It is clear that asthma is not just one disease. Everyone’s asthma is a little different. People react to different triggers. They have different patterns of inflammation and level of severity. People respond differently to asthma medications.
The variation in asthma can make it difficult to diagnose. A person’s medical history and a physical exam provide important information about symptoms and risk factors.1 Spirometry, a lung function test, provides measurable (objective) information about airway blockage and sensitivity. Additional tests are done to:
- Rule out other lung diseases
- Figure out what type of asthma a person has
- Monitor changes in lung function
How is lung function tested?
The most important test of lung function is spirometry. This test checks how much and how quickly you can exhale air. There are three main results from spirometry:2
- FEV1: The amount of air you can forcefully exhale in one second
- FVC: The maximum amount (“vital capacity”) of air you can forcefully exhale
- FEV1/FVC: The percentage your total air capacity that you can forcefully exhale in one second
Spirometry results are used to determine if you have obstructive or restrictive lung disease.3 The results also indicate how obstructed your airways are. Spirometry is often done before and after inhaling a medication that opens the airways. The results are compared to see how “reversible” the obstruction is, that is, how well the airways can open up.
Spirometry is done during doctor’s visits. It should be part of the initial diagnosis. It should be repeated after starting treatment once symptoms have stabilized. Spirometry also should be done anytime symptoms start to worsen, and at least once every one to two years.1
Spirometry can also be done before and after challenge tests, which is also called "bronchoprovocation.” In a challenge test, you intentionally expose your airways to a trigger to measure how sensitive (“hyperresponsive”) they are. Airway sensitivity is a sign of asthma. An inhalable spray called methacholine and exercise are usual triggers used for challenge tests. These tests are done in a controlled way. Medications are given at the end of the test to open up the airways.
What conditions are similar to asthma?
- Hay fever (allergic rhinitis)
- Sinus infection
- Foreign object blocking the airway
- Hardening or collapse of airways
- Vocal cord dysfunction
- Enlarged lymph nodes
Obstructive vs restrictive lung disease testing
Asthma is an obstructive lung disease. In obstructive lung disease, the airflow through the small airways is blocked (obstructed). The blockage makes it difficult for the air to get out of your lungs. Chronic bronchitis and emphysema—known together as “chronic obstructive pulmonary disease”—are two other obstructive lung diseases.
Spirometry results distinguish obstructive lung disease from restrictive lung disease.3 People with restrictive lung disease have a hard time getting enough air into the lungs to fill them fully. This usually happens because the lungs have gotten stiff and they cannot expand enough. It can also happen when there are problems with the muscles in the chest. For example, fibrosis (tissue scarring) due to asbestos exposure or radiation can cause restrictive lung disease.
Challenge tests and chest x-ray may also used to rule out conditions that mimic asthma.
Testing for asthma type
Allergy tests and tests of inflammation are helpful for figuring out what type of asthma you have. Your provider may recommend these tests if inhaled corticosteroids are not enough to control your symptoms and more information is needed about what kind of asthma you have.4 The results of these tests can help to adjust your treatment plan.
Allergy testing may be useful if it seems that year-round, indoor allergens trigger your asthma.1 Examples of these kinds of allergens are dust mites, pet dander, and cockroaches. It takes effort to avoid these triggers, so it is important to know specifically what you are allergic to. People who are thinking of taking allergy shots also need allergy testing.1
Three tests are used to measure inflammation: nitric oxide testing, mucus (sputum) eosinophils, and blood eosinophils. These tests are often done in research about severe asthma. More information is needed before these tests are used in everyday practice.1
Testing asthma at home
Lung function can be monitored with spirometry or a peak flow meter. The National Heart, Lung, and Blood Institute recommends checking whether lung function has changed over time.1 A decline in lung function increases the risk of an asthma attack.
You can monitor your lung function at home with your peak flow meter. You can see how your lungs are currently working, compared with times when they are working the best.1 Decreased lung function could be a sign that a change in treatment is needed. Written Asthma Action Plans may include instructions on what steps to take based on your peak flow measurements. At-home monitoring is especially important for people who:1
- Have moderate to severe asthma
- Have had a severe asthma attack
- Have a hard time noticing that their symptoms are getting worse