Pulmonary function testing (PFTs) are administered for people who have persistent asthma. They measure how well the lungs are functioning and how serious the asthma is. In addition, PFTs can provide feedback on the work of the asthma treatment.

PFTs can diagnose asthma. It does this by differentiating between the two major types lung diseases: obstructive and restrictive.  Obstructive lung disease is the one where the flow of air through the lungs is obstructed. It is the condition responsible for asthma.

There are three major types of PFTs: spirometry, lung volumes and diffusion capacity. They can measure:
1.  FVC (Forced Vital Capacity): The amount of total air that is exhaled after inhaling air deeply.
2. FEV1 (Forced Expiratory Volume in 1 Second): The amount of air that is expelled in the first second after a deep inhalation.

The ratio of FEV1 to FVC can indicate the type of lung disease. If the ratio is lower than expected, it is “obstructive”. If the ratio is normal (but the value of FVC is lower), it means “restrictive”. If both the rate and the FV are low, it indicated both “obstructive” and “restrictive” lung diseases.
In some cases, a person with asthma has normal PFT measurements. Thus PFTs are not a completely reliable method.