Pulmonary Function Testing To Diagnose Asthma
Last updated: July 2022
Some refer to it as spirometry. Where I work it's called a pulmonary function test (PFT). Whatever you refer to it as, it's a breathing test that can help diagnose asthma. The Global Initiative for Asthma (GINA) released new asthma guidelines in 2017.
Here's what they say about PFTs
Forced Vital Capacity. This is the total amount of air you can exhale after a full inhalation. You inhale as deep as you can. Then you blow as hard and fast as you can. You exhale until you have no air left to exhale.
Flow Volume Loop. This will appear on the computer as you do this test. The expiratory loop will appear concave if you are having asthma symptoms. This is indicitive of airflow limitation. It means your airways are obstructed. If you are not having asthma symptoms, the expiratory loop will not appear concave. You can see this in the picture above.
Forced Expiratory Volume in one second (FEV1). The computer uses your FVC to calculate how much air you exhaled in one second. This is the best indicator of asthma. It cannot be faked. Normal is 80% or better. It may be less than 80% if you're having asthma symptoms. The goal of any asthma treatment program is to keep this above 80%. Your lung function should be normal or close to normal between attacks.
FEV1/ FVC. This is a very senstive calculation. It is a tel-tale indicator of airflow limitation. Normal is 80%. Less than 80% may means airflow limitation or asthma.
Bronchodlator. After performing the test, you will be given a breathing treatment. The most common medicine used is albuterol. It relaxes airway smooth muscles to open airways. This treatment takes 5-10 minutes. After the treatment you will wait about 15 minutes. Then, you will repeat the FVC.
Post bronchodilator FVC. This should improve if you have asthma. If this value improves by more than 10-12%, you may have asthma. This is because asthma is reversible with treatment. The reversibility of symptoms is a tel-tale sign of asthma. The greater the improvement, "the more confident you can be of a diagnosis of asthma." That's what the GINA guidelines say.
Please note that there is no one test for confirming a diagnosis of asthma. But, a PFT can point the finger in that direction. The value that should improve are FEV1 and FEV1/FVC. The flow volume loop should be closer to normal (less concave).
Important note. It is best to do a PFT before starting treatment. If you already take asthma medicine, you might have to hold taking them the day of the test. But, sometimes this is not possible.
In such cases, it may not be prudent to wait for a PFT to begin treatment.
Bronchoprovocation. The goal of any asthma treatment program is to have normal lung function. It's to have no asthma symptoms at all. Even if you are breathing easy, your doctor may be suspicious of asthma due to your reported history. If this is true, your doctor may order this special test. Before performing the PFT, you will inhale a medicine called methacholine. If you have asthma, this medicine will cause asthma symptoms. A post bronchodilator FVC will show improvements in lung function if you have asthma. So, this is another option for diagnosing asthma.
What else are PFTs good for?
They are helpful for diagnosing. They are also helpful for monitoring your progress over time. Your doctor may order a test to see how your treatment regimen is working. This can help you and your doctor make necessary tweaks to your treatment regimen.
What to make of this?
So, if your doctor suspects you have asthma, you can probably plan on doing a PFT at some point. Based on my personal experience, I can tell you it's not easy to do with asthma. But, it's an ideal test for diagnosing asthma. It's also useful for monitoring your progress over time.
Has laughter ever triggered your asthma?