Pulmonary function tests, commonly called PFTs, are a group of breathing tests that measure how well your lungs move air in and out and how efficiently they transfer oxygen into your blood. The most familiar PFT is spirometry, where you blow as hard and fast as you can into a mouthpiece, but the full suite includes lung volume measurements, gas diffusion tests, bronchial challenge tests, and more. Depending on the clinical question, a doctor may order one test or several, and the results hinge on surprisingly nuanced details of technique, reference values, and interpretation.
What Spirometry Actually Measures
Spirometry is the workhorse of pulmonary function testing. You take the deepest breath you can, seal your lips around a mouthpiece, and blast the air out as fast and completely as possible. The device records two key numbers. The first is how much total air you expelled, called forced vital capacity (FVC). The second is how much of that air came out in the first second, called forced expiratory volume in one second (FEV1). The ratio between them, FEV1 divided by FVC, is the single most important number for distinguishing between the two broad categories of lung disease. A low ratio suggests obstruction, meaning the airways are narrowed and air can’t get out quickly. A normal or high ratio paired with a low FVC suggests restriction, meaning the lungs can’t expand fully.
This sounds straightforward, but interpreting the ratio is trickier than it appears. For decades, guidelines used a fixed cutoff: if the ratio fell below 70%, the patient was considered obstructed. The problem is that the ratio naturally declines with age and varies with height. A study using a large U.S. reference dataset found that applying the fixed 70% threshold misclassifies a large number of results at the extremes of age and height compared with using a statistically derived lower limit of normal.1PubMed. FEV1/FVC ratio of 70% misclassifies patients with obstruction at the extremes of age In practice, this means healthy older adults get told they have obstruction when they don’t, and young adults with genuine obstruction get missed. One study found that nearly half of young adults whose ratio fell below the statistical lower limit of normal were missed because their ratio still sat above 70%.2PubMed. Spirometric criteria for airway obstruction: Use percentage of FEV1/FVC ratio below the fifth percentile, not < 70% Those missed individuals weren’t just borderline cases. A longitudinal study tracking them over time showed they had a significantly higher risk of developing COPD and used more emergency healthcare resources for breathing problems than people without airflow obstruction.3Thorax. Underestimation of airflow obstruction among young adults using FEV1/FVC <70% as a fixed cut-off
Most pulmonary function labs now report both the fixed ratio and the lower limit of normal. If your report shows a ratio of, say, 68% but the lower limit of normal for your age and height is 65%, you and your doctor should have a conversation about whether that result truly indicates disease or simply reflects normal variation. The distinction matters for whether you end up on inhalers or reassurance.
When Spirometry Isn’t Enough to Measure Lung Size
Spirometry tells you how much air you can blow out, but it can’t tell you how much air remains trapped in your lungs after you exhale. That leftover air, called residual volume, is invisible to spirometry. To measure it, you need a separate test. The two main approaches are body plethysmography, where you sit inside a sealed glass booth and breathe against a closed shutter while pressure changes reveal your lung volumes, and helium dilution, where you breathe a known concentration of helium in a closed circuit until it equilibrates throughout your lungs.
