Preparing for a pulmonary function test (PFT) comes down to removing anything that could artificially inflate or deflate your lung measurements: certain medications, recent exercise, tight clothing, a full stomach, and cigarettes. The test itself is straightforward, but your results can shift meaningfully based on what you did in the hours beforehand. Because PFTs compare your numbers against predicted values for someone of your age, sex, height, and ethnicity, even small preparation missteps can push your readings into an abnormal range or mask a real problem.
What PFTs Actually Measure and Why Preparation Matters
A PFT is not one single test. The most common component is spirometry, where you blow as hard and fast as you can into a mouthpiece. Spirometry measures how much air you can exhale in total (forced vital capacity, or FVC) and how much you can push out in the first second (FEV1). These two numbers, and their ratio, are the foundation for diagnosing conditions like asthma and COPD. More specialized tests exist as well: body plethysmography measures total lung volumes including air trapped in the lungs, and a diffusion capacity test (DLCO) checks how efficiently oxygen crosses from your lungs into your bloodstream.1Ukrainian Pulmonology Journal. METHODS OF PULMONARY FUNCTION TESTING IN THE DIAGNOSIS AND MONITORING OF CHRONIC OBSTRUCTIVE LUNG DISEASE (SPIROMETRY, BODYPLETHISMOGRAPHY, DLCO) ACCORDING UPDATED GOLD-2024
What makes preparation so important is that each of these measurements is sensitive to interference. A bronchodilator inhaler used too recently can make obstructed airways look temporarily normal. Exercise right before the test can change your airflow numbers in confusing ways. Even something as seemingly trivial as wearing a snug waistband can restrict how deeply you inhale. The goal of preparation is to present your lungs in their baseline state so the results reflect what is actually going on with your breathing day to day.
Medications You May Need to Pause
This is the single most consequential preparation step, and the one most often done incorrectly. If your PFT includes a bronchodilator reversibility assessment, your doctor or the testing lab will want you to withhold certain inhaled medications beforehand. The reason is straightforward: bronchodilators open your airways, and the test needs to see how your airways perform without that assistance so it can then measure whether a bronchodilator makes a meaningful difference.
Standard withholding windows recommended by clinical guidelines break down by how long each medication acts:
- Short-acting bronchodilators (4 hours): This includes rescue inhalers like albuterol (salbutamol) and short-acting anticholinergics like ipratropium.
- Long-acting bronchodilators (12 hours): This covers long-acting beta-agonists (LABAs) like salmeterol and formoterol, as well as aminophylline and theophylline pills.
Notably, there are no established withholding recommendations for some newer or ultra-long-acting agents like tiotropium (a long-acting anticholinergic) or indacaterol (an ultra-long-acting beta-agonist), and the same applies to beta-blocker medications.2Dove Press. Drugs potentially affecting the extent of airways reversibility on pulmonary function testing are frequently consumed despite guidelines Research has found that patients frequently take medications that could affect reversibility results despite these guidelines being in place, which can muddy the interpretation of results and potentially lead to misdiagnosis.
A critical caveat: never stop a medication without your prescribing doctor’s explicit instruction. Some people with severe asthma or COPD cannot safely skip their inhalers for half a day. Your doctor and the testing facility should coordinate this. If you accidentally took your inhaler that morning, tell the technician before starting. The test can often be rescheduled or interpreted with that information noted.
Inhaled corticosteroids (like fluticasone or budesonide) are treated differently from bronchodilators. These are anti-inflammatory medications, and most labs do not require you to withhold them before spirometry. However, many combination inhalers contain both a corticosteroid and a long-acting bronchodilator in one device, so skipping the combination inhaler may be necessary. Ask your doctor specifically which of your inhalers to hold and which to continue.
Skip Exercise the Morning of the Test
You might assume that exercise would “open up” your lungs and help you perform better on a breathing test, but the reality is more complicated. A study of healthy young men found that submaximal exercise (comparable to a brisk treadmill session) caused FVC to drop from about 3.77 liters to 3.54 liters, a statistically significant decrease. At the same time, FEV1 actually increased slightly, from 3.26 to 3.41 liters, and peak expiratory flow rate jumped considerably.3Cureus. Immediate Spirometric Response to Submaximal Exercise in Healthy Young Adult Males In other words, exercise pushes some numbers up and others down, creating a distorted snapshot that does not represent your baseline lung function.
The practical advice is simple: avoid vigorous physical activity for at least four to six hours before your appointment. A gentle walk from the parking lot to the clinic is fine. Running, cycling, swimming, heavy lifting, or even an energetic house-cleaning session could shift your results enough to matter. If you work a physically demanding job and have a morning appointment, mention this to the technician so they can factor it in.
