Pulmonary function tests, commonly called PFTs, are a group of breathing tests that measure how well your lungs take in air, move it through your airways, and transfer oxygen into your bloodstream. They are used to diagnose conditions like asthma and chronic obstructive pulmonary disease (COPD), track how a disease is progressing, and guide treatment decisions. Research has consistently shown that PFTs are more precise than symptoms or a physical exam alone when it comes to gauging the severity of lung disease.1Mayo Clinic Proceedings. Current Practice in Pulmonary Function Testing The umbrella term “PFT” covers several distinct tests, each measuring something different about your respiratory system.
Spirometry Is the Starting Point
If you are sent for lung function testing, spirometry is almost certainly the first test you will do. You breathe into a mouthpiece connected to a device that records how much air you can blow out and how quickly. The test asks you to inhale as deeply as possible, then blast the air out as hard and fast as you can. This forced effort generates the numbers your doctor cares about most.
The two headline measurements are forced vital capacity (FVC), which is the total volume of air you can exhale after a full breath in, and forced expiratory volume in one second (FEV1), which is how much of that air comes out in the first second.2PubMed Central. Spirometry in the evaluation of pulmonary function Those two numbers, and the ratio between them, tell clinicians a surprising amount about what is happening inside your lungs. A low FEV1/FVC ratio, for example, suggests obstruction: your airways are narrowed, so air takes longer to get out. A proportionally low FVC with a preserved ratio suggests restriction: your lungs cannot expand fully, but the airways themselves are not blocked.
Spirometry is recognized as essential for both diagnosing and managing respiratory diseases, a status it has earned over 175 years of clinical use since John Hutchinson introduced the spirometer and coined the term “vital capacity” in the mid-1800s.3PubMed Central. Exploring the 175-year history of spirometry and the vital lessons it can teach us today
How Your Results Are Compared to “Normal”
Your raw spirometry numbers on their own do not mean much. A healthy 25-year-old man who is six feet tall will blow out considerably more air than a healthy 70-year-old woman who is five feet tall, and that difference is purely anatomical, not pathological. So your results are compared against predicted values based on your age, height, sex, and sometimes ethnicity. If your FEV1 is, say, 85% of the predicted value for someone with your demographics, that is interpreted very differently than if it is 50% of predicted.
The traditional cutoff for the FEV1/FVC ratio was a fixed number of 0.70: anything below that was labeled obstruction. The problem is that the ratio naturally drifts lower as you age. A strong mathematical relationship exists between FEV1 and FVC, but because a small constant in that relationship differs from zero, the ratio inherently depends on lung size, which changes with age.4PubMed Central. A mathematical reason for FEV1/FVC dependence on age Using a fixed 0.70 cutoff means that older adults who are perfectly healthy can be told they have obstruction, while younger adults with genuinely narrowed airways can be told they are fine. A large study of over 40,000 adults found that using the lower limit of normal, a statistical boundary set at the fifth percentile for your age group, reduces these misclassifications.5Thorax. Using the lower limit of normal for the FEV1/FVC ratio reduces the misclassification of airway obstruction
The Race-Neutral Reference Equation Debate
For decades, predicted spirometry values were calculated using race-specific equations. Different equations existed for white, Black, Hispanic, and Asian populations, reflecting observed average differences in lung volumes across groups. In recent years, a significant push has been made toward race-neutral reference equations, partly because race is a social category rather than a precise biological one, and partly because race-specific equations can perpetuate health disparities by setting a lower bar for what counts as “normal” in certain populations.
The transition is not purely philosophical; it changes real diagnoses. A 2023 study found that switching from race-specific to race-neutral Global Lung Initiative equations increased the overall prevalence of restriction from about 27% to about 38% and roughly doubled the prevalence of a nonspecific pattern of impairment, while the prevalence of obstruction stayed largely unchanged. Among white individuals specifically, the shift went the other direction: restriction diagnoses dropped from about 23% to 18%.6PubMed Central. Global, Race-Neutral Reference Equations and Pulmonary Function Test Interpretation In plain terms, which equation your lab uses can determine whether you walk out with a diagnosis or a clean bill of health. The Global Lung Function Initiative has published all-age reference equations for spirometry, diffusing capacity, and lung volumes that many labs worldwide have adopted or are in the process of adopting.7ERJ Open Research. Evaluation of the Global Lung Function Initiative reference equations in Belgian adults
Beyond Spirometry: Measuring Lung Volumes
Spirometry tells you how air moves through your airways, but it cannot measure the air that stays trapped in your lungs after you exhale completely. That leftover air, called residual volume, matters clinically. In diseases like emphysema, trapped gas inflates the lungs far beyond normal, and tracking that hyperinflation helps doctors understand how the disease is progressing.
Two common techniques measure these fuller lung volumes. Body plethysmography has you sit in a sealed, phone-booth-sized chamber while pressure changes as you breathe are used to calculate total lung capacity and residual volume. Helium dilution is simpler: you breathe a known concentration of helium, and because helium is not absorbed by the lungs, how much the concentration drops tells you how much air was already in your lungs. The two methods generally agree, though body plethysmography tends to pick up trapped gas that helium dilution can miss, particularly in severe obstruction.8Plucne bolesti. Correlation of the size of the residual volume measured by whole body plethysmography and the single breath helium dilution method
Diffusing Capacity: How Well Oxygen Crosses into Your Blood
Even if air flows freely in and out of your lungs, the oxygen in that air still has to cross a thin membrane to reach your bloodstream. Diffusing capacity testing, usually abbreviated DLCO, evaluates this gas-transfer step. You inhale a small, harmless amount of carbon monoxide, hold your breath for roughly ten seconds, and then exhale. By measuring how much carbon monoxide was absorbed during that breath-hold, the test gauges the health of the membrane between your air sacs and your capillaries.
