Your doctor may use several tests to determine how well your lungs are working.
This is the main test doctors generally use to diagnose asthma in people 5 years or older.
To help determine how well your lungs are working (pulmonary function), you take a deep breath and forcefully breathe out (exhale) into a tube connected to a spirometer. This records both the amount (volume) of air you exhale and how quickly you exhale. If certain measurements are below normal for a person your age, it may indicate that asthma has narrowed your airways.
After taking lung test measurements, your doctor may ask you to inhale an asthma drug to open air passages and then do the test again. Showing significant improvement after taking the medication could mean you have asthma.
If your spirometer results are normal or near normal, your doctor might try to trigger asthma symptoms by having you inhale a substance that causes the airways to narrow in people with asthma, such as methacholine (meth-uh-KO-leen).
If you appear to have asthma triggered by exercise (exercise-induced asthma), you may be asked to do physical activity to see whether it triggers symptoms.
After either action, you'll retake the spirometry test. If your spirometry measurements remain normal, you probably don't have asthma. But if your measurements have fallen significantly, it's possible you do.
Lung tests in children
Doctors seldom do lung tests in children under age 5. Instead, diagnosis is generally based on a child's signs and symptoms, medical history, and physical examination. It can be especially difficult to diagnose asthma in young children because there are many conditions that cause asthma-like symptoms in this age group.
If your child's doctor suspects asthma, the doctor may prescribe a bronchodilator — a drug that opens the airways. If your child's signs and symptoms improve after using the bronchodilator, your child may have asthma.
Exhaled nitric oxide test
You breathe into a tube connected to a machine that measures the amount of nitric oxide gas in your breath. Nitric oxide gas is produced by the body normally, but high levels in your breath can mean your airways are inflamed — a sign of asthma.