Pulmonary function tests (PFTs) measure how well your lungs move air in and out, how much air they can hold, and how efficiently they transfer oxygen into your blood. The results boil down to a few key numbers and patterns that tell you whether your lung function is normal, whether there’s a problem with airflow (obstruction), a problem with lung volume (restriction), or both. Here’s how to read those numbers and understand what they mean.
The Key Numbers on Your Report
A PFT report typically includes three core measurements. FEV1 is the amount of air you can force out in one second. FVC is the total amount of air you can force out in one full breath. The FEV1/FVC ratio compares these two values and tells clinicians whether air is flowing out of your lungs at a normal rate. Most reports also include DLCO, which measures how well oxygen passes from your lungs into your bloodstream.
Each result is expressed as a percentage of a “predicted” value, which is what someone of your age, sex, height, and ethnicity would normally produce. If your FEV1 is 90% of predicted, your lungs are pushing out 90% of the air expected for someone like you. The critical question is whether your numbers fall within the normal range or below it.
What Counts as Normal
For years, a simple rule of thumb said anything above 80% of predicted was normal. Current guidelines from the European Respiratory Society and American Thoracic Society no longer recommend that cutoff. Instead, they use the Lower Limit of Normal (LLN), defined as the 5th percentile of a healthy reference population. Think of it like a height chart: if your value falls below the bottom 5% of what healthy people produce, it’s considered abnormal. The LLN accounts for the natural decline in lung function with age, so it’s more accurate than a one-size-fits-all 80% threshold, especially for older adults and younger people at the extremes.
Obstructive Pattern: Trouble Getting Air Out
An obstructive pattern means your airways are narrowed, making it harder to push air out quickly. The hallmark is a low FEV1/FVC ratio. You can fill your lungs reasonably well, but the air comes out too slowly because of narrowing or inflammation in the bronchial tubes. Asthma, COPD, chronic bronchitis, and emphysema all produce this pattern.
The current recommendation is to use the LLN to define obstruction rather than a fixed FEV1/FVC ratio of 0.70. The fixed 0.70 cutoff, still used by the GOLD criteria for diagnosing COPD, tends to overdiagnose obstruction in older adults (whose ratio naturally drops with age) and underdiagnose it in younger people. If your report uses LLN-based thresholds, it’s following the most up-to-date standard.
Once obstruction is identified, severity is graded by how much your FEV1 has dropped. The widely used GOLD staging system breaks it down this way:
- Mild (Stage 1): FEV1 at or above 80% of predicted
- Moderate (Stage 2): FEV1 between 50% and 79% of predicted
- Severe (Stage 3): FEV1 between 30% and 49% of predicted
- Very severe (Stage 4): FEV1 below 30% of predicted
A person with mild obstruction may feel fine at rest and only notice symptoms during heavy exercise. Someone in the severe range typically struggles with daily activities like climbing stairs or walking uphill.
Restrictive Pattern: Trouble Expanding the Lungs
A restrictive pattern means your lungs can’t expand fully. The air that does get in flows out at a normal rate, so the FEV1/FVC ratio stays normal or even increases. What drops instead is the total amount of air your lungs can hold.
Spirometry alone can suggest restriction if the FVC is low and the ratio is preserved, but it can’t confirm it. Confirmation requires measuring Total Lung Capacity (TLC), which is done through a separate test (body plethysmography or gas dilution). A TLC below 80% of predicted confirms restrictive disease. Conditions that cause restriction include pulmonary fibrosis, sarcoidosis, obesity, chest wall deformities, and neuromuscular diseases that weaken the breathing muscles.
Mixed Pattern
Some people show both obstruction and restriction at the same time. The FEV1/FVC ratio is low (indicating obstruction), and the TLC is also reduced (indicating restriction). This combination can appear in conditions like sarcoidosis or in someone who has both COPD and pulmonary fibrosis. Interpreting a mixed pattern requires looking at the full set of PFT data together rather than any single number.
What DLCO Tells You
DLCO, sometimes written as TLCO, measures how efficiently gas crosses from your lung’s air sacs into the tiny blood vessels surrounding them. It’s reported as a percentage of predicted, with a normal range of roughly 75% to 140%. A low DLCO means something is interfering with that gas exchange, either damage to the air sacs themselves or a problem with blood flow through the lungs.
