Pulmonary function tests (PFTs) follow a logical sequence: first you determine whether airflow is obstructed, restricted, or both, then you assess severity and look at gas exchange. The key numbers are your FEV1/FVC ratio (which detects obstruction), your total lung capacity (which confirms restriction), and your DLCO (which measures how well oxygen crosses into your blood). Once you know what pattern you’re looking at, the diagnosis narrows quickly.
The Three Numbers That Matter Most
A PFT report contains dozens of values, but interpretation starts with three. FEV1 is the volume of air you can force out in the first second of a hard exhale. FVC is the total volume you can force out. The ratio between them, FEV1/FVC, tells you whether air is having trouble getting out of your lungs. A healthy person can blow out roughly 70 to 85 percent of their total breath in that first second.
Each of your results is compared to a “predicted” value based on your age, sex, height, and ethnicity. Your results are expressed as a percentage of predicted. An FEV1 of 80% predicted means your first-second airflow is 80% of what’s expected for someone like you. The lower limit of normal (LLN) is a statistically derived cutoff that accounts for natural variation. Results below the LLN are considered abnormal. Experts generally recommend using the LLN rather than fixed percentage cutoffs, because a single number like “70%” can over-diagnose obstruction in older adults and miss it in younger ones.
Step 1: Check for Obstruction
The first question in any PFT interpretation is whether the FEV1/FVC ratio is low. If it falls below the lower limit of normal, you have an obstructive pattern. This means air is having difficulty leaving the lungs, typically because the airways are narrowed, inflamed, or floppy. Conditions that cause obstruction include asthma, COPD, emphysema, and chronic bronchitis.
Once obstruction is identified, the FEV1 percentage of predicted grades its severity. The widely used scale looks like this:
- Mild: FEV1 70% or above predicted
- Moderate: FEV1 60 to 69% predicted
- Moderately severe: FEV1 50 to 59% predicted
- Severe: FEV1 35 to 49% predicted
- Very severe: FEV1 below 35% predicted
So if your report shows an FEV1/FVC ratio below the LLN and an FEV1 of 62% predicted, you’re looking at moderate obstruction.
Step 2: Check for Restriction
If the FEV1/FVC ratio is normal or even high, the next question is whether your lungs can hold a normal amount of air. A low FVC on spirometry can suggest restriction, but it doesn’t confirm it. The gold standard is total lung capacity (TLC), measured through a separate test called body plethysmography or gas dilution. A TLC below 80% of predicted (or more precisely, below the 5th percentile) confirms a restrictive defect.
Restriction means the lungs can’t fully expand. This happens with scarring inside the lungs (interstitial lung disease), chest wall problems, obesity compressing the lungs, or weak breathing muscles from a neuromuscular condition. The distinction matters because each cause has a very different treatment path.
Some people have both obstruction and restriction at the same time. This “mixed” pattern shows a low FEV1/FVC ratio along with a low TLC. It’s common in conditions like combined pulmonary fibrosis and emphysema, or in smokers who also have another lung condition.
Step 3: Evaluate Gas Exchange With DLCO
DLCO measures how efficiently your lungs transfer gas from inhaled air into your bloodstream. You breathe in a tiny, harmless amount of carbon monoxide and the test measures how much your lungs absorb. This value acts as a window into the health of the tiny air sacs (alveoli) and the blood vessels surrounding them.
DLCO is especially useful for telling similar-looking conditions apart. In someone with an obstructive pattern on spirometry, the DLCO helps distinguish emphysema from chronic bronchitis or asthma. Emphysema destroys the walls of the air sacs, reducing the surface area for gas exchange, so the DLCO drops. A smoker with airflow obstruction but a normal DLCO typically has chronic bronchitis rather than emphysema. Asthma, on the other hand, generally produces a normal or even elevated DLCO because the air sacs themselves are intact.
For restrictive patterns, DLCO separates lung scarring from problems outside the lung. Low lung volumes with a low DLCO point toward interstitial lung disease, where scar tissue thickens the barrier between air and blood. Low lung volumes with a normal DLCO suggest something external is limiting expansion, like obesity, a stiff chest wall, or muscle weakness.