What to Wear
This one catches people off guard, but it has real research behind it. Tight clothing around the chest and abdomen restricts diaphragmatic breathing and significantly reduces how much air you can inhale.4PubMed. The effect of clothing on inhalation volume Your diaphragm is the primary muscle driving deep inhalation, and anything that prevents your belly from expanding freely during a maximal breath will shave volume off your results.
Wear loose-fitting clothes. Skip tight belts, compression garments, restrictive waistbands, corsets, and binding undergarments. If you realize at the appointment that your clothing is snug around the midsection, loosen your belt or unbutton your waistband before testing. The technician will usually suggest this, but it helps to arrive prepared. A loose t-shirt and comfortable pants or a relaxed-fit dress are ideal.
Eating, Drinking, and Smoking
Avoid eating a large meal within about two hours of your test. A full stomach pushes up on the diaphragm, reducing how far your lungs can expand downward. You do not need to fast entirely; a light snack is fine. The goal is just to avoid that heavy, stuffed feeling.
Alcohol should be avoided for at least four hours before testing and ideally longer, as it can relax smooth muscle in the airways and slightly depress respiratory drive, neither of which represents your normal state.
Smoking and vaping are the most important substances to avoid. Do not smoke for at least four to six hours before a PFT, and ideally not at all the day of the test. Smoking acutely constricts airways, increases mucus production, and temporarily reduces how well gas transfers across the lung lining. If your test includes a DLCO measurement, smoking is especially problematic because carbon monoxide from cigarettes directly competes with the test gas and will artificially lower your diffusion result. If you smoked within a few hours of the test, tell the technician. They need to know.
Caffeinated drinks like coffee and tea are mild bronchodilators. Most guidelines suggest avoiding caffeine for a few hours before the test, though the effect on spirometry results is small in most people. If you are a heavy coffee drinker and have an early morning appointment, skipping your usual cup is a minor sacrifice that removes one more variable.
Dentures, Mouthpieces, and Oral Fit
If you wear dentures, you may wonder whether to keep them in or take them out. Research shows that denture status significantly affects spirometric readings. A study comparing various oral conditions found meaningful differences in FVC and FEV1 depending on whether participants wore their upper dentures, lower dentures, or no dentures at all.5PubMed Central. Effects of complete dentures on respiratory performance: spirometric evaluation The general recommendation at most labs is to keep well-fitting dentures in during the test. Dentures help maintain the shape of your mouth and provide a firmer surface to seal around the mouthpiece, preventing air leaks. Poorly fitting dentures that are loose or uncomfortable can be a different story, so mention this to the technician ahead of time.
Regardless of denture status, the mouthpiece seal matters. Any air escaping around the sides of your mouth during the forced exhalation will reduce your measured volumes and make the test unreliable. The technician will place a nose clip on you to ensure all air goes through your mouth. If you feel the seal slipping during a blow, say so. You will get multiple attempts.
Your Height Matters More Than You Think
PFT results are interpreted by comparing your measured values against predicted values for someone of your age, sex, height, and ethnicity. Height is the strongest predictor of lung size, so even a small error in height measurement can shift your predicted values enough to change the interpretation. Research comparing different methods of estimating height found that using arm span as a substitute produced misclassification rates as high as roughly 24% in taller individuals.6Europe PMC. Using arm span to derive height: impact of three estimates of height on interpretation of spirometry
The practical takeaway: when the technician measures your height, stand up as straight as you can, shoes off, looking straight ahead. Do not slouch. If you have significant spinal curvature (from osteoporosis, kyphosis, or a vertebral fracture) that has reduced your standing height, mention it. Labs have methods to estimate your “true” height from arm span or other body measurements, though these estimates introduce their own variability. If you were tested at a different facility previously and want to compare results, make sure the height recorded was similar both times. A two-centimeter discrepancy between visits can shift your predicted FVC enough to change a “normal” result to “mildly reduced” or vice versa.
How to Actually Perform the Maneuver Well
Preparation is not just about what you avoid beforehand. The biggest source of unreliable PFT results is technique during the test itself. The single most common error is not inhaling fully before starting the forced exhalation. This is called submaximal inhalation error, and it is widespread enough that researchers have developed mathematical corrections for it. One analysis found that the error introduces excessive variability in FEV1 results, though a correction factor brought about 95% of affected measurements back within acceptable range of the true value.7American Journal of Respiratory and Critical Care Medicine. C108-05 The FEV1 From Spirometry Efforts Demonstrating Submaximal Inhalation Error (SIE) Can Be Corrected to Produce an Accurate Estimate of the FEV1 Generated From Full Inflation
The technician will coach you through the steps, but knowing what to expect helps you perform better:
- Breathe in completely: Fill your lungs as absolutely full as you can. Think of inflating yourself like a balloon. The breath in matters as much as the breath out.
