person with asthma typically experiences bronchoconstriction, leading to reduced airflow, which can be measured by a decrease in FEV1. When a bronchodilator is used, it relaxes the airway smooth muscles, resulting in an increase in airflow and, consequently, an increase in FEV1.

A marked improvement (generally defined as an increase of 12% and at least 200 mL in FEV1) following the bronchodilator indicates that the airway obstruction is reversible, which aligns with the pathophysiology of asthma. This improvement following bronchodilator therapy is not typically seen in other chronic respiratory conditions, like COPD, where airflow limitation tends to be more fixed.

Normal FEV1/FVC ratio does not indicate the presence of asthma since, in asthma, the ratio may be low during obstructive episodes but can normalize with bronchodilator use. Low FEV1 without bronchodilator response does not provide sufficient information to distinguish asthma from other conditions, as it only indicates a reduction in airflow without