What Are Pulmonary Function Tests?
To diagnose asthma, your doctor will review your symptoms, your medical and family history, and may do 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 they happen.
Your doctor will also give you a physical exam and listen to your heart and lungs.
Along with pulmonary function tests for asthma, your doctor might do allergy tests, blood tests, and chest and sinus X-rays. They might refer you to a pulmonologist, a lung specialist, to further test for asthma and other conditions that might be affecting it.
What Are the Different Lung Function Tests for Asthma?
There are several lung function tests for asthma. The most common include:
- Spirometry: This simple breathing test measures how much air you can breathe in and how quickly you can blow it back out. It shows the amount of airway obstruction you have. You’ll take a deep breath and exhale into a hose attached to a device called a spirometer. It records how much air you blow out (FVC, or forced vital capacity) and how quickly you do it (FEV, or forced expiratory volume). You can do spirometry before and after you inhale a short-acting medication called a bronchodilator, like albuterol. The bronchodilator makes your airways expand, which lets air pass through freely. This test can also help the doctor check your progress and decide if and how to change your treatment plan.
- Peak flow meter tests: These tests measure how well your lungs push out air. Although less accurate than spirometry, they’re a good way to check your lung function at home -- even before you feel any symptoms. A peak flow meter can help you learn what makes your asthma worse, whether treatment is working, and when you need emergency care.
- Challenge tests: Your doctor might call them provocation tests. There are three types:
- Exercise challenge test: This helps show if you have exercise-induced bronchoconstriction (EIB), which means exercise triggers your asthma symptoms. The doctor will watch your oxygen and heart rate while you run on a treadmill. This will show the doctor if exercise triggers your symptoms.
- Irritant challenge: The doctor exposes you to an asthma trigger, like smoke, perfume, or chemicals. Then you take a breathing test to see how your lungs respond. This helps them figure out what triggers your asthma.
- Methacholine challenge test: This test is more commonly used for adults than children. You might get it if your symptoms and screening spirometry don’t clearly show a diagnosis of asthma. When you have asthma and inhale methacholine, your airways spasm (contract as a reflex) and narrow. During this test, you inhale more methacholine mist before and after spirometry. If your lung function drops by at least 20%, you have asthma. The doctor will give you medicine at the end of the test to reverse the effects of the methacholine.
- FeNO test: You might hear it called the exhaled nitric oxide test. If you have allergic or eosinophilic asthma, it can help show how much inflammation you have and how well inhaled steroids are keeping it controlled. You’ll blow in to a handheld device that measures the amount of nitric oxide in your breath.
How Do I Prepare for a Lung Function Test?
Before your appointment, ask your doctor if there’s anything you need to do to prepare for spirometry.
Before taking a methacholine challenge test, let your doctor know if you recently had a viral infection, such as a cold, or any shots or immunizations, since these might affect the test results.
Other things to do on the day of a lung function test:
Can I Use Asthma Drugs Before a Lung Function Test?
Be prepared to adjust your asthma drugs. Some medications can affect the test results. You’ll need to stop different medications at different times. For example, you can take a short-acting inhaled bronchodilator like albuterol up to 8 hours before the test. But you should avoid long-acting inhaled bronchodilators for 48 hours before the test. Your doctor will tell you when to stop taking medication. Don't stop taking anything without talking to them first.