A complete guide to lung function testing: what FEV1, FVC and DLCO are really telling you
Division of Pulmonary Medicine, Taipei Veterans General Hospital Hsinchu Branch · Health information on reading a lung function test
You have your reportAn abnormal lung function report: self-assessment and what to do next Item by itemSmart report system （中文）
Ms. Chen is 32. Four words on her health-check report — “abnormal lung function” — kept her awake all night. The next day she brought the report to clinic. It said FVC 68% of predicted, and the conclusion read “possible restrictive ventilatory defect”.
The first thing her doctor looked at was not that 68%, but the flow curve and the quality grade in the corner of the report — the curve had collapsed partway through, and the three attempts were a long way apart. On a repeat test her FVC came back at 92%. It turned out that at the first test she had just got over a cold and had not dared blow out hard. “Before the number, ask whether this report can be trusted” is the first lesson in reading lung function.
Key points on this page
- A lung function test uses blowing out to measure what the lungs can do. It is not a way of looking for a lump in the lung — that needs imaging.
- Three core numbers: FEV1 (how much you blow out in the first second), FVC (how much you blow out in total) and FEV1/FVC (the ratio). The ratio decides whether the pattern is “obstructive” or “restrictive”.
- Quality decides everything. Not blowing hard enough, stopping too early or a leak will all produce a report that looks abnormal but cannot actually be trusted.
- Normal lung function does not mean nothing is wrong. Asthma can be completely normal between attacks; early interstitial lung disease may show nothing but a fall in DLCO.
- The current international standards recommend interpreting results against population reference values and the lower limit of normal (LLN), using z-scores, rather than holding to a single fixed ratio.
What a lung function test measures
A lung function test measures what the lungs can do, not what they look like. It answers three kinds of question:
- Can air get in and out? — whether airflow is obstructed (an obstructive pattern).
- How much air can the lungs hold? — whether lung volume has fallen (a restrictive pattern).
- Can oxygen get from the alveoli into the blood? — how efficient gas exchange is (gas transfer, DLCO).
These three things are independent of one another. Some people have completely normal airflow but very poor gas transfer; others have severe obstruction and near-normal transfer. So a full assessment usually needs more than one test.
What a lung function test cannot do
It cannot see a tumour, a nodule, pneumonia or a pleural effusion — that is the job of a chest X-ray and a CT scan. It also cannot diagnose asthma on its own: what characterises asthma is “variable” airflow limitation, and between attacks it can be completely normal. Lung function is a tool for narrowing a clinical question down, not the final answer.
How to read the numbers on the report
| Abbreviation | Full name | In plain words | What a low value may mean |
|---|---|---|---|
| FVC | Forced vital capacity | How much you blow out in total when you take a deep breath in and then blow out hard all the way | Low: lung volume may have fallen — or you simply did not finish blowing out |
| FEV1 | Forced expiratory volume in one second | How much of that breath you blow out in the first second | Low: the airways may have narrowed, or lung volume may have fallen |
| FEV1/FVC | The one-second ratio | The proportion blown out in the first second | A low ratio is the key marker of obstruction |
| FEF25-75% | Mid-expiratory flow | The flow rate in the middle of the blow, weighted towards the small airways | Varies a great deal; on its own it is not enough to draw a conclusion |
| DLCO | Diffusing capacity for carbon monoxide | How efficiently gas passes from the alveoli into the blood | Often becomes abnormal earlier than the other numbers |
| TLC | Total lung capacity | The most air the lungs can hold (needs lung volume measurement) | Only a low TLC genuinely supports a restrictive pattern |
The “% of predicted” on the report compares your value with a reference population of the same sex, age and height. The ERS/ATS 2022 technical standard recommends interpreting results instead against the lower limit of normal (LLN) and the z-score, because a fixed percentage threshold over-diagnoses in older people and misses cases in the young.
The common kinds of lung function test
- Basic spirometry Strong evidence Covered by the NHI— the one done most often; it measures FEV1, FVC and the ratio.
- Bronchodilator test (BDT) Strong evidence Covered by the NHI— you blow once before and once after inhaling the medicine, which answers the question “is the obstruction reversible?”. Reversible points towards asthma, irreversible towards COPD.
- Lung volume measurement (body box or gas dilution) Strong evidence Covered by the NHI, conditions apply— measures total lung capacity (TLC), and is the only way to confirm a restrictive pattern.
- Gas transfer (DLCO) Strong evidence Covered by the NHI, conditions apply— assesses gas exchange, and is particularly important in interstitial lung disease, emphysema and pulmonary vascular disease.
- Impulse oscillometry (IOS) Moderate Covered by the NHI, conditions apply— done with quiet breathing alone, so it suits people who cannot blow well, and small airway assessment.
- Fractional exhaled nitric oxide (FeNO) Moderate Covered by the NHI, conditions apply— what it measures is not airflow but whether there is allergic inflammation in the airways.
- Six-minute walk test Strong evidence Covered by the NHI, conditions apply— measures what you can actually do, and how your blood oxygen changes with activity.
Coverage conditions and availability differ from hospital to hospital. Strength of evidence and National Health Insurance (NHI) coverage are two independent axes; see how we grade evidence for more.
