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Your doctor may need to test your lungs to figure out if you have chronic obstructive pulmonary disease (COPD). To do this, doctors usually use a machine called a spirometer.

When you have this test, you take as big a breath as you can and then blow out, as hard as you can, into a mouthpiece. The machine will print out the results, including two very important measurements:¹

• The total amount of air you were able to breathe out, which is called forced vital capacity, or FVC
• The amount of air you breathed out during the first second of the test, which is called forced expiratory volume, or FEV 1 .

After you do this test once, your doctor will ask you to breathe in a drug called a bronchodilator. This kind of drug can help make your airways wider so you can breathe more easily. Then you do the same test again, to see if your lungs work any better.

Having a spirometry test before and after you take a bronchodilator drug helps the doctor tell whether you have COPD or a different lung disease called asthma. This is important because it will affect your treatment.

If you have COPD, there may not be much difference between the test results before and after the bronchodilator. There is likely to be a greater difference if you have asthma.

When you have done the test twice, your doctor will use the results to work out your lung function. The way the doctor does this is by dividing the FEV1 (the measurement of the air you breathed out in the first second of the test) by the FVC (the total amount of air you breathed out). You may see this written out as FEV1/FVC.

You may hear the results given as a percentage. That percentage is a way of comparing the results of your spirometry test with the results for other people of your age, height and weight. So, for example, your doctor may tell you that 70 percent and above is normal.²

If you have COPD, you will probably have regular spirometry tests. For example, if you're doing very well, you may have them once or twice a year. If your COPD causes you a lot of trouble, you'll probably have tests more often.

Your doctor will check the results to see how well your lungs are working. Everyone's lung function goes down as they get older. But the lung function of people with COPD usually goes down more quickly.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.