A pulmonary function test measures how well your lungs actually work, so there is no trick to “passing” one the way you might cram for an exam. What you can do is make sure the test captures your lungs at their honest best, rather than producing artificially low numbers because of poor technique, bad timing, or avoidable preparation mistakes. The difference between a clean result and a misleading one often comes down to how you breathe into the mouthpiece, what you did in the hours beforehand, and whether you are testing on a day your body is ready for it.
What the Technician Needs From You
Most pulmonary function testing starts with spirometry, where you take the deepest breath you can and then blow out as hard and as long as possible into a tube. Two numbers matter most: the total volume of air you can force out (your forced vital capacity, or FVC) and how much of that volume comes out in the first second (FEV1). The ratio between them tells your doctor whether air is flowing freely or getting trapped.
The single biggest factor under your control is effort. A half-hearted blow produces results that look like lung disease even in a perfectly healthy person. The technician coaching you through the test is not being dramatic when they shout “keep going, keep going!” They need you to blast the air out as explosively as possible at the start, then sustain that exhalation until your lungs are truly empty. Most adults need to exhale for at least six seconds, and some need considerably longer. The test also requires you to fill your lungs completely before each blow. A shallow inhale means you are measuring a partial breath, not your actual capacity.
You will repeat the maneuver several times. This is not because you failed; the technician needs at least two blows where the FVC values land within 150 milliliters of each other and the FEV1 values do the same. That consistency proves the results are reliable rather than a product of one lucky or unlucky effort.1PubMed Central. Standardization of Spirometry 2019 Update. An Official American Thoracic Society and European Respiratory Society Technical Statement Expect to do at least three rounds, and sometimes up to eight, before the technician is satisfied.
Mistakes That Invalidate a Blow
Each individual blow has to meet several quality checks before it counts. Understanding what disqualifies one can help you avoid repeating it. The major pitfalls include:
- Hesitating at the start: If you pause or stutter before the blast, the machine registers a slow takeoff. The volume of air expelled before the true explosive start must be tiny relative to your total exhaled volume.
- Coughing in the first second: A cough during the initial burst corrupts the FEV1 reading. Coughing later in the exhalation is less damaging but can still render the blow unusable if the technician judges it affected your effort.
- Stopping too soon: The test needs you to reach a plateau where almost no additional air is coming out, or to exhale for at least 15 seconds. Cutting the effort short produces an artificially low FVC.
- Closing your throat: Some people instinctively close their glottis mid-blow, especially when they feel like they have run out of air. This creates a sudden stop in airflow that the equipment detects.
- Leaking around the mouthpiece: A poor seal lets air escape without being measured. Make sure your lips are wrapped tightly around the tube, and use a nose clip if one is offered.
These criteria come from international standards that testing labs follow worldwide.2European Respiratory Journal. Standardisation of spirometry The technician grades each blow in real time and will tell you what went wrong so you can fix it on the next attempt. Do not feel embarrassed about needing several tries. Even experienced patients sometimes need coaching.
How to Prepare Before the Test
The choices you make in the 24 hours before your appointment can meaningfully shift your numbers. None of these steps will turn unhealthy lungs into healthy ones, but they prevent your results from being worse than they should be.
If you smoke, stop for at least four to six hours before the test, and ideally for 24 hours. Cigarette use worsens spirometry values and diffusing capacity in a dose-dependent way, and it also increases residual volume, meaning more air gets trapped in your lungs where the spirometer cannot measure it.3CHEST. Smoking and Health (Abstracts from CHEST 2016) Stopping a few hours before testing will not undo years of damage, but it avoids the acute bronchospasm and airway irritation that a recent cigarette causes.
Avoid a heavy meal for at least two hours beforehand. A full stomach pushes your diaphragm upward and restricts how deeply you can inhale. For similar reasons, wear loose, comfortable clothing that does not squeeze your chest or abdomen. A tight belt or restrictive waistband can cost you measurable volume.
Ask your doctor ahead of time whether to skip any inhaled medications. Short-acting bronchodilators are typically withheld for four to six hours before testing, and long-acting ones for 12 to 24 hours, depending on the drug. The goal is to measure your baseline lung function without medication masking or boosting the result. If you are testing specifically to see how you respond to a bronchodilator, the lab will have you do a baseline set of blows first and then repeat them after administering the medication. Do not stop medications without your doctor’s approval, though, especially oral steroids or controller inhalers that are unsafe to interrupt.
Caffeine can mildly dilate airways, and vigorous exercise within 30 minutes of the test can temporarily change your breathing mechanics. Neither effect is huge, but if you want your results to represent a true resting baseline, skip the espresso and the gym that morning.
