In short
A lung function report gives your numbers as a percentage of what someone your age, sex and height is predicted to manage. The ratio between two of them separates blocked airways from small lungs.
Emergency if: Struggling to speak in full sentences, blue lips, exhaustion with breathing, or a reliever inhaler that is not working — go to an emergency department now.
On your report
Hemoglobin, NT-proBNP, Absolute Eosinophil Count, Total IgE +1 more
On this page
01
What the test does
Spirometry measures how much air you can breathe out in one breath, and how fast. You take the deepest breath you can and blow out as hard and as long as possible into a tube, usually three times, and the best technically acceptable effort is the one reported.
Because it depends entirely on your effort, a poor result can mean poor technique rather than poor lungs. Reports usually comment on whether the effort met quality criteria, and a test described as suboptimal or not reproducible is worth repeating before anything is concluded from it.
02
The three numbers that matter
FVC, forced vital capacity, is the total volume you can blow out. FEV1, forced expiratory volume in one second, is how much of that comes out in the first second. The ratio of the two, FEV1 divided by FVC, describes how freely air leaves the lungs rather than how much there is.
Your values are reported alongside predicted results that compare you with peers of your age, sex, height and weight, and expressed as a percentage of that prediction. This is why the raw litres matter less than the percentage — a value of 2.4 litres can be excellent in one person and poor in another.
03
Obstructive versus restrictive
Obstructive disorders such as asthma and emphysema mainly affect the airways carrying air to and from the lungs, and can leave the lungs holding too much air and taking longer to empty. On the report this shows as a disproportionately low FEV1 and therefore a reduced ratio: the air is there, but it cannot get out quickly. A ratio below roughly 0.7 is the usual signal of airflow obstruction and is the cutoff COPD guidance uses.
Restrictive diseases instead leave the lungs scarred and smaller, holding too little air and transferring oxygen poorly — pulmonary fibrosis and sarcoidosis are examples. Here FVC falls, FEV1 falls with it, and the ratio stays normal or even rises. Spirometry can suggest restriction, but confirming it needs lung volume measurement rather than spirometry alone.
04
Reversibility, and telling asthma from COPD
If obstruction is found, the test is often repeated about fifteen minutes after an inhaled bronchodilator. A substantial improvement in FEV1 afterwards is called significant reversibility and points towards asthma; obstruction that barely shifts points towards COPD. The distinction changes treatment, which is why the second set of numbers is worth looking for on the report.
The separation is not absolute. Asthma can become less reversible over years, some people have both conditions, and a single normal spirometry does not exclude asthma, because asthma is variable by nature and the airways may be open on the day you were tested. Peak flow readings kept over two weeks often settle that question better than one visit.
05
What can make your result look worse than it is
Technique dominates. Not sealing your lips around the mouthpiece, stopping the blow early, coughing during the manoeuvre, or taking an incomplete breath in beforehand all lower the numbers. So does testing during a chest infection or a flare, which is why routine spirometry is postponed until several weeks after one.
Preparation matters too: inhalers are usually withheld for a set period beforehand if reversibility is being assessed, and a heavy meal, tight clothing or recent smoking just before the test all affect it. Follow whatever instructions you were given precisely, and say if you could not.
06
Reading the report as a whole
Spirometry is used to diagnose asthma, COPD and pulmonary fibrosis, to monitor known lung conditions over time, and as part of targeted lung health checks offered to people over 55 who smoke. Its greatest value is as a trend: one set of numbers describes today, while the same test yearly shows the direction of travel, which is what actually predicts how someone will do.
A normal report in someone with genuine breathlessness does not close the question — it redirects it. Heart failure, anaemia, deconditioning, obesity and anxiety all cause breathlessness with normal spirometry, and the next tests differ completely. Take the report to whoever ordered it rather than reading a single percentage in isolation.
When to act
Routine — see a doctor
A mildly reduced value with good technique and no symptoms: repeat as advised and treat the trend across years as the meaningful result.
Same-day — call promptly
Breathlessness that is clearly worse than usual, needing your reliever inhaler far more often, or waking at night short of breath.
Emergency — act now
Struggling to speak in full sentences, blue lips, exhaustion with breathing, or a reliever inhaler that is not working — go to an emergency department now.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- Spirometry — NHSnhs.uk
- Pulmonary Function Tests — MedlinePlusmedlineplus.gov
- COPD — NHSnhs.uk
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