These two methods don’t always agree, and the disagreement itself carries clinical information. In people with severe COPD, some regions of the lung are so poorly ventilated that helium never reaches them, so helium dilution underestimates true lung volume. Plethysmography, meanwhile, captures all the gas in the chest regardless of ventilation. A study of COPD patients found that the gap between the two measurements widened as the disease got worse, and the size of that gap could actually help differentiate severe COPD from less severe disease.4PubMed Central. The measurement of lung volumes using body plethysmography and helium dilution methods in COPD patients: a correlation and diagnosis analysis A separate comparison including CT scans found that in people with obstruction, plethysmography overestimated lung volume compared with both helium dilution and CT-derived volumes, with the overestimation greatest among those with the most severe obstruction.5PubMed Central. Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD? Even in non-disease states the methods can diverge. In late-stage pregnancy, plethysmography produced larger values than helium dilution, likely because physiological changes at term create small pockets of poorly ventilated lung.6PubMed. Comparison of helium dilution and plethysmographic lung volumes in pregnant women
Why does this matter for your care? If spirometry alone suggests restriction but your lung volumes haven’t been directly measured, the finding may be misleading. A population-based study confirmed the need for direct lung volume measurement to diagnose true restrictive lung function, since spirometry alone can flag restriction that turns out not to exist once volumes are checked.7PubMed. Prevalence of restrictive lung function in children and adults in the general population
Bronchodilator Reversibility Testing
After your baseline spirometry, the technician may hand you an inhaler containing a fast-acting bronchodilator and ask you to repeat the test fifteen minutes later. The idea is to see whether your airways open up with medication, which helps distinguish conditions like asthma (where reversibility is expected) from COPD (where it’s typically more limited). A positive response has traditionally been defined as an increase of at least 12% and at least 200 mL in either FEV1 or FVC.8PubMed Central. Spirometry and Bronchodilator Test
That dual threshold sounds precise, but it has biases. The percentage change relative to baseline is influenced by how low your starting value is. In people with very severe obstruction whose starting FEV1 is tiny, even a modest absolute change can produce a large percentage swing. A study of over a thousand patients with low baseline FEV1 found that many showed percentage increases of 12% to nearly 45% relative to baseline while still not reaching the 200 mL absolute cutoff.9PubMed. Bronchodilator Response in FVC Is Larger and More Relevant Than in FEV(1) in Severe Airflow Obstruction That same study found that expressing the response as a percentage of the predicted value or as a statistical z score reduced biases related to age, sex, and height. In people with severe airflow limitation, changes in FVC may actually be more meaningful than changes in FEV1, because the bronchodilator allows trapped air to escape, increasing the volume the patient can exhale even if the flow rate doesn’t change much.
Population-based reference data for “normal” bronchodilator response are surprisingly sparse. An epidemiological study of healthy, asymptomatic non-smokers found that the upper 95th percentile of bronchodilator response, expressed as a fraction of the predicted baseline FEV1, averaged about 9% and stayed remarkably stable across sex, age, and height groups.10American Review of Respiratory Disease. Clinical Interpretation of Airway Response to a Bronchodilator: Epidemiologic Considerations That’s well below the 12% threshold commonly used in clinics, which means the standard cutoff may be too generous, potentially labeling some responses as “positive” when they fall within normal variation.
Bronchial Challenge Tests
When spirometry is normal but asthma is still suspected, a bronchial challenge test can provoke the airways to see if they’re abnormally twitchy. The most common version uses methacholine, a drug that causes airway narrowing in everyone but does so at much lower doses in people with hyperreactive airways. You inhale progressively higher doses while your FEV1 is tracked. The test ends when FEV1 drops by 20% or the maximum dose is reached without a drop. The dose that triggers that 20% fall is the key result. Updated European Respiratory Society standards now recommend reporting this as a delivered dose rather than a concentration, so results are comparable across different nebulizer devices and protocols.11European Respiratory Journal. ERS technical standard on bronchial challenge testing: general considerations and performance of methacholine challenge tests
The test is best at ruling asthma out rather than ruling it in. In someone with recent symptoms, a normal methacholine test essentially excludes current asthma with high confidence. A positive test at moderate-to-high sensitivity doses carries strong specificity, but mildly positive results can be seen in people with allergic rhinitis, recent respiratory infections, or even COPD.12PubMed. Direct challenge tests: Airway hyperresponsiveness in asthma: its measurement and clinical significance Some labs supplement FEV1 tracking with whole-body plethysmography during the challenge because airway resistance measured by plethysmography may pick up early airway narrowing that FEV1 misses. One study found that an airway resistance parameter detected two to nearly four times more change per dose step than FEV1 did, with a better overall diagnostic performance.13PubMed Central. Diagnostic accuracy of methacholine challenge tests assessing airway hyperreactivity in asthmatic patients – a multifunctional approach
Detecting Small Airway Disease
Standard spirometry is relatively insensitive to problems in the smallest airways, defined as those less than 2 mm in diameter. These tiny passages make up the majority of the branching tree deep in the lungs, and disease there can be well advanced before FEV1 budges.14CHEST. Ascertainment of Small Airways Dysfunction Using Oscillometry to Better Define Asthma Control and Future Risk Impulse oscillometry (IOS) offers an alternative. Instead of requiring a forceful blow, it superimposes tiny pressure pulses on your normal tidal breathing and analyzes how the airways respond. The test is effort-independent and takes only a few quiet breaths.