Pairing the DLCO result with spirometry narrows the diagnosis considerably:
- Obstruction with low DLCO: Points toward emphysema, which destroys the walls of the air sacs, reducing the surface area for oxygen transfer. Also seen in cystic fibrosis and alpha-1 antitrypsin deficiency.
- Restriction with low DLCO: Suggests interstitial lung disease or other conditions that scar or thicken the lung tissue.
- Normal spirometry with low DLCO: Can indicate pulmonary vascular disease (such as blood clots in the lungs), early interstitial lung disease that hasn’t yet affected volumes, anemia, or elevated carbon monoxide levels from smoking.
A DLCO below 50% of predicted signals a high risk of oxygen levels dropping during physical activity. Below 40% of predicted, a person may qualify for disability determination. Patients with emphysema and a DLCO below 60% of predicted have higher mortality rates overall, making this number an important prognostic marker.
Bronchodilator Response
If your spirometry shows obstruction, the lab will often repeat the test after giving you an inhaled bronchodilator (typically albuterol). This reveals whether the airway narrowing is reversible, which helps distinguish asthma from COPD.
The older standard defined a positive response as an improvement of at least 12% and at least 200 mL in FEV1 or FVC. The updated 2022 criteria use a simpler formula: a change greater than 10% of the predicted value. A significant response suggests the airways can open up with medication, which is more typical of asthma. A minimal or absent response is more characteristic of COPD, where the airway damage is structural and less reversible. Neither result is absolute on its own, though. Some people with asthma don’t reverse fully during testing, and some COPD patients show partial reversibility.
Reading the Flow-Volume Loop
Most PFT reports include a graph called a flow-volume loop, which plots airflow speed against lung volume during a forced breath out and in. The shape of this curve carries diagnostic information beyond the raw numbers.
A normal loop looks like a tall, sharp peak on expiration that tapers smoothly downward, followed by a rounded inspiratory curve below the line. In obstructive disease, the expiratory portion scoops inward, creating a concave or “scooped out” shape because airflow slows disproportionately as the breath continues. In restriction, the loop looks like a smaller version of normal, with the same shape but compressed because lung volumes are reduced.
Certain shapes point to problems outside the lungs entirely. A fixed upper airway obstruction, such as a tracheal tumor or scar tissue, flattens both the expiratory and inspiratory curves into a characteristic box pattern. An obstruction inside the chest (intrathoracic) flattens only the expiratory loop, while the inspiratory portion looks normal. Recognizing these patterns on the graph can flag an airway blockage that spirometry numbers alone might not explain.
How to Prepare for Accurate Results
PFT results are only useful if the test is performed correctly, and certain medications can mask your baseline lung function. If you’re being tested to see how your lungs perform without treatment, you’ll typically be asked to withhold inhalers beforehand. The timing depends on the type:
- Short-acting rescue inhalers (albuterol): Stop 6 hours before
- Short-acting anticholinergic inhalers (ipratropium): Stop 12 hours before
- Long-acting inhalers (formoterol, salmeterol): Stop 24 hours before
- Ultra-long-acting inhalers (tiotropium, indacaterol): Stop 36 hours before
Beyond medications, avoid smoking for at least 4 to 6 hours before testing, don’t eat a heavy meal right beforehand, and skip vigorous exercise on the day of the test. Tight clothing that restricts your chest or abdomen can also affect results. The test requires maximal effort, so the technician will coach you to blow as hard and as long as you can. A halfhearted effort produces artificially low numbers that mimic disease.
Putting It All Together
Interpreting PFTs follows a logical sequence. First, look at the FEV1/FVC ratio to check for obstruction. If the ratio is low, assess severity using FEV1 percent predicted and check for bronchodilator reversibility. If the ratio is normal but the FVC is low, suspect restriction and confirm with TLC. Then layer in the DLCO to evaluate gas exchange and narrow the differential. Finally, examine the flow-volume loop shape for any patterns the numbers might miss.
No single number on a PFT report gives a diagnosis. The power of the test comes from reading the values as a set, in the context of symptoms, imaging, and clinical history. A low FEV1 means something very different in a 25-year-old nonsmoker than in a 65-year-old with a 40-pack-year history. The numbers provide the objective framework, but interpretation always requires the full picture.