Checking for asthma involves breathing tests

Q. My doctor thinks I may have asthma. He wants to do a pulmonary function test. What can I expect?

A. Pulmonary function testing examines different aspects of how your lungs function. During the test, you will breathe in and out through a tube that is connected to various machines. A test called spirometry measures how forcefully you can inhale and exhale while taking as large a breath as possible. Give this test your best effort. You can skew the results by not trying hard.

A separate test will measure your lung volume: how much air your lungs can hold, and how much air is left in your lungs when you breathe out as much as you can. This may be done in one of two ways. One way is to have you inhale a small amount of a specific gas (such as helium).

You can also measure lung volume by sitting inside an airtight cubicle. You breathe in and out through a pipe in the wall. Your breathing will cause the air pressure inside the box to change. This pressure change is measured and used to calculate the amount of air you are breathing.

Your doctor will also want to measure how efficiently your lungs deliver oxygen and other gases to your bloodstream. To measure this, you breathe in a small quantity of carbon monoxide (too little to do any harm). The amount of carbon dioxide you breathe back out is measured. Your ability to absorb carbon monoxide is representative of your ability to absorb other gases into your bloodstream.

Today, doctors can quite accurately measure the oxygen in your blood with a simple device that clips to your finger. The old way of doing it required sticking a needle into an artery; you should be glad it can be done so simply now.

What do doctors learn from pulmonary function testing? As in your case, they can learn whether you have signs of asthma. They also can determine if you have chronic obstructive pulmonary (or lung) disease, known as COPD or COLD. This condition is seen most often in long-term smokers. They also can measure the severity of a group of diseases that make the lung stiff, chronic restrictive lung disease.

You should hear back from your doctor within a week.

Full-scale pulmonary function testing requires you to go to a hospital or medical facility. But if your doctor finds that you do have asthma, you may find yourself doing your own simple pulmonary function testing at home. You may be given a small device that you hold in your hand and blow into. It's called a peak flow meter, and it measures how hard you can blow air out of your lungs. When you get an asthma attack, this simple device can help you judge how bad it is.

• Dr. Komaroff is a physician and professor at Harvard Medical School. Submit questions or get additional information at AskDoctorK.com.