bronchodilator reversibility
Bronchodilator reversibility is a before-and-after experiment: do spirometry, give an inhaled drug that relaxes airway muscle, wait about fifteen minutes, then repeat. If the airways were clenched and the drug loosened them, the second blow is noticeably bigger.
A meaningful improvement — by current criteria, a gain of more than 10 percent of the predicted FEV1 or FVC — shows that at least part of the airflow obstruction can be opened up. Because this kind of variable, reversible narrowing is the essence of asthma, a positive test strongly supports that diagnosis.
The reverse is not so clean. COPD usually shows little or no reversibility, but some patients with COPD do respond, and some asthmatics on a good day do not, so a single negative test never rules asthma out. Reversibility is one piece of evidence, weighed alongside symptoms, history and sometimes repeat testing.
A patient's FEV1 climbs from 2.0 L to 2.4 L (a 400 mL gain, about 13 percent of predicted) after salbutamol — a positive reversibility test favouring asthma.
A clear rise after the drug supports a diagnosis of asthma.
Reliever inhalers and some other medications must be withheld for a set time before the test, or pre-existing bronchodilation will blunt and mask the response.