Correctly diagnosing asthma is a multi-faceted task. First, your doctor or respiratory therapist will take a complete medical history, including any family history of asthma, allergies, or the skin disease eczema, and ask you about any symptoms you are having. A physical exam will also be performed, to listen to your lungs and heart. Then you will likely be asked to have one or more laboratory tests aimed at confirming the asthma diagnosis.

Pulmonary Function Tests
In most people suspected of having asthma, pulmonary function tests (PFTs) are considered the “gold standard” necessary to correctly diagnose the disease. The two most common PFTs for asthma are spirometry and methacholine challenge.

Spirometry is used to determine the amount of airflow obstruction you have and is most often performed in a doctor’s office or pulmonary function laboratory by a respiratory therapist who is specially trained to ensure the accuracy of the test. During spirometry, you will be asked to blow forcefully into a device called a spirometer several times over a 10 to 15 minute period. Each time the respiratory therapist note the results, and a spirogram will be produced that will be reviewed by your doctor to determine how your readings compare to those of non-asthmatics of a similar age, sex, height, and race. Based on the degree of difference between your readings and those of people similar to you without asthma, your doctor will determine whether you have the disease and, if so, how severe it is.

In a few cases, spirometry alone is not enough to clearly determine whether a person has asthma. In these cases, your doctor may also order a methacholine challenge test, which is usually also performed in a pulmonary function laboratory by a respiratory therapist. In this common and safe test, you will first perform spirometry to measure the level of obstruction, then be asked to inhale a substance called methacholine, which causes the airways to narrow and spasm in people with asthma. Your respiratory therapist will slowly increase the amount of methacholine you inhale over the duration of the test. After the test is complete, you will once again be asked to perform the spirometry test, to gauge any changes in airflow obstruction. Your respiratory therapist will also probably give you a bronchodialator treatment as well, to reverse any symptoms caused by the methacholine test. The results will then be reviewed by your doctor. Normally, asthma is diagnosed when lung function drops by at least 20 percent following the methacholine challenge.

It is important to note that your doctor or respiratory therapist will provide you with a list of instructions to follow prior to coming in for your methacholine challenge, such as not smoking on the day of the test and avoiding certain foods, exercise, cold air, and medications. Following these instructions is paramount to ensuring the accuracy of the test.

Other Tests Your Doctor May Order
Some doctors will also want to take a chest x-ray in the process of diagnosing asthma, and in some cases, special x-rays, scans, and tests may be done to determine the role other medical conditions may be playing in your symptoms. Two common conditions that can trigger or exacerbate asthma symptoms are sinusitis, an inflammation or swelling of the sinuses due to infection, and gastroesophageal reflux disease, or heartburn, which is characterized by stomach acid flowing upward into the throat. If these conditions are found to be causing or contributing to your asthma symptoms, treating them is essential to effectively treating your asthma.