A normal pulmonary function test (PFT) means your lungs can hold a healthy amount of air, move it in and out efficiently, and transfer oxygen into your blood at an expected rate. Your results are compared to predicted values based on your age, height, and sex, and “normal” generally falls above the 5th percentile of what healthy people with your characteristics can do. Here’s what each part of the test measures and what the numbers mean.
The Three Parts of a PFT
A full pulmonary function test has three components: spirometry, lung volumes, and diffusing capacity. Spirometry measures how well you can push air out of your lungs and how fast you can do it. Lung volume testing measures how much air your lungs can hold at maximum inflation. The diffusing capacity test measures how efficiently gas crosses from your lungs into your bloodstream.
Not every patient gets all three. Spirometry alone is the most commonly ordered test and is often enough to screen for problems. Your doctor adds lung volumes and diffusing capacity when they need a more complete picture, such as distinguishing between different types of lung disease or tracking how a known condition is progressing.
Normal Spirometry Numbers
Spirometry produces three key numbers:
- FVC (forced vital capacity): the total amount of air you can forcefully blow out after taking the deepest breath possible.
- FEV1 (forced expiratory volume in one second): how much of that air comes out in the first second.
- FEV1/FVC ratio: the proportion of your total air that you can exhale in that first second.
Traditionally, results were considered normal if your FEV1 and FVC were both above 80% of the predicted value and your FEV1/FVC ratio was above 0.70. You may still see these thresholds on your report. However, the American Thoracic Society and European Respiratory Society now recommend using the “lower limit of normal” (LLN), defined as the 5th percentile of healthy people, rather than the fixed 80% cutoff or the 0.70 ratio.
The reason: those fixed cutoffs don’t work equally well for everyone. For older or shorter adults, 80% of predicted is actually above the 5th percentile, meaning the old rule flags them as abnormal when they may be fine. For taller, younger adults, the opposite happens. They could fall below the true 5th percentile while still appearing “normal” at 80% of predicted. The percentile-based approach adjusts for this, giving a more accurate picture regardless of body size or age.
Normal Lung Volumes
Lung volume testing looks at how much space your lungs have to work with. The main measurement is total lung capacity (TLC), which averages around 6 liters in adults but varies widely depending on height, sex, and age. Residual volume (RV), the air that stays trapped in your lungs even after you exhale as hard as you can, normally runs between 1 and 1.2 liters. Functional residual capacity (FRC) is the air left in your lungs after a relaxed, normal breath out.
These volumes matter because they shift in predictable patterns with disease. In obstructive conditions like COPD, asthma, or bronchiectasis, airways collapse or narrow during exhalation, trapping extra air. This inflates TLC, FRC, and RV above normal. Residual volume is typically the first measurement to rise in early obstructive disease, making it a useful early marker. In restrictive conditions, where the lungs can’t fully expand (from scarring, chest wall problems, or muscle weakness), TLC, FRC, and RV all drop below normal.
Normal Diffusing Capacity
The diffusing capacity test, often abbreviated DLCO, checks how well oxygen moves from your air sacs into your blood. During the test, you breathe in a tiny, harmless amount of carbon monoxide, hold your breath briefly, then exhale. The equipment measures how much of that gas was absorbed.
A normal DLCO falls between 75% and 140% of the predicted value. Results below 75% suggest that something is interfering with gas exchange. This could be damage to the air sacs (as in emphysema), thickening of the tissue between the air sacs and blood vessels (as in pulmonary fibrosis), or reduced blood flow through the lungs. A low DLCO alongside normal spirometry can sometimes be the first clue that a lung condition is developing.
How Your Predicted Values Are Calculated
Your results are never judged in isolation. The testing software calculates a predicted value for someone of your age, sex, and height, then compares your actual performance to that benchmark. Height has the largest influence on lung size: taller people naturally have bigger lungs. Sex matters because, on average, males have larger lung volumes than females of the same height. With aging, the lungs gradually lose elastic recoil, so predicted values decrease over time.
The current gold-standard equations come from the Global Lung Function Initiative (GLI), which collected data from tens of thousands of healthy people worldwide. These equations also account for ancestral background, since lung volumes differ across populations. The older practice of applying a fixed racial adjustment factor is no longer recommended. Instead, the GLI provides specific reference equations based on self-reported ancestry, or a general equation when ancestry is unknown.
Weight is not a direct factor in predicting lung size, though very high body weight can compress the lungs and reduce measured volumes in practice.
What Abnormal Patterns Look Like
When results fall outside the normal range, they tend to follow one of two broad patterns. An obstructive pattern means air has trouble getting out. The FEV1/FVC ratio drops below the lower limit of normal because the airways are narrowed or blocked. This is the pattern seen in asthma, COPD, and chronic bronchitis. If a bronchodilator is given during the test and FEV1 improves by more than 10% of the predicted value, that reversibility suggests asthma rather than a fixed obstruction like COPD.
A restrictive pattern means the lungs can’t fully fill. FVC and TLC are both reduced, but the FEV1/FVC ratio stays normal or even increases because the airways themselves aren’t blocked. This pattern shows up with pulmonary fibrosis, obesity, neuromuscular diseases, or chest wall deformities. Some people show a mixed pattern with features of both.
Severity is graded using z-scores, which express how far your result falls from the predicted value in statistical terms. A z-score above negative 1.645 is normal. Between negative 1.65 and negative 2.5 is mild impairment. Between negative 2.51 and negative 4 is moderate. Below negative 4 is severe. Your report may present this as a severity grade rather than a raw z-score.
How to Prepare for the Test
A few things can skew your results if you don’t plan ahead. On the day of testing, don’t smoke at all and avoid alcohol for at least four hours. Skip heavy exercise for at least 30 minutes beforehand, and don’t eat a large meal within two hours. Wear loose, comfortable clothing that won’t restrict your breathing.
If you use inhalers, your testing center will likely ask you to hold certain medications. Short-acting bronchodilators like albuterol are typically stopped 6 hours before testing. Long-acting inhalers need to be held for 24 to 36 hours. Your doctor’s office should give you specific instructions, since the timing depends on which medications you take and whether the test is specifically checking your response to a bronchodilator.
What the Test Feels Like
The spirometry portion requires you to seal your lips around a mouthpiece, take the deepest breath you can, then blast the air out as hard and fast as possible for at least six seconds. A technician will coach you through it and will usually ask you to repeat the maneuver at least three times to get consistent results. It can feel physically demanding, and some people get lightheaded briefly.
For lung volume testing using a body plethysmograph, you sit inside a clear, phone-booth-sized chamber and breathe against a closed shutter while the box measures pressure changes. It’s not painful, but some people find the enclosed space uncomfortable. The diffusing capacity test is simpler: you take a single deep breath of a special gas mixture, hold it for about 10 seconds, then exhale. A full battery of all three tests typically takes 30 to 60 minutes, though spirometry alone can be done in about 15 minutes.