There are two main types of tests that are routinely carried out to see how the lungs are working.
Spirometry and peak flow meter testing can give the doctor information about how much air is getting in and out of the lungs and how fast it is doing so. This in turn gives them an idea of how congested or narrowed the airways are. Spirometry testing is done in a doctor’s office whereas peak flow meter testing can be used at home to monitor lung function on a daily basis.
When doctors suspect that a person has asthma, they perform a spirometry test. This lets them know how much air can be maximally exhaled in a certain period of time (volume) as well as how fast it is being exhaled (flow). A patient may be asked to perform several steps so that doctors can get different measures that can then help them make a diagnosis. Doctors can compare readings against charts that have been made using the readings of healthy individuals.
This is a test where the person blows air into the device as fast and as hard as possible. The rate (speed) at which they blow air out is then measured and is known as PEF for peak expiratory flow rate. In people with asthma, the rate will be lower than in people without asthma. Just as for spirometry, doctors can compare readings against charts that have been made using the readings of healthy individuals.
It is very common for doctors to keep track of something called PEF variability. If a person’s PEF values vary a lot over time, it is likely to mean they have asthma. But if they have low variability, that could mean they have COPD, which stands for chronic obstructive pulmonary disease and basically includes diseases like emphysema and chronic bronchitis.