Pulmonary function tests

The two most common lung-function tests used to diagnose asthma are spirometry and methacholine challenge tests.

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• Spirometry: This simple breathing test measures how much air you can blow out of your lungs and how fast. 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. The bronchodilator causes your airways to expand, allowing for air to pass through freely. The test is also used to monitor patient progress and help doctors determine if and how to adjust a treatment plan.
• Methacholine challenge test: This test is more commonly used for adults than for children. It may 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. Before taking a methacholine challenge test, be sure to tell your doctor if you have recently had a viral infection or any shots or immunizations, since these may affect the test's results.

Here are other general preparations to make before the methacholine test:

• No smoking on the day of the test
• No caffeine (coffee, tea, cola, or chocolate) for 24 hours before the test
• Avoid exercise and cold-air exposure on the day of test.
• Medications taken to treat asthma can affect the test results. Your doctor will tell you how long before testing you should discontinue any medications you are taking.