Asthma Diagnosis and Tests

Medically Reviewed by Jennifer Robinson, MD on May 11, 2023
Written by WebMD Editorial Contributors
5 min read

Your doctor will use tests to diagnose asthma. Some measure how well your lungs work. Others can tell if you’re allergic to mold, pollen, or other things.

All of these asthma tests help your doctor decide if you have asthma and other conditions that often come with it, like allergies, GERD, and sinusitis.

Your doctor will start with a physical exam. They will:

They’ll also ask you about signs of asthma such as:

Next, your doctor will ask about your symptoms and overall health to figure out if asthma or something else is causing your problem. Some questions might include:

  • What are your symptoms?
  • When do you have them?
  • What seems to trigger them? What about cold air, exercise, or allergies?
  • Do you have hay fever or allergies?
  • Does a family member have hay fever, asthma, or allergies?
  • What other health problems do you have?
  • What medications do you take?
  • Do you often come into contact with tobacco smoke, pets, dust, or chemicals in the air?
  • What do you do for a living?

Lung function tests are a way to check how well your lungs are working. Doctors use them 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 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. It's a simple, quick, and painless way to check your lungs and airways. You take a deep breath and exhale into a hose attached to a device called a spirometer. It records how much air you blow out (called forced vital capacity or FVC) and how quickly you do it (called forced expiratory volume or FEV). 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 have them done only 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. The 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.
  • Exhaled nitric oxide test. You’ll breathe into a tube connected to a machine that measures the amount of nitric oxide in your breath. Your body makes this gas normally, but levels could be high if your airways are inflamed.

Even if your lung function tests are normal, your doctor may order other tests to see what could be causing your asthma symptoms.

  • 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 has 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.

Nasal polyps or sinusitis may make asthma harder to treat and control. Sinusitis, also called a 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 may order a special sinus X-ray, called a CT scan, to examine your sinuses if they think you have an infection. If you have sinusitis, you will be treated with antibiotics for at least 10 to 12 days. Treating the sinusitis may help prevent asthma symptoms.

The doctor may also do tests for other conditions that can make asthma worse, like:

  • Gastroesophageal reflux disease (GERD)
  • Allergies

Based on these tests and your symptoms, your doctor may diagnose you with asthma. The next step is to find out if your asthma is severe. This will help them decide your treatment. You’ll have one of these four types of asthma:

  1. Mild intermittent asthma. Mild symptoms less than twice a week. Nighttime symptoms less than twice a month. Few asthma attacks.
  2. Mild persistent asthma. Symptoms three to six times a week. Nighttime symptoms three to four times a month. Asthma attacks might affect activities.
  3. Moderate persistent asthma. Symptoms every day. Nighttime symptoms five or more times a month. Asthma attacks affect your activities.
  4. Severe persistent asthma. You have ongoing symptoms both day and night. They’re so frequent that you have to limit your activities.

If a doctor diagnoses you with asthma, they can prescribe medications to help manage your condition and prevent attacks.