When I was 14, after performing a pulmonary function test (PFT), I had a hundred questions for the respiratory therapist who performed the test.  After I basically forced him to tell me, he said, "You have about a 35% lung function."

"Cool!" I said.  "What does that mean?"

So, what is a PFT?  Basically, it's a test where you breath into a mouthpiece to a device called a spirometer.  The spirometer measures your lung function, and helps your doctor

determine if you have asthma and how severe it is. This test is also called spirometry.

There are a lot of tests you do, yet the most important in diagnosisng asthma is performing a forced vital capacity (FVC).  In this test where you take in a breath as deep as you can, and then blow out as hard as you can until you can't exhale any more.  While you're performing the FVC you'll note some pretty cool loops on the computer, from which the following calculations are made:

1.  PEFR:  Peak Expiratory Flow Rate.  This is the maximum flow that can be generated with a forced exhalation. This is simlar to what is obtained when you blow into your peak flow meter, but more accurate and reliable.

2.  FEV1: This is a test that measures the amount of exhaled air during the first second of the FVC.  This number cannot be faked, and is the most important value for diagnosing asthma. 

3. FEV1/FEV6:  This test is used as a substitute for FEV1 in adults who have significant air trapping and who get "light headed" while trying to forcibly do spirometry.

4. FEV1/FVC:  This may be a more accurate measure of severity of asthma in children as compared with FEV1.

So, what use are the above measurements?  To answer this question, we must have a basic definition of what asthma is.  Therefore, asthma is a reversible obstructive airway disease. 

Also, before you take the test, the RT will estimate your predicted normal results for the values listed above based on your height and weight.

Airflow obstruction: If your actual FEV1 is less than 80% of your predicted FEV1, you are considered to have airflow obstruction.  And your lung function will be considered to be 80%.

If you happen to be a hardluck asthmatic as I was as a kid, and your FEV1 is 35%, then you are considered to have a 35% lung function. 

So that's the first component of asthma.  Now we must consider reversibility.

Reversibility:  After you do your initial sequence of tests, you will take a breathing treatment with the bronchodilator of your doctor's choice.  In most cases it will be an Albuterol breathing treatment. 

If your FEV1 increases by 10-15% within 15 minutes following a bronchodilator, this is indicative of airflow obstruction that is reversible.