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Lung Function Tests for Asthma

To diagnose asthma, your doctor will review your asthma symptoms, your medical and family history, and may perform lung function tests (also called pulmonary function tests). Your doctor will be interested in any 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 of asthma in detail (coughing, wheezing, shortness of breath, chest tightness), including when and how often these symptoms occur.

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

Along with the pulmonary function tests for asthma, your doctor might perform allergy tests, blood tests, and 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 the Different Lung Function Tests for Asthma?

Lung function tests for asthma include numerous procedures to diagnose lung problems. The two most common pulmonary function tests for asthma are spirometry and methacholine challenge tests.

  • Spirometry: This pulmonary function test for asthma is a simple breathing test that measures how much air you can blow out of your lungs and how quickly. 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 check your progress and to help your doctor determine if and how to adjust your treatment plan.
  • Methacholine challenge test: This lung function test for asthma is more commonly used in adults than in 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 (contract involuntarily) 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%. A bronchodilator is always given at the end of the test to reverse the effects of the methacholine.

How Do I Prepare for a Lung Function Test?

Ask your doctor if there is anything you need to do to prepare for spirometry if you're having lung function tests done.

Before taking a methacholine challenge test, be sure to tell your doctor if you have recently had a viral infection, such as a cold, or any shots or immunizations, since these might affect the test results.

Other general preparations to follow before lung function tests for asthma include the following:

  • Do not smoke on the day of the test.
  • Do not have coffee, tea, cola, or chocolate on the day of the test.
  • Avoid exercise and cold air exposure on the day of the test.

Can I Use Asthma Drugs Before a Lung Function Test?

Be prepared to adjust your asthma drugs. Some asthma medications can affect the test results. Different medications must be stopped at different intervals. For example, a short-acting inhaled bronchodilator like albuterol (Ventolin, Proventil) should be stopped eight hours before testing, but long-acting inhaled bronchodilators cannot be taken for 48 hours. Your doctor will tell you how long before testing you should discontinue any drugs you are taking. Don't stop taking any of your asthma medications without first talking to your doctor.

WebMD Medical Reference

Reviewed by Roy Benaroch, MD on February 11, 2014
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