IOS has shown promise in picking up small airway dysfunction that spirometry misses. In patients with respiratory symptoms but preserved spirometry values, IOS was more sensitive at detecting small airway dysfunction than the mid-expiratory flow measure (FEF25-75%) that spirometry provides.15PubMed. Small Airway Dysfunction by Impulse Oscillometry in Symptomatic Patients with Preserved Pulmonary Function And in patients with small airway disease or COPD, IOS parameters like overall and peripheral resistance were significantly elevated compared with healthy controls.16Scientific Reports. Diagnosis and evaluation of small airway disease and COPD using impulse oscillometry Because oscillometry requires only relaxed breathing, it’s also well suited for young children and anyone who can’t perform the explosive exhalation spirometry demands.
Flow-Volume Loops and Upper Airway Obstruction
One of the most visually informative PFT outputs is the flow-volume loop, a graph that plots airflow rate against volume during both a maximal exhalation and a maximal inhalation. In most lower-airway diseases like asthma or COPD, the expiratory limb shows a characteristic scooped-out shape. Upper airway problems produce something altogether different. A landmark study of 43 patients with tracheal strictures, tumors, or vocal cord paralysis identified three distinct patterns on the flow-volume loop.17American Review of Respiratory Disease. Evaluation of Obstructing Lesions of the Trachea and Larynx by Flow-Volume Loops
A fixed obstruction, like a rigid tracheal scar, flattens both the inspiratory and expiratory curves roughly equally. A variable extrathoracic obstruction, such as a floppy vocal cord, primarily flattens the inspiratory curve because negative pressure during inhalation sucks the obstruction inward. A variable intrathoracic obstruction, like a tracheal tumor inside the chest, flattens the expiratory curve because the positive pressure of exhalation compresses the airway around it. These patterns give clinicians a heads-up that the problem is in the large central airway, not the small peripheral ones, which can redirect the workup toward imaging or direct airway inspection.18Otolaryngologic Clinics of North America. Flow Volume Loops in the Evaluation of Upper Airway Obstruction One caveat: if the patient also has COPD or asthma, the upper airway pattern can be masked, and small lesions may not show up at all in people with otherwise normal lungs.
Race-Neutral Reference Equations
Every PFT result is compared against a predicted value for a healthy person of the same age, height, and sex. Historically, those predictions also factored in race, with separate equations for Black, white, Asian, and other groups. The rationale was that average lung volumes differ across populations. The consequence was that a Black patient could have genuinely impaired lung function yet be told their result was “normal for their race,” obscuring real disease.
A major shift is underway. The Global Lung Function Initiative released race-neutral reference equations in 2022, and studies applying them to large datasets have found meaningful reclassifications. One analysis found that switching to the race-neutral equations altered the interpretation of about 8% of spirometry tests overall, rising to roughly 15% among Black patients.19American Journal of Respiratory and Critical Care Medicine. Application of Global Lung Function Initiative Global Spirometry Reference Equations across a Large, Multicenter Pulmonary Function Lab Population Another study found that the race-neutral equations identified more Black individuals as having restrictive or nonspecific impairments, and at greater severity, than the old race-specific equations had. The researchers concluded that race-specific equations may promote health disparities by inflating the apparent lung function of Black individuals and obscuring impairments a race-neutral approach would catch.20JAMA Network Open. Global, Race-Neutral Reference Equations and Pulmonary Function Test Interpretation A separate analysis found that applying the race-neutral equations reclassified roughly 40-49% of patients as having normal physiology or less severe disability.21Scientific Reports. Race-neutral versus race-specific GLI reference equations on spirometry interpretation in the general population This transition is still in progress across labs, and if you’re comparing recent PFT results against older ones, it’s worth asking which reference equations were used.
Exhaled Nitric Oxide
Not every PFT involves blowing hard. Fractional exhaled nitric oxide (FeNO) is a quick, non-invasive breath test that measures a specific gas your airways produce when a certain type of inflammation is present. Elevated FeNO levels point toward eosinophilic (type 2) airway inflammation, the kind that responds well to inhaled corticosteroids. Guidelines recommend FeNO as a supplement to the diagnostic workup when asthma is suspected, and it’s increasingly used to monitor whether inflammation is controlled on treatment.22PubMed Central. Update on the Role of FeNO in Asthma Management In children especially, FeNO adds information that spirometry and bronchial challenge testing alone can’t provide, giving a window into the nature of the underlying inflammation rather than just the mechanical consequences.23PubMed. Exhaled nitric oxide in childhood asthma: a review FeNO isn’t specific to asthma, though. It can be elevated in eosinophilic bronchitis, atopy without asthma, or even in certain infections, and levels drop if you’ve recently used corticosteroids. Context matters for interpretation.