Why Sitting Upright Matters
Spirometry is almost always performed while you are seated, and there is a good reason for that. Lying back reduces your functional residual capacity by roughly 25%, driven by abdominal organs pushing the diaphragm upward and blood pooling in the chest.4Archivos de Bronconeumología. Effects of Posture on the Inspiratory and Expiratory Components of Vital Capacity in Healthy Humans Standing and sitting produce comparable lung volumes, so either position works, but standing can introduce wobbliness and lightheadedness during a prolonged forced exhalation, which is why most labs prefer the chair.5PubMed Central. The effect of body position on pulmonary function: a systematic review
Sit up straight with both feet flat on the floor. Do not lean forward or slump. If you are in a wheelchair or have a spinal condition that prevents full upright posture, tell the technician; they will note it so your results are interpreted in context. Any deviation from standard posture can lower your numbers without reflecting actual lung disease.
When to Reschedule
Testing while you are acutely sick almost guarantees worse results. Respiratory viral infections can trigger increased airway sensitivity that lasts for weeks after the cold itself is gone.6European Respiratory Journal. Association of bronchial hyperresponsiveness and acute respiratory viral infections (ARVI) If you have had a significant cold, flu, or chest infection in the past few weeks, your airways may still be twitchier than usual, and your results will reflect the infection rather than your true baseline. Most guidelines suggest waiting at least two to three weeks after a respiratory illness before testing, and longer if you had pneumonia or bronchitis.
Certain medical events make the test unsafe, not just inaccurate. Forced exhalation briefly raises pressure inside your chest and abdomen, which is risky if you have recently had a heart attack, a pulmonary embolism, an aneurysm, or major surgery on your chest, abdomen, or head.7PubMed. An update on contraindications for lung function testing If any of these apply, your doctor should clear you before you attempt the test. Eye surgery, including cataract procedures, is another common reason to delay because the straining can raise intraocular pressure.
How Your Results Are Interpreted
Your raw numbers on their own mean little. The lab compares your FEV1 and FVC to predicted values based on your age, height, sex, and ethnicity. Height is the strongest predictor of lung size because taller people have bigger rib cages and longer airways.8PubMed Central. Multi-ethnic reference values for spirometry for the 3–95 year age range: the global lung function 2012 equations Ethnic background also plays a role: proportional differences in lung size exist between populations, reflecting differences in body build rather than disease. The FEV1-to-FVC ratio, however, is generally independent of ethnicity, which makes it a particularly reliable indicator of airflow obstruction across all groups.8PubMed Central. Multi-ethnic reference values for spirometry for the 3–95 year age range: the global lung function 2012 equations
A result is not “pass” or “fail” in a binary sense. Instead, it falls on a spectrum. If your FEV1 and FVC are both above the lower limit of normal for someone of your age, height, sex, and background, your spirometry is considered normal. Values below that threshold prompt further investigation but do not automatically mean you have a disease. A single low reading could be the result of poor effort, a lingering cold, or just a bad day. Doctors usually look at the pattern across multiple values and, if needed, repeat the test.
The Bronchodilator Part of the Test
If your initial spirometry shows airflow limitation, the technician will often have you inhale a short-acting bronchodilator (usually albuterol) and then repeat the test about 15 minutes later. The question being asked is whether your narrowed airways can open back up. A positive response is traditionally defined as an increase in FEV1 or FVC of at least 12% and at least 200 milliliters above your pre-bronchodilator values.9PubMed. Is an increase in FEV₁ and/or FVC ≥ 12% of control and ≥ 200 mL the best way to assess positive bronchodilator response? Yes
A strong bronchodilator response hints at asthma, where the obstruction is largely reversible. A poor response is more typical of COPD, where structural damage has narrowed the airways permanently. But these are tendencies, not rules. In large population studies, roughly one in five people with either asthma or COPD showed significant reversibility, and about one in twenty people with no airway disease at all met the threshold too.10European Respiratory Journal. Bronchodilator reversibility in asthma and COPD: findings from three large population studies No single test result clinches a diagnosis on its own.
If you took your own bronchodilator inhaler before arriving, the post-bronchodilator portion of the test becomes meaningless because your airways are already dilated. This is the main practical reason labs ask you to hold your inhalers before the appointment.
Beyond Spirometry
Depending on what your doctor is looking for, you may undergo additional tests during the same visit. Two common ones are the diffusing capacity test and lung volume measurement, and each has its own quirks worth knowing about.
