Spirometry: how the test is performed and what a spirogram shows

Spirometry is a lung function test. It measures how much air a person exhales in a single breath and how quickly. A referral for spirometry and the result of a spirogram raise the same questions: how the procedure is performed and what the numbers mean.

What spirometry is and what it measures

Spirometry (spiro — to blow, metreo — to measure) is a method of examining external respiratory function. It includes measurement of volumetric and flow-related respiratory parameters. The device is called a spirometer, and the recording of the result is called a spirogram.

Spirometry is part of the group of external respiratory function tests. Besides it, the group includes measurement of lung volumes and diffusion capacity. Spirometry measures air flow: how much a person exhales and at what speed. This is how a wide range of lung diseases is assessed.

How spirometry is performed

The test is performed by a healthcare professional in a polyclinic, clinic, or hospital. The procedure takes no more than an hour.

Before the test, patients are usually asked not to smoke for 24 hours, not to eat a heavy meal for at least 2 hours, and not to drink alcohol for at least 4 hours, and to wear loose clothing. A healthcare professional advises on stopping medications and inhalers.

Before the test, it is checked that it is safe for the person to undergo it. Height and weight are measured — they are needed to calculate the results.

During the test, the patient sits next to the device to which a tube is attached; a soft clip is placed on the nose. Then the person takes a deep breath, closes their lips tightly around the tube, and exhales as quickly and forcefully as possible to completely empty the lungs. The test is repeated two more times. A weak seal of the lips around the tube gives an inaccurate result.

Dizziness and coughing are possible, but this should not last long. The result is analyzed by a specialist and passed to the doctor; usually the response is ready within a few weeks.

Spirometry with a bronchodilator: why it is needed

After the first series of exhalations, the healthcare professional may ask the patient to use an inhaler and repeat the test after a short pause. This is how it is checked whether the inhaler affects the result.

The bronchodilator challenge is one of the types of functional tests that are part of spirometry along with provocation tests. Inhaling the medication before or during the test helps to understand whether the parameters change under its effect.

What a spirogram shows

The parameters are compared with predicted values for age, sex, height, and weight and are expressed as a percentage of them. A significant decrease is considered a deviation. Normal ranges differ slightly between laboratories.

• FVC — forced vital capacity of the lungs.
• FEV1 — forced expiratory volume in the first second.
• Tiffeneau index — the FEV1/VC ratio as a percentage, normally 75–80%.
• PEF — peak expiratory flow.
• MEF 25, 50, 75 — maximal instantaneous expiratory flows at a fraction of FVC.

Other values also appear in the report: DLCO, TLC, RV.

Obstructive diseases (asthma, emphysema, chronic bronchitis, infections) affect the airways: too much air remains in the lungs, and they empty more slowly. Other diseases make the lungs scarred and smaller in volume, so there is too little air in them and they transfer oxygen to the blood poorly. Examples are severe obesity, pulmonary fibrosis, sarcoidosis, and scleroderma. Muscle weakness can also cause deviations even if the lungs are normal.

Who is referred for the test and what the limitations are

A referral is given to find the cause of symptoms and to suspect asthma, COPD, or pulmonary fibrosis, to monitor a known disease and assess the effect of treatment, and to check the lungs before surgery. The test is offered to smokers over 55 as part of lung cancer screening.

In some cases, to assess whether a person has asthma, a methacholine challenge is performed: the person inhales the irritant methacholine, and a decrease in parameters of 20% or more from baseline may indicate asthma.

The test is not performed in case of chest infection, certain heart diseases and uncontrolled hypertension, recent heart attack or stroke, or after certain operations. In certain lung diseases there is a small risk of pneumothorax.

When to see a doctor

If the result has not arrived after several weeks, call the place where the test was performed.

A spirogram is not a diagnosis. It is interpreted by a doctor together with symptoms and other examinations.

Spirometry alone is not a reason to call an ambulance. 103/112 — if signs of a heart attack or stroke appear.

This material is for reference: it explains the procedure and terms but does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer