So how can you tell if your adult patient has asthma or COPD.  Sometimes you can’t, and sometimes it might be both.  But here are some clues.

helpful conceptualization from Medscape

Clinical course: The hallmark of asthma is intermittent episodes of obstruction or bronchospasm. These patients, particularly at onset, often have asymptomatic periods.  COPD, on the other hand, is slowly progressive and NOT reversible. Patients clearly have exacerbations, but are rarely completely symptom free.

Age at onset: Asthma patients tend to be younger. They may be able to identify specific triggers: allergens (pets), irritants (tobacco smoke), exercise, temperature. COPD is due to long-standing exposures, so patients are older, and almost always have a smoking history.

PFTs: Asthma and COPD both have an obstructive pattern on PFTs. Think low FEV1/FVC ratio, larger lung volumes. The difference is that the obstruction seen in asthmatics is REVERSIBLE with bronchodilators. The bronchodilator administration is necessary for the diagnosis of asthma. Specific criteria are that FEV1 increases more than 12% over baseline or Peak Flow increases > 20% over baseline with bronchodilator. Normal PFTs essentially rule out COPD, but PFTs can be normal in an asymptomatic asthmatic.

Wondering what else is in the differential?  Check out this post from 2013.