Lung function tests for asthma
Lung functions tests, also called pulmonary function tests, measure how well the lungs are working and can help doctors diagnose asthma.
The two most common lung function tests for asthma are spirometry and methacholine challenge tests.
- Spirometry: this pulmonary function test for asthma is a simple breathing test that measures how much air you can blow out of your lungs and how quickly. It is often used to determine the amount of airway obstruction you have. Spirometry can be done before and after you inhale a short-acting medication called a bronchodilator, such as salbutamol. The bronchodilator causes your airways to expand, allowing air to pass through freely. This test might also be done at future doctor visits to check your progress and to help your doctor determine if and how to adjust your treatment plan.
- Methacholine challenge test: this lung function test for asthma is more commonly used in adults than in children. It might be performed if your symptoms and screening spirometry do not clearly or convincingly establish a diagnosis of asthma. Methacholine is an agent that, when inhaled, causes the airways to spasm (contract involuntarily) and narrow if asthma is present. During this test, you inhale increasing amounts of methacholine aerosol mist before and after spirometry. The methacholine test is considered positive, meaning asthma is present, if the lung function drops by at least 20%. A bronchodilator is always given at the end of the test to reverse the effects of the methacholine.
How do I prepare for pulmonary function tests for asthma?
Ask your doctor if there is anything you need to do to prepare for spirometry.
Before taking a methacholine challenge test, be sure to tell your doctor if you have recently had a viral infection, such as a cold, or any injections or immunisations, since these might affect the test results.
Other general preparations to follow before lung function tests for asthma include the following:
- Do not smoke on the day of the test.
- Do not have coffee, tea, cola, or chocolate on the day of the test.
- Avoid exercise and cold air exposure on the day of the test.
Can I use asthma medications before the pulmonary function test?
Be prepared to adjust your medications. Some asthma medications can affect the test results. Different medications must be stopped at different intervals. For example, short-acting inhaled bronchodilators like salbutamol or terbutaline should be stopped eight hours before testing, but long-acting inhaled bronchodilators like salmeterol or formoterol cannot be taken for 48 hours. Your doctor will tell you how long before testing you should discontinue any medications you are taking. Don't stop taking any of your asthma medications without first talking to your doctor.