Spirometry

Spirometry measures how much air you can breathe out in one breath and how fast you can blow it out, using a tube joined to a machine called a spirometer. The two main numbers are FVC, the total amount of air you blow out, and FEV1, the amount you blow out in the first second.

A doctor may ask for spirometry to find out whether breathlessness, a long-standing cough, wheeze or chest tightness is caused by a lung condition such as asthma or COPD, to check how a lung condition and its treatment are doing, or to check the lungs before an operation. It is also used to keep watch on conditions such as interstitial lung disease (scarring of the lungs).

The lab will tell you what to do. In general:

Tell the staff first if you have a chest infection, a recent heart attack, stroke or operation, active tuberculosis, a recently collapsed lung, uncontrolled high blood pressure, or late pregnancy. The test may be put off.

A spirometry test goes like this:

1. Your height and weight are noted, as the results are worked out from them.
2. You sit next to the machine. A soft clip goes on your nose, and you close your lips tightly around the tube.
3. You breathe in as deeply as you can, then blow out as fast and as hard as you can until your lungs feel empty.
4. This is repeated at least twice more, so that your best efforts can be compared.
5. You may be given an inhaler and, after a short break, asked to blow again to see whether it opens the airways.

A trained person does it in a clinic or hospital. It takes about 15 minutes, or about 30 with an inhaler.

You can usually go home and carry on as normal. Blowing hard can leave you dizzy, breathless or coughing for a short while, so rest until you feel steady. A specialist reviews the tracing and sends the report to your doctor, which can take up to a few weeks, so ask when to expect it.

Spirometry carries little risk, but it is not suitable for everyone, which is why you are asked about recent illness, heart problems and surgery first. The usual effects are brief dizziness, cough or breathlessness. Rarely, in people with certain lung conditions, it can cause a collapsed lung.

Your numbers are compared with those expected for a person of your age, sex and height. Normal ranges can differ slightly between labs, so the report prints the lab's own. A low FEV1 compared with FVC points to narrowed airways, as in asthma or COPD. A low FVC with a normal ratio can mean smaller or stiffer lungs, and needs more tests to confirm.

If the result improves after the inhaler, the airways can open up, as in asthma. This alone does not separate asthma from COPD.

What spirometry cannot show: it takes no pictures of the lungs, it cannot diagnose a disease on its own, and a normal result does not rule out asthma. Related tests include a chest X-ray or CT scan, tests of lung volume and of how well oxygen passes into the blood, and a methacholine challenge for asthma.