At times, asthma can be difficult to diagnose because the symptoms are sometimes similar to other lung conditions. Initially, your doctor will ask about your symptoms and medical history. A physical exam will be done.

In addition, the following tests may be performed:

Chest x-ray is the initial imaging test for most people with symptoms of asthma. A chest x-ray can reveal other causes of wheezing (other than asthma) or complications (such as pneumonia ). But, the x-ray itself cannot be the only way to diagnose asthma.

Pulmonary function tests (PFTs) are done using an instrument known as a spirometer. The spirometer measures how much air is inhaled and expelled as you breathe. This test will measure both the amount and rate of air that can pass through your airways.

The diagnosis of asthma is usually made when reversible lung airway obstruction is demonstrated on PFTs. Lung obstruction means that the air passes too slowly through the airways. When the obstruction is corrected after you are given an inhaled dose of “rescue medicine” (eg, albuterol ), the obstruction is termed reversible. At this point, asthma is typically diagnosed.

A peak flow meter measures the fastest rate at which you expel air. Peak flow meters can be used at home to assess if the degree of airway obstruction indicates an imminent asthma attack or a need to change your medicine.

People with asthma will experience a mild constriction of the airways when the drug methacholine is inhaled. If asthma is suspected, but there are no obvious symptoms of airflow obstruction on PFTs, methacholine is administered and the tests are run again. Alternative provocation tests include administration of histamine or inhalation of cold air. The diagnosis of asthma is usually made if the PFTs indicate lung airway obstruction during a provocation tests.

You may have allergy tests if allergic asthma is suspected. A tiny allergen particle is placed under the skin with a needle. In the majority of cases, an allergic response is confirmed if the skin becomes raised or red within 20 minutes. Under some circumstances, the less accurate radioallergosorbent (RAST) test can be used to detect allergic responses.

Last Reviewed September 2011