- Blast out immediately: As soon as you are completely full, explode the air out as hard and fast as possible. Any hesitation at the top of the breath can reduce your FEV1.
- Keep blowing until empty: Do not stop when you feel like you are out of air. The technician will encourage you to keep pushing for at least six seconds, sometimes longer. That last trickle of air affects your FVC measurement.
- Repeat without frustration: You will likely do the maneuver at least three times, sometimes more. The lab needs at least two consistent efforts to consider the results reliable. If you feel lightheaded between blows, take a break. This is normal.
Sitting up straight with both feet flat on the floor also helps. Slouching compresses the lungs, and crossing your legs can tense your abdominal muscles in ways that interfere with a full exhalation.
What Happens During a Bronchodilator Reversibility Test
Many PFT appointments include a second round of spirometry after you inhale a bronchodilator, usually albuterol. This is the reversibility test, and its purpose is to see whether your airways open up with medication. A positive response is generally defined as an improvement of at least 12% and at least 200 mL in either FEV1 or FVC compared to your pre-bronchodilator baseline.8PubMed Central. Spirometry and Bronchodilator Test This distinction helps clinicians differentiate between asthma (which tends to be more reversible) and COPD (which tends to be less reversible), though there is plenty of overlap.
After you complete the initial spirometry, the technician will give you a metered dose of bronchodilator through a spacer device or nebulizer. Then you wait, typically 15 to 20 minutes, before repeating the spirometry. During that waiting period, stay seated and relaxed. Do not get up and walk briskly around the waiting room or start doing stretches, as any physical exertion during the waiting window can muddy the post-bronchodilator results for the same exercise-related reasons discussed earlier.
If you were told to hold your usual bronchodilator medications before the test, this is why. The reversibility test needs to compare your un-medicated baseline to the effect of a standardized dose of bronchodilator administered in the lab. If you already took your inhaler that morning, the baseline measurement already reflects the drug’s effect, and the test loses much of its diagnostic value.
Things People Worry About That Usually Do Not Matter
A few common anxieties about PFTs are worth addressing because they cause unnecessary stress. First, the test is not painful. You may feel lightheaded from the forceful exhalations, and some people get a mild headache, but these sensations pass quickly. Second, the test cannot “damage” healthy lungs. The pressures generated during a forced blow are high, but your ribcage is designed to handle them. Third, having a mild cold is usually okay as long as your doctor still wants to proceed. However, if you are actively wheezing, have a fever, or recently had a chest infection, mention this before the test starts. The results may still be useful for your doctor, but they will interpret them in context.
Finally, anxiety itself can affect the test. People who are nervous tend to take shallow, hesitant breaths and struggle with the “blast out as hard as you can” instruction. If you know you tend to be anxious in medical settings, arrive a few minutes early, sit in the waiting room, and take some slow deep breaths. Getting comfortable with the sensation of a truly maximal inhalation before you are sitting in front of the machine helps more than you would expect. The technician has coached hundreds or thousands of patients through this, and they are very good at reading when someone needs an extra moment to settle in.
A Quick Checklist for Test Day
Rather than trying to remember scattered details, here is what test-day preparation looks like in practice:
- Medications: Follow your doctor’s specific instructions on which inhalers or pills to skip and for how long. When in doubt, call the prescribing clinic the day before.
- Food and drink: Eat lightly. Skip alcohol and limit caffeine. Do not smoke or vape.
- Clothing: Loose-fitting shirt and pants. Ditch the tight belt.
- Exercise: No vigorous activity for several hours before. A gentle pace getting to the appointment is fine.
- Dentures: Keep them in if they fit well. Mention it if they are loose.
- Arrive informed: Know that you will need to blow hard and long, multiple times. Expect the whole visit to take 30 to 90 minutes depending on which tests are ordered.
Bring a list of all your current medications, including over-the-counter supplements and any inhalers you use, even occasionally. The technician will ask about these, and having the list handy prevents the awkward moment of trying to remember the name of a medication you take only when symptoms flare up. If you have had a PFT at a different facility, bringing those results (or having them sent to your current provider) allows for a direct comparison, which is often more informative than any single test in isolation.