A doctor will diagnose asthma based on a person's symptoms, and may also arrange some tests.
These include tests to measure lung function and sometimes allergy tests.
Lung function tests
Lung function tests assess lung function. The most common pulmonary function tests used to diagnose asthma are peak flow measurements and spirometry. Methacholine challenge (also called histamine challenge) tests are also available but more rarely used in clinical practice.
Spirometry is a simple breathing test that measures how much and how fast you can blow air out of your lungs. It is often used to determine the amount of airway obstruction you have. Peak flow meters measure the maximum speed at which you breathe air out of your lungs, called peak expiratory flow rate (PEFR). If you have untreated or uncontrolled asthma, your peak flow measurements will tend to be lower than they should be for your age and size (because your airways are narrowed) and they will vary during the course of the day.
The methacholine challenge test may be performed if your symptoms and screening spirometry do not clearly or convincingly establish a diagnosis of asthma. Your doctor will know which test is best for your situation.
While a chest X-ray is not an asthma test, it may be used to make sure that nothing else is causing your asthma symptoms. An X-ray is an image of the body that is created by using low doses of radiation reflected on special film or a fluorescent screen. X-rays can be used to diagnose a wide range of conditions, from bronchitis to a broken bone. Your doctor may arrange an X-ray examination in order to see the structures inside your chest, including the heart, lungs and bones. Viewing your lungs helps your doctor to determine if asthma might be causing your symptoms.
Evaluation for heartburn and GORD
Gastro-oesophageal reflux disease, commonly called GORD, is another condition that may aggravate asthma. If your doctor suspects this problem, he or she may recommend specific tests to identify it.
Allergy testing may be recommended to identify any allergens that trigger asthma symptoms.
Evaluation of the sinuses
The presence of nasal polyps or sinusitis may make asthma harder to treat and control. Sinusitis is an inflammation or swelling of the sinuses often due to infection. When the sinuses become blocked and filled with fluid, bacteria grow, causing infection and inflammation. Your doctor may arrange a CT scan to evaluate your sinuses. Once acute sinusitis is diagnosed, you may sometimes be treated with antibiotics. Treating the sinusitis may help prevent asthma symptoms.
Judging the severity of asthma
Based on these asthma tests and your symptoms, your doctor may determine that you have asthma. The next step is for the doctor to determine the best asthma treatment for you as an individual.