Respiratory Muscle Strength Testing
When lung volumes are low but the airways seem fine, the problem might not be the lungs themselves but the muscles that inflate them. Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) tests measure how much negative or positive pressure you can generate by breathing against a sealed valve. These volitional tests are the most commonly performed assessments of respiratory muscle strength because the equipment is simple and widely available.24PubMed Central. Diagnostic methods to assess inspiratory and expiratory muscle strength
MIP and MEP are particularly valuable in neuromuscular diseases like ALS, muscular dystrophy, or myasthenia gravis, where respiratory failure can develop insidiously. Performing both tests together increases the probability of catching respiratory insufficiency, since some neuromuscular conditions preferentially weaken inspiratory muscles while others affect expiratory muscles more.25PubMed. Utility of maximum inspiratory and expiratory pressures as a screening method for respiratory insufficiency in slowly progressive neuromuscular disorders In patients who can’t form a seal around a mouthpiece, such as those with facial muscle weakness or a tracheostomy, alternative maneuvers like the sniff nasal pressure test or cough pressure measurement can substitute.26European Respiratory Journal. Maximal inspiratory and expiratory pressure measurement in tracheotomised patients
Cardiopulmonary Exercise Testing
Sometimes resting PFTs look normal, but a patient is still short of breath with exertion. Cardiopulmonary exercise testing (CPET) puts the whole system under stress by having you pedal a stationary bike or walk a treadmill while continuously measuring your breathing, heart rate, oxygen consumption, and carbon dioxide output. It’s the most comprehensive way to sort out whether breathlessness stems from the lungs, the heart, deconditioning, obesity, or a combination. In a study of 100 patients whose routine tests failed to explain their shortness of breath, CPET identified ventilatory abnormalities in about a quarter, cardiovascular disease in nearly as many, and deconditioning or obesity in the rest, while 16 turned out to have a completely normal exercise response.27PubMed Central. Investigation of dyspnea using cardiopulmonary exercise test among patients resorting to the respiratory tests CPET is also used before major surgeries, especially lung resection, to predict whether a patient can tolerate the loss of lung tissue.
PFTs Before Surgery and During Drug Monitoring
Pulmonary function testing has been used to evaluate surgical risk since the 1950s. For lung cancer surgery in particular, the predicted post-operative FEV1 is a central part of the risk assessment. Early work showed that a maximal voluntary ventilation below 50% of predicted or an FVC below 70% of predicted carried roughly a 40% risk of death after lung resection.28Respiratory Medicine. Preoperative evaluation of patients for lung resection Modern algorithms are more refined, incorporating diffusion capacity and exercise testing alongside spirometry, but the principle remains: PFTs help estimate how much lung function the patient will have left after tissue is removed.