Why quality matters more than the numbers
Lung function is one of the few tests that depend heavily on the person being tested. In the same person on the same day, blowing hard and not blowing hard can differ by more than 20%. The ERS/ATS 2019 technical standard lays down explicit criteria for acceptability and repeatability, and reports usually present this as a grade (A, B, C and so on) or as a note.
What a report you can trust looks like
- The flow curve has a steep start, with no hesitation and no weak initial effort
- The breath out lasts long enough, so that the end of the curve tails off gently rather than being cut off suddenly
- At least three acceptable attempts, with the best two close enough together
- No coughing, no leak, and no tongue blocking the mouthpiece
If the quality is poor and the conclusion is “abnormal”, the thing to do is repeat the test, not simply accept that diagnosis — and certainly not go on to arrange a CT scan because of it.
Three common patterns
1. Obstructive: the air will not come out
A low FEV1/FVC (in adults the cut-off used is often 0.70, or the LLN). It means the airways have narrowed. The next step is a bronchodilator test: clearly reversible points towards asthma, while irreversible with a history of smoking or exposure to air pollution points towards COPD.
2. Restrictive: the air will not go in
A low FVC, but a normal or high FEV1/FVC. But blowing alone cannot confirm this — a low FVC may simply mean the blow was not finished. Confirming it requires lung volume measurement, to see whether TLC really has fallen. If FVC and DLCO fall together, interstitial lung disease has to be taken seriously.
3. Mixed: both at once
Both FEV1/FVC and FVC are low. There may genuinely be obstruction and restriction at the same time, or severe obstruction may be trapping air and making the FVC look smaller than it is. Telling those two apart again needs lung volume measurement.
One combination that is often overlooked
The blowing numbers are completely normal, but the DLCO is low. This combination deserves to be taken seriously — early interstitial lung disease, emphysema and pulmonary vascular disease (including pulmonary hypertension and chronic pulmonary embolism) can all look like this. Please do not stop here just because “the lung function report was normal”.
When to seek help immediately
Situations not to put up with while you wait for a test
A lung function test is something that can be arranged in due course. But in the following situations, see a doctor first; do not book a test first:
- Sudden breathlessness, breathlessness at rest, or not being able to finish a sentence
- Lips or fingernails turning purple, or a change in consciousness
- Chest pain together with difficulty breathing — a heart attack, a pneumothorax and a pulmonary embolism have to be ruled out first
- Coughing up blood
- Clear weight loss over a few weeks, or night sweats, together with a long-standing cough
One more thing to note: blowing for a lung function test takes effort, so anyone who has recently had a pneumothorax, a heart attack, eye or abdominal surgery, or coughed up blood should say so first and let their doctor decide whether to put the test off.
Common questions
My report says “abnormal” — does that mean I am ill?
Not necessarily. By far the commonest reason is that the blowing was not done well. Next most common, if the reference values used to interpret it do not suit your population, a normal result can be read as abnormal. When you are handed an abnormal report, the sensible order is: check the quality and the reference values first, repeat the test if necessary, and then have your doctor interpret it together with your symptoms and history. You can start by narrowing things down with the abnormal lung function report self-assessment.
My lung function was normal, so why does my doctor still think I might have asthma?
Because the airflow limitation in asthma is variable. Between attacks, lung function can be completely normal. Your doctor will also look at the pattern of your symptoms, your FeNO, your blood eosinophil count and the day-to-day variation in your peak expiratory flow, and will arrange a bronchial challenge test if needed.
What do I need to do before the test?
The general advice is: stop your bronchodilator beforehand as your doctor instructs (if a reversibility test is planned), avoid a large meal on the day, avoid strenuous exercise and smoking, and wear loose clothing. If you have a cold at the moment, or have just had a respiratory infection, tell the person doing the test — it will affect the result noticeably.
How often should it be repeated?
It depends on the disease. In asthma and COPD it is usually done after a change of treatment and once a year; in interstitial lung disease, because the changes in FVC and DLCO are what decide whether the disease is progressing, follow-up is closer and is arranged by your doctor.
Does a lung function test involve any radiation?
None at all. A lung function test is only blowing and breathing; it uses no X-rays or any other ionising radiation, and no contrast injection is needed. That is one of the reasons it suits being repeated again and again.
References
- Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J. 2022;60(1):2101499.
- Graham BL, Steenbruggen I, Miller MR, et al. Standardization of spirometry 2019 update: an official American Thoracic Society and European Respiratory Society technical statement. Am J Respir Crit Care Med. 2019;200(8):e70-e88.
- Quanjer PH, Stanojevic S, Cole TJ, et al. Multi-ethnic reference values for spirometry for the 3-95-yr age range: the global lung function 2012 equations. Eur Respir J. 2012;40(6):1324-1343.
- Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49(1):1600016.
- Global Initiative for Chronic Obstructive Lung Disease. Global strategy for the diagnosis, management, and prevention of chronic obstructive pulmonary disease: 2026 report. GOLD; 2025. Available from: https://goldcopd.org/2026-gold-report/
References follow the ICMJE (Vancouver) style. Guidelines and official documents are given with their version or year of publication; legislation and National Health Insurance (NHI) rules are given with the responsible authority and the version date. Last checked: 8 October 2026.
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