An abnormal lung function report: look at three numbers first, then decide whether to worry
Division of Pulmonary Medicine, Taipei Veterans General Hospital Hsinchu Branch · Symptom triage
Related pagesFind your symptomSmart report system (full interpretation) （中文）Breathlessness, difficulty breathing
Three sentences first
- Look at FEV1/FVC first — it decides whether your report is obstructive or restrictive, and the two mean entirely different things.
- An abnormal report does not mean you have a disease. Not blowing hard enough, or not for long enough, produces a false abnormality. A report that fails on quality should not be used to make a diagnosis.
- A normal report does not mean all is well either. Lung function in asthma can be completely normal between attacks, and early interstitial lung disease may show nothing but a fall in DLCO.
Three numbers, three questions
It answers: how much air can your lungs hold? You take a deep breath in, blow out as hard as you can until there is nothing left, and the total is measured.
A low value means the lungs have “got smaller”, or that you cannot fill them — which may be pulmonary fibrosis, fluid in the pleural space, a deformed chest wall, neuromuscular weakness or obesity, or may only mean you did not blow hard enough.
It answers: can the air get out? How much you blow out in that first second.
A low value means the airways are not letting air through freely, or that the overall lung volume has fallen. On its own it means little; it has to be read together with the FVC.
It answers: are the airways obstructed? This is the first cut in classifying a report.
The threshold commonly used in adults is < 0.70 after a bronchodilator (the standard the GOLD guideline uses to diagnose COPD). A fixed ratio does, however, tend to over-diagnose in older people and to miss cases in the young, so the “lower limit of normal (LLN)” is now also widely used as the criterion.
It answers: can the oxygen actually get from the alveoli into the blood?
What it measures is the efficiency of gas exchange. It falls in interstitial lung disease, in emphysema and in pulmonary vascular disease, and it often becomes abnormal earlier than the other numbers. If you are “breathless on exertion but with normal ordinary lung function”, DLCO is particularly worth doing.
The two main abnormal patterns
| Obstructive | Restrictive | |
|---|---|---|
| FEV1/FVC | Reduced (< 0.70, or < LLN) | Normal or high |
| FVC | Normal or low | Reduced |
| Where the problem is | The air “will not come out” — the airways have narrowed | The air “will not go in” — the lungs or the chest wall will not expand |
| Common causes | Asthma, COPD, bronchiectasis, bronchiolitis | Interstitial lung disease, fluid in the pleural space, a deformed chest wall, neuromuscular disease, obesity, ascites |
| Next step | A bronchodilator test — to see whether the obstruction is reversible | Lung volume measurement and DLCO — to confirm whether the restriction is real or a measurement problem |
| Reversible means | Points towards asthma; irreversible points towards COPD | — |
Blowing alone (spirometry) cannot truly confirm a restrictive pattern — a low FVC may only mean you did not blow enough. Confirming it needs lung volume measurement (by body box or gas dilution) to see whether total lung capacity (TLC) really has fallen. That is also why “abnormal lung function” on a health check so often has to be repeated in a chest medicine clinic.
Before you read the report, make sure it can be trusted
A report that fails on quality should not be used to make a diagnosis
Lung function testing is a test that needs your co-operation. Not blowing hard enough, stopping half way, a leak, a cough — any of these will produce a false abnormality. A formal report usually states a quality grade (A–F, or acceptable / not acceptable).
- Blow until there is genuinely nothing left (usually at least 6 seconds in an adult), rather than stopping after three.
- At least three acceptable blows, and the best two should be close enough to each other.
- Before the test, stop your bronchodilators as instructed (about 4–6 hours for a short-acting one, longer for a long-acting one), or reversibility will be underestimated.
- Beforehand, do not eat a large meal and do not smoke, and wear loose clothing.
If the report says the quality was poor, the reasonable next step is to do it again, not to accept that number.
What might an abnormal report be?