A DLCO below 60% of predicted in someone with emphysema is associated with higher mortality risk, making it a meaningful marker for prognosis, not just diagnosis.
What the Flow-Volume Loop Tells You
Your PFT report usually includes a flow-volume loop: a graph that plots how fast air moves against how much air has been exhaled and inhaled. The shape of this loop carries diagnostic information that numbers alone can miss.
A normal loop has a sharp peak on the exhale side that tapers smoothly downward, and a rounded curve on the inhale side. In obstructive diseases like COPD, the exhale portion scoops inward, creating a concave or “caved-in” shape. This reflects the airways collapsing as the lungs empty. In restrictive diseases, the loop looks like a smaller version of normal: the overall shape is preserved, but the loop is narrower because the lungs hold less air.
The loop is particularly valuable for spotting upper airway obstruction, which spirometry numbers can underestimate. A fixed obstruction (like a tracheal tumor or scarring that doesn’t change with breathing) flattens both the inhale and exhale curves, producing a characteristic box shape. A variable obstruction inside the chest flattens only the exhale curve, while one outside the chest (like vocal cord dysfunction) flattens only the inhale curve. These patterns can flag problems that would otherwise require imaging to detect.
Bronchodilator Response Testing
If your spirometry shows obstruction, the next step is often a bronchodilator response test. You inhale a short-acting bronchodilator (typically albuterol), wait 15 to 20 minutes, and repeat spirometry. The question is whether your airflow improves, which helps distinguish asthma from fixed airway diseases like COPD.
A positive response is traditionally defined as an increase of at least 12% and at least 200 mL in either FEV1 or FVC. A newer set of criteria defines a positive response as a 10-percentage-point increase in FEV1 or FVC when expressed as percent predicted. Either way, a significant improvement suggests the obstruction is at least partially reversible, which is a hallmark of asthma. Keep in mind that some people with COPD also show partial reversibility, so the result is interpreted alongside your symptoms and history, not in isolation.
Preparing for Accurate Results
PFT results are only useful if the test is performed correctly, and preparation starts before you arrive. If your test includes bronchodilator response or bronchoprovocation testing, you’ll need to withhold your inhalers beforehand so they don’t mask your baseline lung function. The withholding times vary by medication type:
- Short-acting rescue inhalers (like albuterol): stop 6 hours before
- Short-acting anticholinergics (like ipratropium): stop 12 hours before
- Long-acting inhalers (like formoterol or salmeterol): stop 24 hours before
- Ultra-long-acting inhalers (like tiotropium): stop 36 to 48 hours before
The test itself requires maximal effort. You’ll be asked to breathe in as deeply as possible and blow out as hard and as long as you can. Incomplete effort is the most common reason for unreliable results. Technicians will typically have you repeat the maneuver at least three times to ensure consistency.
Certain medical situations make PFTs unsafe. The forced exhale generates significant pressure in your chest and abdomen, so testing is generally postponed for at least one month after a heart attack, and for appropriate recovery time after chest, abdominal, brain, or eye surgery. Active pneumothorax (collapsed lung), an ascending aortic aneurysm, coughing up blood, and severe uncontrolled hypertension (systolic above 200 or diastolic above 120) are all reasons to delay.
Putting It All Together
Interpreting a PFT follows a decision tree. Start with the FEV1/FVC ratio. If it’s low, you have obstruction: grade the severity with FEV1 percent predicted, check DLCO to see if emphysema is present, and look at bronchodilator response to assess reversibility. If the ratio is normal but volumes are low, measure TLC to confirm restriction, then use DLCO to determine whether the problem is inside the lungs or outside them. If both the ratio is low and TLC is reduced, you’re dealing with a mixed pattern.
No single PFT number gives a diagnosis on its own. The power of the test comes from reading the pattern across all the values together, combined with your symptoms, imaging, and clinical history. But understanding the logic behind the numbers puts you in a much stronger position to have a meaningful conversation about what your results actually mean.