Lung function tests are a way to check how well your lungs are working. Doctors use lung function tests to diagnose asthma and to monitor its progression. Monitoring asthma with lung function tests is helpful, because you may not always be able to tell -- just from your symptoms -- whether or not your asthma is under control.
In most cases, you have lung function tests in an exam room that contains special devices to measure lung function. A specially trained respiratory therapist or technician is likely to do the tests.
Ask your doctor if you should do anything to prepare for your lung function tests. For instance, you might need to adjust your medication. You may also need to avoid heavy meals, smoking, and any irritants or other substances that might trigger an asthma attack.
Types of Lung Function Tests
These lung function tests are commonly used to diagnose and monitor asthma:
- Spirometry is the most common of the lung function tests used for asthma. It's a simple, quick, and painless way to check your lungs and airways. You just 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). Your score is lower if your airways are swollen or constricted because of asthma or other lung diseases. Your doctor may want you to have several spirometry lung function tests to monitor your asthma over time. You might have spirometry before and after you take medication to see if the medication helps. Your doctor may also want readings taken during exercise to see how your airways react to exercise.
- Challenge tests are lung function tests used to help confirm a diagnosis of asthma. You inhale a small amount of a substance known to trigger symptoms in people with asthma, such as histamine or methacholine. After inhaling the substance, someone tests your lung function. Because challenge tests can trigger an asthma attack, you should only have them done by someone with experience.
- Peak flow meter tests measure how well your lungs push out air. Although they are less accurate than spirometry, these lung function tests can be a good way to regularly test your lung function at home -- even before you feel any symptoms. A peak flow meter can help you know what makes your asthma worse, whether treatment is working, and when you need to seek emergency care.
A peak flow meter is a handheld plastic tube with a mouthpiece on one end, which you breathe into. Your doctor might ask you to use the peak flow meter each day and write down the readings. After a couple of weeks, you report the results to your doctor.
Other Tests You May Need if You Have Asthma
Even if your lung function tests are normal, your doctor may order other tests to see what could be causing your asthma symptoms.
- Allergy tests are used to pinpoint substances called allergens to which you might be allergic. Your doctor might do such tests by pricking your skin with a tiny amount of the allergen, by injecting it under your skin, or by giving you a blood test.
- Gas and diffusion tests can measure how well your blood absorbs oxygen and other gases from the air you breathe. You breathe in a small amount of a gas, hold your breath, then blow out. The gas you exhale is analyzed to see how much your blood absorbed.
- X-rays may tell if there are any other problems with your lungs, or if asthma is causing your symptoms. High-energy radiation creates a picture of your lungs. You may be asked to briefly hold your breath while you stand in front of the X-ray machine.