Spirometry (PFT) Report: FEV1, FVC and the Ratio Explained

Short answer: a spirometry report rests on three numbers. FVC is how much air you can blow out after the deepest breath, FEV1 is how much of it leaves in the first second, and the FEV1/FVC ratio shows whether the airways are narrowed. A low ratio points to an obstructive pattern such as asthma or COPD; a normal ratio with a low FVC hints at a restrictive pattern, where the lungs can’t fill fully. Your doctor puts that together with your symptoms.

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The printout looks like a school maths worksheet. There’s a loop shaped like a shark fin, a curve that climbs and flattens, and three columns: Pre, Post and % Pred. Somewhere near the bottom a line says “moderate obstruction with significant reversibility.” What does any of that mean for your breathing? Quite a lot, once you know which three rows to read.

What does a spirometry test measure?

Spirometry, the main part of a pulmonary function test (PFT), measures how much air you can move and how fast. You take the biggest breath you can, seal your lips around a mouthpiece, and blast it all out as hard and as long as possible. A clip on your nose makes sure no air escapes the other way.

ELI5: imagine emptying a balloon through a straw. A big balloon with a wide straw empties fast. A big balloon with a pinched straw empties slowly. A small balloon empties fast but there isn’t much in it. Spirometry tells the doctor which of those three is your lungs.

The three key numbers on your report

These are typical ranges, and they vary by lab and by which reference equations the machine uses. Use the range, or “LLN” (lower limit of normal), printed on your report. Predicted values also depend on ethnicity settings, and South Asian lungs are often a little smaller for the same height than the European reference, which is why the lab should use the right equation.

Obstructive vs restrictive patterns

Obstructive means the airways are narrowed, so air gets out slowly. FEV1 falls much more than FVC, and the ratio drops below normal. Asthma, COPD from smoking or years of cooking smoke from wood and dung fires, and chronic bronchitis are the usual causes. The flow-volume loop often looks “scooped out”, like a spoon has taken a bite from the downward slope.

Restrictive means the lungs can’t fill fully. FVC is low, FEV1 falls roughly in step, and the ratio stays normal or even high. Causes include lung scarring (fibrosis), a stiff chest wall, obesity, weak breathing muscles and fluid around the lungs. Spirometry can only hint at restriction; a full lung volume test confirms it.

Mixed patterns exist too, and so does a normal result with symptoms, which may mean the problem comes and goes, as asthma often does.

What “reversibility” or “post-bronchodilator” means

Many labs repeat the test 15 to 20 minutes after you breathe in a reliever inhaler, which opens the airways. The “Post” column shows the new values. If FEV1 rises by at least 12% and 200 mL, the report calls it significant reversibility, a pattern that fits asthma. COPD usually improves less. It’s a clue, not a diagnosis, and the doctor reads it alongside your history of wheeze, triggers, smoking and allergies.

Myth: a normal spirometry rules out asthma

Not always. Asthma narrows the airways in bursts, so on a good day the test can be completely normal. That’s why doctors may repeat it when you have symptoms, ask you to keep a peak-flow diary, or order a challenge test. The myth survives because people expect one test to settle the question once and for all. Breathing doesn’t work that way.

Why results can be wrong

Spirometry depends on effort. A half-hearted blow, a cough in the middle, stopping too early or air leaking round the lips all lower the numbers. A good lab asks for at least three acceptable blows and checks they match. Some reports carry a quality grade (A to F). If yours shows a low grade, the result may not be reliable, and your doctor may want it repeated.

When a breathing result means see a doctor today

How to prepare for spirometry

1. Ask about inhalers in advance. The lab or doctor may ask you to hold some relievers for a few hours before the test. Only do this if they tell you to, and bring your inhalers along.
2. No smoking for at least an hour before, and ideally that whole day.
3. Avoid a heavy meal in the two hours before; a full stomach makes deep breaths hard.
4. Skip heavy exercise for about half an hour before the test.
5. Wear loose clothing, nothing tight across the chest or belly.
6. Tell the staff about a recent heart attack, eye or chest surgery, chest pain or coughing up blood; the test may need to wait.

If a nagging cough is what sent you for the test, when a 3-week cough needs a TB test covers that side. The honest guide to asthma home care explains triggers and what inhalers do. Other report guides are in Lab Reports Decoded.

Surprising facts about lung function

Did you know? Lung function peaks in the early to mid twenties and then falls slowly for the rest of life, faster in smokers. And years of cooking over an open wood or dung fire in a closed room can narrow airways much like cigarettes, which is why many non-smoking women in villages turn out to have COPD.

Spirometry questions people ask most