Asthma
Strong evidenceObstructive, and with a clear improvement after a bronchodilator (usually defined as a rise in FEV1 of ≥ 12% and ≥ 200 mL).
The point: lung function in asthma can be completely normal between attacks. One normal test cannot rule asthma out — FeNO, a bronchial provocation test or home peak flow variability can be added.
Chronic obstructive pulmonary disease (COPD)
Strong evidenceObstructive, with FEV1/FVC still < 0.70 after a bronchodilator. That is the diagnostic threshold for COPD — without this number, COPD cannot be diagnosed.
The point: severity is graded on the post-bronchodilator FEV1 as a percentage of predicted. But symptoms and the number of exacerbations affect the choice of treatment more than FEV1 does.
Interstitial lung disease ／ pulmonary fibrosis
Strong evidenceRestrictive, and with a fallen DLCO. Typically the FVC and the DLCO fall together while FEV1/FVC is normal or even high.
The point: DLCO is often abnormal earlier than FVC. On follow-up, a fall in FVC of ≥ 10% within a year is an important marker of progression and affects treatment decisions.
Small airway dysfunction
ModerateOrdinary lung function is normal, but FEF25-75 is low, or impulse oscillometry (IOS) shows increased small airway resistance. This is particularly valuable in people who have symptoms but in whom nothing abnormal can be found.
The point: FEF25-75 is itself highly variable and cannot be used on its own to make a diagnosis; it has to be read together with the symptoms and the other tests.
Obesity and chest wall factors
Strong evidenceWeight, scoliosis and severe ascites all press on the lungs and produce a restrictive picture, but the lungs themselves are fine.
The point: typically the FVC is low but the DLCO is normal or even high — and that combination is what separates it from interstitial lung disease.
Neuromuscular disease
Strong evidenceWeak respiratory muscles mean the lungs cannot be filled and cannot be emptied completely. A clear fall in FVC on lying down (a difference of > 10–20% between sitting and lying) is an important clue.
The point: maximal inspiratory and expiratory pressures (MIP/MEP) need to be added. If there is difficulty swallowing or weakness of the limbs, referral to neurology is appropriate.
It is actually a problem with the quality of the test
Strong evidenceThis is commoner than people imagine, particularly in the health check setting. Not blowing for long enough, not blowing hard enough, a leak and a cough will all manufacture a false “restrictive pattern”.
The point: look at the quality grade and the shape of the flow–volume curve on the report. If the curve has no peak, or stops early, doing it again should be considered.
What usually comes next?
It answers: is this obstruction reversible? You blow again 15 minutes after inhaling a short-acting bronchodilator.
A clear improvement points towards asthma; no improvement points towards COPD. This is the standard next step for an obstructive report.
It answers: is the restriction real, or was the blow simply poor? It shows whether total lung capacity (TLC) really has fallen.
It also shows gas trapping from emphysema (a raised RV).
It answers: how efficient is gas exchange? It is the key to separating “lungs squashed smaller” from “lung tissue destroyed”.
Interpreting it needs correction for haemoglobin — anaemia makes DLCO look low.
It answers: is there allergic-type inflammation in the airways? Particularly useful when lung function is normal but asthma is suspected.
It answers: is there a problem in the small airways? All it needs is quiet breathing, with no forced blowing, which suits children, older people and anyone who finds co-operating difficult.
It answers: what does the structure of the lung look like? With a restrictive pattern and a fallen DLCO, it is used to confirm whether this is interstitial lung disease.
This is a second-line test. Having abnormal lung function and DLCO figures in hand is exactly the “results of other relevant tests” that the National Health Insurance (NHI) asks for — so the order is to complete the lung function testing first, and talk about CT after. See the explanation below.
