Failing a pulmonary function test (PFT) doesn’t mean you’ve received a diagnosis. It means your lungs aren’t moving air as well as expected for someone your age, sex, and height, and your doctor now has a starting point to figure out why. What happens next depends on the pattern of your results, how far they fall below normal, and the reason you were tested in the first place.
What “Failing” Actually Means
PFTs measure several things: how much air your lungs can hold, how fast you can push it out, and how efficiently oxygen crosses from your lungs into your blood. Your results are compared to predicted values for someone with your demographics. When one or more numbers fall below the normal range, the test is considered abnormal.
The most important numbers are your FEV1 (how much air you can forcefully exhale in one second), your FVC (the total amount of air you can exhale after a full breath), and the ratio between them. A low ratio points toward an obstructive pattern, where air has trouble getting out. A low total lung capacity with a preserved or high ratio points toward a restrictive pattern, where your lungs can’t fully expand. Some people show a mix of both.
There’s also a measurement of how well oxygen passes through the thin membrane between your air sacs and blood vessels, called diffusing capacity. When this number drops below 75% of predicted, it suggests damage to that membrane, even if your other numbers look normal. This can show up in conditions like pulmonary fibrosis, emphysema, blood vessel disease in the lungs, or even anemia.
How Severity Is Classified
Not all abnormal results carry the same weight. For obstructive conditions like COPD, the widely used GOLD classification breaks severity into four stages based on the percentage of predicted FEV1 you achieve:
- Stage 1 (mild): 80% or above
- Stage 2 (moderate): 50% to 79%
- Stage 3 (severe): 30% to 49%
- Stage 4 (very severe): below 30%
Someone in Stage 1 may barely notice symptoms, while someone in Stage 4 likely struggles with basic activities. Your stage shapes every decision that follows, from medication choices to whether you’re referred for pulmonary rehabilitation.
Obstructive vs. Restrictive Patterns
An obstructive pattern means something is narrowing or blocking your airways, making it hard to exhale fully. The most common causes are COPD, asthma, bronchiectasis, and bronchiolitis. Your lungs may actually hold a normal or even increased total volume of air, but you can’t push it out efficiently.
A restrictive pattern means your lungs can’t expand fully in the first place. This can come from scarring inside the lungs (pulmonary fibrosis, sarcoidosis, or damage from radiation or certain medications), but it can also come from outside the lungs entirely. Severe obesity, spinal deformities like kyphoscoliosis, neuromuscular diseases such as muscular dystrophy or ALS, large fluid collections around the lungs, and even massive abdominal swelling can all physically prevent your chest from expanding. Post-COVID lung scarring has also become a recognized cause.
The distinction matters because treatments are fundamentally different. Obstructive diseases often respond to inhalers that open the airways, while restrictive diseases typically require treating the underlying cause, whether that’s reducing inflammation, draining fluid, or managing weight.
The Bronchodilator Response Test
If your PFT shows an obstructive pattern, your doctor will often have you inhale a fast-acting bronchodilator and then repeat the test about 15 minutes later. If your FEV1 improves by at least 10% of the predicted value (or by the older standard of 12% and 200 milliliters), that’s considered a positive response. A significant improvement suggests your airway narrowing is at least partly reversible, which is a hallmark of asthma rather than fixed obstruction from COPD. This single retest can steer your diagnosis and treatment plan in a completely different direction.
What Happens Next Diagnostically
Abnormal PFT results alone rarely give a final diagnosis. They tell your doctor the type and severity of the problem, but not the specific disease. Expect your provider to order follow-up tests based on the pattern they see.
For restrictive patterns or unexplained low diffusing capacity, a high-resolution CT scan of the chest is a common next step. It can reveal scarring, inflammation, fluid around the lungs, or structural abnormalities that spirometry alone can’t identify. For suspected asthma with normal baseline spirometry, a methacholine challenge test (where you inhale a substance that temporarily narrows the airways) may be used to provoke and confirm airway hyperreactivity. Blood tests, oxygen level monitoring during exercise, and sometimes a walking test that measures how far you can go in six minutes also help fill in the picture.
The key point is that an abnormal PFT opens a diagnostic pathway. It’s a first chapter, not a final answer.
Impacts on Surgery Clearance
If you had PFTs done before a planned surgery, abnormal results can change the timeline. Both the American College of Chest Physicians and the European Respiratory Society use PFT values to stratify surgical risk, particularly for lung surgery. FEV1 and diffusing capacity are the two numbers that matter most. Notably, about 40% of patients whose FEV1 looks acceptable (above 80% predicted) still have a reduced diffusing capacity, and 7% of those fall below 40% predicted, putting them in a high-risk category.
Failing to meet the thresholds doesn’t necessarily cancel your surgery. It may mean additional testing like a cardiopulmonary exercise test, a modified surgical approach, or a period of optimization beforehand, such as quitting smoking, treating infections, or starting pulmonary rehabilitation to improve your baseline fitness.
Workplace and Respirator Clearance
For workers in industries requiring respirator use, abnormal PFTs carry practical job consequences. Under OSHA regulations, a healthcare professional must provide a written recommendation on whether you’re medically able to wear a respirator. If your lung function makes a standard negative-pressure respirator risky, your employer is required to provide a powered air-purifying respirator (PAPR) instead, which does the breathing work for you. You aren’t simply barred from working. If a later medical evaluation shows your lung function has improved, you can return to using a standard respirator.
The process is designed to protect you, not to end your employment. But it can mean job reassignment if no safe respirator option exists for your level of impairment.
Treatment Paths After Abnormal Results
Your treatment depends entirely on what’s causing the abnormal results, but there are some common paths. For moderate to very severe COPD (GOLD Stages 2 through 4), pulmonary rehabilitation is a standard referral. These structured programs combine supervised exercise, breathing techniques, and education over several weeks, and they consistently improve exercise tolerance and quality of life even when your numbers on paper don’t change dramatically. Pulmonary rehab is also used for interstitial lung diseases, bronchiectasis, pulmonary hypertension, and persistent respiratory problems after COVID-19.
For asthma confirmed by a positive bronchodilator response, treatment typically centers on inhaled medications that reduce inflammation and open the airways. For restrictive diseases caused by scarring, treatment focuses on slowing progression. For restriction caused by obesity, weight loss can produce measurable improvements in lung volumes. For neuromuscular causes, respiratory support devices may be introduced.
Regardless of the specific diagnosis, abnormal PFTs often prompt lifestyle changes. If you smoke, this is the point where cessation becomes medically urgent. Even in early-stage disease, stopping smoking is the single most effective intervention to slow lung function decline. Your provider will also likely monitor your PFTs periodically to track whether your condition is stable, improving, or worsening over time.