A different application is monitoring for drug-related lung damage. Certain medications, including some chemotherapy agents and the immunosuppressant everolimus, can cause interstitial lung disease. One prospective study of breast cancer patients on everolimus found that a decline in diffusing capacity for carbon monoxide (DLCO) differentiated drug-induced lung disease from alternative diagnoses with about 91% sensitivity.29PubMed Central. Prospective Study of Drug-induced Interstitial Lung Disease in Advanced Breast Cancer Patients Receiving Everolimus Plus Exemestane However, a broader systematic review found that the sensitivity of DLCO monitoring for early detection of drug-induced lung disease varied wildly and was below 80% in most studies examined. The review concluded that routine PFT screening for drug-induced lung disease doesn’t have strong enough evidence to recommend universally, though individual clinical situations may still warrant it.30PubMed. Pulmonary function testing for the early detection of drug-induced lung disease: a systematic review in adults treated with drugs associated with pulmonary toxicity
PFTs in Young Children
Getting a three-year-old to perform a full forced exhalation is exactly as difficult as it sounds. Standard spirometry requires understanding and cooperation that most preschool-age children can’t reliably provide. Specialized pediatric labs have developed strategies to work around this, including role-playing where the child teaches a stuffed animal how to blow, gamified software that rewards effort, and tidal-breathing techniques that don’t require forceful maneuvers at all.31Breathe. Reflections of a paediatric pulmonologist: strategies for optimising lung function tests in preschool children Impulse oscillometry is especially useful here because it only requires the child to breathe normally through a mouthpiece. Along with gas dilution and plethysmography, these tidal-breathing techniques have opened up PFT to children as young as two to five years old who couldn’t have been tested a generation ago.32PubMed. Pulmonary function testing in young children Adapted equipment and experienced pediatric teams remain essential, though. The success rate rises sharply with team expertise and a child-friendly testing environment.33PubMed. Pulmonary function testing in young children
Occupational Lung Surveillance
PFTs play a specific role in workplaces where dust, fumes, or chemical vapors put employees’ lungs at risk. The value here isn’t a single snapshot but tracking changes over time. Comparing each worker’s spirometry to their own prior results detects excessive decline earlier than a single test benchmarked against a population average, because it catches losses that are meaningful for that individual even if the numbers still fall within the “normal” range.34American Journal of Respiratory and Critical Care Medicine. Official American Thoracic Society Technical Standards: Spirometry in the Occupational Setting After outbreaks of constrictive bronchiolitis in flavoring-production workers, longitudinal spirometry proved particularly useful for documenting the extent of disease in the workforce and identifying the worst-affected areas of the facility.35PubMed Central. Medical surveillance for the emerging occupational and environmental respiratory diseases A meta-analysis of population-based studies confirmed that occupational exposure to gases, fumes, vapors, and dusts is significantly associated with accelerated FEV1 decline over time.36Occupational & Environmental Medicine. Ever and cumulative occupational exposure and lung function decline in longitudinal population-based studies
Test quality is critical in this setting. An occupational health study found that roughly 70% of spirometry tests were unacceptable, with the most common error being an exhalation lasting less than six seconds, followed by repeatability errors between trials.37PubMed Central. Quality of spirometry tests in the field of occupational health Poorly performed spirometry doesn’t just waste the worker’s time; it greatly increases the risk of misinterpreting results, potentially masking real occupational injury or triggering unnecessary alarm.38PubMed. How to make sure your spirometry tests are of good quality
Home Spirometry and Remote Monitoring
The COVID-19 pandemic accelerated interest in whether patients with chronic lung disease could reliably perform spirometry at home. Early results are encouraging. A study comparing unsupervised home spirometry with supervised lab spirometry found a very strong correlation between the two, with the home device slightly underestimating FEV1 by about 0.10 L and FVC by 0.03 L, biases that fell within narrow statistical limits of agreement. Nearly all participants completed the home sessions, and exit surveys showed strong willingness to continue using a home device for long-term care.39European Respiratory Journal Open Research. Home spirometry appears accurate and feasible for monitoring chronic respiratory disease Home spirometry doesn’t replace the full battery of lab-based PFTs, but for tracking trends in conditions like cystic fibrosis, lung transplant rejection, or progressive pulmonary fibrosis, it could provide early warning signals between clinic visits.
How Altitude Changes Your PFT Numbers
If you’ve ever had PFTs done at elevation and wondered whether your results were different because of the altitude, the answer is yes. Studies comparing residents of high-altitude and low-altitude environments have found that people living at high altitude tend to have higher FVC and FEV1 values but a lower FEV1/FVC ratio, alongside higher oxygen saturation levels and lower resting heart rates.40PubMed Central. Respiratory function of university students living at high altitude A genotype-controlled study, which compared highland and lowland natives of similar genetic background, confirmed that FVC and FEV1 were roughly 8-15% greater among highlanders.41PubMed Central. A comparative analysis of lung function and spirometry parameters in genotype-controlled natives living at low and high altitude Even short-term stays at altitude shift things. Three weeks at about 5,400 meters increased lung diffusing capacity, the membrane diffusion component, and alveolar volume, though pulmonary capillary blood volume stayed the same.42PubMed. High-altitude exposure of three weeks duration increases lung diffusing capacity in humans Most reference equations were developed at low altitudes, so interpreting PFTs for long-term high-altitude residents using standard predicted values can underestimate their true baseline or, conversely, make a modest impairment look normal. Some labs at elevation apply altitude-specific adjustments, but practice varies.