How to Read Spirometry, From FEV1 and FVC to Their Ratio

Spirometry measures how much air you can blow out after a full breath in, and how fast. It’s the main test for asthma and chronic obstructive pulmonary disease (COPD), and it rests on three numbers: FEV₁, FVC, and the ratio between them. Each is compared with what’s expected for someone of your age, sex, and height, and the comparison matters as much as the number.

This article is general information, not medical advice. Spirometry results are best interpreted by the clinician who ordered the test.

The Three Numbers

• FVC (forced vital capacity): the total volume, in litres, you can blow out as hard and as long as you can after the deepest breath you can take.
• FEV₁ (forced expiratory volume in 1 second): how much of that comes out in the first second.
• FEV₁/FVC: the share of the total that comes out in the first second. Healthy adults usually blow out most of their air in the first second, and the ratio falls slowly with age.

When the airways are narrowed, as in asthma or COPD, air comes out slowly: FEV₁ falls more than FVC, and the ratio drops. That pattern is obstruction. When FEV₁ and FVC are both low but the ratio is normal, the lungs may hold less air, which needs other tests to confirm.

Compared With What’s Expected

Results are compared with reference equations for healthy people. The European Respiratory Society and American Thoracic Society’s 2022 interpretation standard (Stanojevic and colleagues) recommends:

• Using the Global Lung Function Initiative (GLI) reference equations.
• Judging a value abnormal when it’s below the lower limit of normal (LLN), the value only 5% of healthy people fall below, often reported as a z-score below −1.645, rather than below a fixed 80% of predicted.
• Grading severity by z-score.

An official American Thoracic Society statement in 2023 (Bhakta and colleagues) recommended the GLI’s race-neutral equations, published by Bowerman and colleagues, which no longer adjust expected values by race.

The Global Initiative for Chronic Obstructive Lung Disease (GOLD) uses a simpler rule for diagnosing COPD: a post-bronchodilator FEV₁/FVC below 0.70. A fixed cut-off is easy to apply but can label healthy older people as obstructed and miss obstruction in younger people, which is why lung function societies prefer the LLN. Both appear in practice.

A Good Test Is a Hard Test

The ATS/ERS standard for spirometry (Graham and colleagues, 2019) asks for at least three acceptable blows, each a maximal breath in and then a blast out, continued until no more air comes out. A weak or short effort lowers FVC and changes the ratio, so results from a poor test aren’t comparable with others.

Comparing Tests Over Time

• Compare litres, not only percentages of predicted, since predicted values change as you age.
• Note whether a test was before or after a bronchodilator; they’re different measurements.
• Use the same kind of device where you can. Home spirometers vary in accuracy.

Following It in Kaizora

Forced Expiratory Volume in 1 Second, Forced Vital Capacity, and FEV₁/FVC Ratio are metrics on the Vitals page. Log a clinic test’s values with Log, noting before or after a bronchodilator in its note, or bring in a home spirometer’s results through Apple Health’s export, which carries FEV₁ and FVC. Each value keeps its device, so a clinic test and a home device stay apart on the chart.

The Short Version

• FVC is how much you can blow out; FEV₁ is how much in the first second.
• A low FEV₁/FVC ratio means obstruction, as in asthma or COPD.
• Lung societies judge results against the lower limit of normal; GOLD uses a ratio of 0.70 for COPD.
• Only maximal, repeatable blows give comparable results.

Sources

• Stanojevic S et al., “ERS/ATS technical standard on interpretive strategies for routine lung function tests,” European Respiratory Journal 60, no. 1 (2022): 2101499. doi:10.1183/13993003.01499-2021
• Bowerman C et al., “A race-neutral approach to the interpretation of lung function measurements,” American Journal of Respiratory and Critical Care Medicine 207, no. 6 (2023): 768–774. doi:10.1164/rccm.202205-0963OC
• Bhakta NR et al., “Race and ethnicity in pulmonary function test interpretation: an official American Thoracic Society statement,” American Journal of Respiratory and Critical Care Medicine 207, no. 8 (2023): 978–995. doi:10.1164/rccm.202302-0310ST
• Graham BL et al., “Standardization of spirometry 2019 update: an official American Thoracic Society and European Respiratory Society technical statement,” American Journal of Respiratory and Critical Care Medicine 200, no. 8 (2019): e70–e88. doi:10.1164/rccm.201908-1590ST
• Global Initiative for Chronic Obstructive Lung Disease, 2025 GOLD Report.