Diagnosing Asthma


To diagnose asthma, your doctor will review your medical history, family history, and symptoms. He or she will be interested in any history of breathing problems you might have had, as well as a family history of asthma or other lung conditions, allergies, or a skin disease called eczema. It is important that you describe your symptoms in detail (coughing, wheezing, shortness of breath, chest tightness), including when and how often they occur.

Your doctor will also perform a physical examination and listen to your heart and lungs.

There are many tests your doctor might perform, including:

  • pulmonary function tests
  • allergy tests
  • blood tests
  • chest and sinus X-rays

All of these tests help your doctor determine if asthma is indeed present and if there are other conditions affecting it.

What are pulmonary function tests?

Pulmonary function tests (or lung function tests) include numerous procedures to diagnose lung problems. The two most common lung function tests used to diagnose asthma are spirometry and methacholine challenge tests.

  • Spirometry — This 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. Spirometry can be done before and after you inhale a short-acting medication called a bronchodilator, such as albuterol. The bronchodilator causes your airways to expand, allowing for air to pass through freely. This test might also be done at future doctor visits to monitor your progress and help your doctor determine if and how to adjust your treatment plan.
  • Methacholine challenge test — This test is more commonly used in adults than 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 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 percent. A bronchodilator is always administered at the end of the test to reverse the effects of the methacholine.

How do I prepare for pulmonary function tests?

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, like a cold, or any shots or immunizations, since these might affect the test's results.

Other general preparations to follow before the test include:

  • No smoking on the day of the test
  • No coffee, tea, cola, or chocolate on the day of test
  • Avoid exercise and cold air exposure on the day of test

Medicines taken to treat asthma can affect the test results. Different medicines must be stopped at different intervals. Your doctor will tell you how long before testing you should discontinue any medicines you are taking.

What is a chest X-ray?

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 might perform an X-ray exam on you in order to see the structures inside your chest, including the heart, lungs, and bones. By viewing your lungs, your doctor can see if asthma is causing your symptoms.

Other tests

There are some medical conditions that might make asthma harder to treat and control. Two of these conditions are sinusitis and gastroesophageal reflux disease, commonly called GERD. If you are diagnosed with asthma, your doctor might also test you for these conditions so that they can be treated.

Sinusitis, also called sinus infection, is an inflammation or swelling of the sinuses due to infection. When the sinuses become blocked and filled with fluid, bacteria grow, causing infection and inflammation. Your doctor might order a special X-ray, called a CT scan, to evaluate your sinuses if he or she suspects an infection. Once acute sinusitis is diagnosed, you will be treated with antibiotics for at least 10 to 12 days.