About CT of the chest: in Taiwan it is not a test you can simply ask for
CT has a high diagnostic value, but the strength of the evidence and whether the NHI will pay for it are two different things. In Taiwan it is governed by fairly explicit rules:
- It is second-line, not first-line. The NHI’s points to note in the review of medical expenses require the request form or the report for an imaging examination to state the clinical diagnosis, the purpose of the examination, the relevant history, the physical examination and the results of other relevant tests — which is to say that there usually has to be a chest X-ray first (with lung function testing and sputum tests if needed) that still does not explain things, before CT’s turn comes.
- There has to be a clear indication. The reviewers work out the purpose of the examination from the records. Simply wanting “a clearer look”, “to have a check-up while I am here” or “peace of mind” is not covered, and in that situation you usually pay out of pocket.
- Repeating it within a short period is reviewed strictly. Performing CT or MRI again within a short period (the rules give 12 weeks as the example) requires the clinical picture and the necessity to be set out in the records.
- The doctor has to look at the cloud images before ordering. Since 2019, if the images and reports uploaded for a patient in the previous 28 days have not been reviewed before CT or MRI is ordered in an outpatient clinic, that examination is not covered. So bringing your old reports, or saying clearly which hospital did them, is of real practical benefit to you.
So this page labels CT as Strong evidence Conditions apply: the diagnostic value is clear, but whether it is done and whether it is covered is for your doctor to judge from your clinical situation and the rules in force.
Based on: the clauses on imaging examinations in the Points to Note in the Review of National Health Insurance Medical Expenses, National Health Insurance Administration, Ministry of Health and Welfare, together with the NHI Administration’s rules on reviewing cloud images. The clauses are revised from time to time; what actually applies should follow the NHI Administration’s latest notices and the explanation given by your hospital or clinic.
Common questions
The health check said my lung function is only 70%. Is that serious?
Not necessarily, and “70%” is usually too imprecise to work with — the question to ask is 70% of which number. 70% of FVC, 70% of FEV1, and an FEV1/FVC of 0.70 mean entirely different things. You also need to check whether the quality grade on the report was acceptable. Take the whole report (the table of values and the flow–volume curve included) to a chest medicine clinic, or compare it first in the smart report system （中文）.
Why am I being asked to do it again? Wasn’t it done at the health check?
There are three common reasons: the quality was not acceptable (not blowing hard enough, or not for long enough), no bronchodilator test was done (so reversibility cannot be judged, and COPD therefore cannot be diagnosed), and DLCO or lung volumes are missing (so a restrictive pattern cannot be confirmed). Lung function at a health check is mostly the screening version; diagnosis needs a fuller set of measurements.
I have no symptoms. Does an abnormal number need treating?
It needs assessing, but not necessarily treating. In many people lung function has already begun to fall before any symptom appears, particularly in smokers — and that is exactly when intervention works best. The sensible order is: confirm the quality of the report → find the cause → deal with the reversible factors. Of those, stopping smoking is the only intervention proved to change the rate at which FEV1 falls, and it does far more than any drug.
My lung function is normal but I really am breathless. Does my doctor not believe me?
Normal lung function with symptoms is very common, and there are several reasonable explanations: asthma can be completely normal between attacks; small airway dysfunction needs FEF25-75 or IOS before it can be seen; early interstitial lung disease may show only a fall in DLCO; and the heart, anaemia, the thyroid and general fitness can all cause breathlessness too. Describe the symptoms concretely (how far you can walk before you are breathless, when it is worst), and ask whether DLCO, FeNO or IOS should be added.
Is the blowing test tiring? Does it carry any risk?
It takes effort, and you may feel a little light-headed or want to cough afterwards, but the risk is very low. A few situations call for it to be postponed: a recent myocardial infarction, unstable angina, recent eye, chest or abdominal surgery, a pneumothorax that has not healed, or coughing up a large amount of blood. If any of these apply to you, say so before the test.
What to do next
Bring the whole report — not just the line with the conclusion, but the table of values and the flow–volume curve. Bring any older reports as well: the trend over time means more than any single set of numbers.
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.
- 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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