The diffusing capacity test (DLCO) measures how efficiently gas crosses from your lungs into your blood. You breathe in a tiny, harmless amount of carbon monoxide, hold your breath for about 10 seconds, and then exhale. The machine calculates how much of the gas your lungs absorbed. The result depends on both the health of the membrane between your air sacs and blood vessels and on how much blood is flowing through the capillaries at that moment.11Clinical Science. Angiotensin-converting enzyme inhibition restores the diffusing capacity for carbon monoxide in patients with chronic heart failure by improving the molecular diffusion across the alveolar capillary membrane Smoking right before the test is especially harmful here because carbon monoxide from cigarettes is already occupying the binding sites that the test gas needs, which artificially lowers your score. High altitude and anemia can also suppress the result without reflecting lung disease.
Lung volume measurement tells the doctor how much air your lungs hold in total, including the air you cannot voluntarily exhale (residual volume). This is done either by sitting in a sealed booth called a body plethysmograph and panting gently, or by breathing a tracer gas like helium. The two methods can give different results, especially in people with obstructive lung disease, where trapped air is harder for the helium method to reach. Studies comparing the techniques found that the gap between them grows wider as obstruction becomes more severe.12PubMed Central. The measurement of lung volumes using body plethysmography and helium dilution methods in COPD patients: a correlation and diagnosis analysis The plethysmograph tends to capture total lung capacity more completely in those cases. If you are asked to sit in the booth, the main thing you need to do is stay relaxed and follow the panting rhythm the technician demonstrates. Claustrophobia can be a barrier; mention it ahead of time if enclosed spaces bother you.
Methacholine Challenge Tests
If your spirometry comes back normal but your doctor still suspects asthma, you may be referred for a bronchial challenge test. This involves inhaling incrementally increasing doses of methacholine, a substance that temporarily narrows the airways in people who are sensitive to it. The test measures how little methacholine it takes to make your FEV1 drop by 20%.13European Respiratory Journal. ERS technical standard on bronchial challenge testing: general considerations and performance of methacholine challenge tests A low threshold suggests hyperreactive airways consistent with asthma; a high threshold or no response makes asthma much less likely.
The preparation rules for a methacholine challenge are stricter than for standard spirometry. You will be asked to avoid caffeine, antihistamines, and bronchodilators for longer periods because each of these can blunt your airway response and produce a falsely negative result. The test is also explicitly avoided in anyone whose baseline FEV1 is already significantly reduced, since provoking further narrowing on top of existing obstruction can be dangerous. The lab will check your baseline spirometry first and cancel the challenge if your starting airflow is too low.
Occupational and Clearance Testing
Some people face pulmonary function testing not because they have symptoms but because a job requires it. Firefighters, military personnel, and workers who wear respirators often need periodic spirometry to confirm their lungs can handle the physical demands and the equipment. In these contexts, the stakes feel higher because “failing” can mean being pulled from duty or denied clearance.
The test itself is identical to a clinical spirometry, and the same preparation advice applies. The difference is in how the result is used. Occupational standards may set a hard cutoff, such as a minimum FEV1 percentage predicted, rather than the clinical approach of looking at patterns and context. If your results come in borderline, ask whether a retest is allowed. A single session of suboptimal effort or a day when allergies are flaring can push you just below a threshold that you would clear on another occasion. Many occupational health programs allow retesting after a short interval, especially if the technician noted quality issues with the first attempt.
For respirator clearance specifically, the evaluation is less about hitting a precise lung function number and more about confirming you do not have a condition that would make wearing a tight-fitting mask dangerous. The medical questionnaire that accompanies the spirometry often matters as much as the numbers themselves.
Common Misconceptions
One persistent myth is that you can “train” for a pulmonary function test the way you would train for a fitness test. Lung capacity is largely determined by your anatomy and the health of your lung tissue. No amount of breathing exercises in the days before the test will expand your rib cage or grow new alveoli. Deep breathing practice can help you get comfortable with the sensation of maximal inhalation and forceful exhalation, which may improve your technique, but it will not change the underlying physiology being measured.
Another misconception is that a normal spirometry result means your lungs are completely healthy. Spirometry is excellent at detecting airflow obstruction, but it can miss early restrictive disease, pulmonary vascular problems, and gas exchange issues that only show up on DLCO or exercise testing. A normal FEV1 and FVC are reassuring, but they are not a comprehensive clean bill of lung health.
Finally, some people worry that the test itself is harmful or will trigger an asthma attack. Standard spirometry is extremely safe. The forced exhalation can occasionally cause lightheadedness, and people with very reactive airways may feel mild tightness afterward, but serious adverse events are vanishingly rare in the absence of the cardiovascular contraindications mentioned earlier. If you do experience significant wheezing or distress during the test, the lab has bronchodilator medication on hand to relieve it quickly.