The short answer
Spirometry is a breathing test that measures how much air you can exhale and how quickly. It is the main test used to support a COPD diagnosis and to monitor how the lungs are working over time. Results are reported as numbers and compared with expected values — but interpretation depends on the pattern across several measurements and on your clinical history, not on a single figure.1
What spirometry measures
During a spirometry test, you breathe into a mouthpiece connected to a device called a spirometer. You will be asked to take a deep breath in and then blow out as hard and as long as you can. The test is usually repeated a few times to check consistency.
The two most commonly reported measurements are:
- FEV1 — the volume of air you can force out in one second. This is often used as a general indicator of how much airflow is limited.
- FVC — the total volume of air you can force out in one full breath.
The ratio of FEV1 to FVC is also calculated. A reduced ratio is one of the patterns used to identify airflow obstruction.
How results are reported
Results are usually expressed as a percentage of predicted values. Predicted values are based on reference populations and take into account age, height, sex, and sometimes ethnicity. This is why two people with the same FEV1 in liters may have different percentage results.
| Measurement | What it describes | Common reporting |
|---|---|---|
| FEV1 | Volume exhaled in the first second of a forced breath. | Liters and percentage of predicted. |
| FVC | Total volume exhaled in a full forced breath. | Liters and percentage of predicted. |
| FEV1/FVC ratio | Proportion of the breath exhaled in the first second. | Decimal or percentage. |
| Peak flow | Fastest speed of exhalation. | Liters per minute (sometimes). |
How results are interpreted
Interpretation usually follows a structured approach:
- Whether the FEV1/FVC ratio is reduced, suggesting obstruction.
- Whether the FEV1 percentage of predicted suggests a severity category.
- Whether the pattern changes after a bronchodilator — sometimes called reversibility testing.
- How the results compare with previous tests, if available.
- How the results fit with your symptoms and history.
Severity categories based on FEV1 percentage are commonly used in clinical guidance. They are a broad descriptor, not a personal prognosis, and they can change over time or with treatment.2
Limits and variability
Spirometry results can vary between tests for several reasons: effort, technique, recent illness, medication timing, and the equipment used. This is why clinicians usually look at trends over several tests rather than reacting to a single result.
Spirometry also does not capture everything. It does not measure how breathless you feel, how far you can walk, or how much your daily life is affected. Those are separate assessments — often done through questionnaires, walking tests, or conversation.
For related reading, see our guide A Personal Learning Map for COPD and our article Pulse Oximeter Readings.
What to discuss with your care team
- Can you explain my most recent spirometry results in plain language?
- How do these results compare with my previous tests?
- What pattern do the numbers show — obstruction, restriction, or something else?
- Does the pattern change after a bronchodilator?
- How do these numbers relate to how I actually feel day to day?
- When would you like to repeat the test?
Sources
- American Lung Association. Spirometry and COPD: Understanding Your Lung Function Test.
- Global Initiative for Chronic Obstructive Lung Disease (GOLD). Global Strategy for the Diagnosis, Management, and Prevention of COPD. Recent reports.
- National Heart, Lung, and Blood Institute. COPD: Diagnosis.
- Centers for Disease Control and Prevention. About COPD.