DLCO is actually a product of two components: the rate at which carbon monoxide is taken up from alveolar gas (related to how healthy those membranes and capillaries are) and the volume of accessible lung, since a smaller lung obviously absorbs less gas overall.9American Journal of Respiratory and Critical Care Medicine. Examination of the Carbon Monoxide Diffusing Capacity (DlCO) in Relation to Its Kco and Va Components A common mistake in clinical interpretation is dividing DLCO by lung volume and assuming the result “corrects” for a small lung. It does not, because the relationship between diffusing capacity and lung volume is not a simple straight line. Two patients can have the same DLCO but for very different reasons: one might have damaged membranes in a full-sized lung, the other might have perfectly healthy membranes in a lung that just cannot expand completely. Adjusting properly for lung volume gives a better picture of what is actually going wrong.10PubMed. Importance of adjusting carbon monoxide diffusing capacity (DLCO) and carbon monoxide transfer coefficient (KCO) for alveolar volume
A low DLCO shows up in conditions that damage or destroy the membrane itself (emphysema, pulmonary fibrosis), reduce blood flow through the lungs (pulmonary hypertension, blood clots), or lower hemoglobin (anemia, since hemoglobin is what carries the gas away). DLCO is also a key part of preoperative planning. Before lung cancer surgery, if either FEV1 or DLCO falls below 80% of predicted, surgeons estimate how much lung function will remain after the operation. Estimated postoperative values below 40% predicted signal a substantially increased risk of complications.11The Annals of Thoracic Surgery. Preoperative assessment of the high-risk patient for lung resection
Bronchodilator Reversibility Testing
In many PFT sessions, spirometry is performed twice: once at baseline, and again about 15 minutes after you inhale a short-acting bronchodilator like salbutamol. The idea is to see whether your airways open up in response to the medication. If FEV1 improves by a certain threshold (often 12% and at least 200 milliliters), the obstruction is considered “reversible,” which has traditionally been used to distinguish asthma from COPD.
The real picture is messier than that clean diagnostic split suggests. A large analysis of over 35,000 people found that bronchodilator reversibility was at least as common in participants with COPD as in those with asthma, at about 18% and 17% respectively, while roughly 5% of people with no airway disease at all also met the reversibility threshold.12PubMed. Bronchodilator reversibility in asthma and COPD: findings from three large population studies The upshot is that reversibility testing is useful for confirming that bronchodilators help you, but it cannot reliably tell asthma and COPD apart on its own. Clinicians increasingly use it as one piece of a larger diagnostic puzzle rather than a standalone differentiator.
Provocation Tests and Exhaled Nitric Oxide
Sometimes your spirometry is completely normal, yet you have coughing, wheezing, or shortness of breath that suggests asthma. In that scenario, a bronchial challenge test can provoke the airways to narrow in a controlled setting. You inhale increasing concentrations of an agent like methacholine while spirometry is repeated after each dose. If your FEV1 drops by 20% or more, the test is considered positive for airway hyperresponsiveness. A meta-analysis found that methacholine challenge has high specificity for asthma but moderate sensitivity, meaning a positive result is a strong signal, but a negative result does not completely rule asthma out.13PubMed. Diagnostic comparison of methacholine and mannitol bronchial challenge tests for identifying bronchial hyperresponsiveness in asthma
A newer and much simpler test measures fractional exhaled nitric oxide (FeNO). You breathe steadily into a handheld device for about ten seconds, and it reports how much nitric oxide is in your exhaled breath. Elevated levels are a marker of a specific type of airway inflammation driven by eosinophils, a pattern strongly associated with asthma.14PubMed Central. Update on the Role of FeNO in Asthma Management FeNO is particularly valuable because it predicts whether your airways will respond to inhaled steroids. The correlation between FeNO and eosinophilic inflammation means it acts as an indirect indicator of steroid responsiveness.15American Journal of Respiratory and Critical Care Medicine. An Official ATS Clinical Practice Guideline: Interpretation of Exhaled Nitric Oxide Levels (FeNO) for Clinical Applications If your FeNO is high and your symptoms improve on steroids, that confirms the inflammation type. If your FeNO is low, your doctor may look for other causes of your symptoms rather than escalating steroid therapy.
Exercise Testing for Lung and Heart Function
Resting lung function tests sometimes look reassuring even when people feel short of breath during physical activity. Exercise-based tests bridge that gap. The six-minute walk test is the simplest: you walk as far as you can in six minutes along a flat corridor, and the distance covered serves as a measure of functional capacity. It is reliable and easy to administer, making it a common first-line test for patients with heart failure or pulmonary hypertension.16PubMed. Six-minute walk test and cardiopulmonary exercise testing in patients with chronic heart failure: a comparative analysis on clinical and prognostic insights
For a more detailed look, cardiopulmonary exercise testing (CPET) puts you on a treadmill or stationary bike while simultaneously measuring your breathing, heart rate, blood pressure, oxygen consumption, and carbon dioxide production. CPET can tease apart whether exercise limitation is due to your lungs, your heart, deconditioning, or some combination. In children with pulmonary hypertension, research suggests that the six-minute walk test reflects maximal exercise capacity mainly in those who walk less than 300 meters, while those who perform better benefit from the additional detail CPET provides.17Archives of Disease in Childhood. Comparison of 6-min walk test distance and cardiopulmonary exercise test performance in children with pulmonary hypertension
When Standard Spirometry Is Not Possible
Spirometry demands a big, forceful effort: a full breath in, then a blast out sustained for several seconds. That is manageable for most adults, but young children, elderly patients, and people with cognitive impairment or severe breathlessness often cannot produce a reliable forced maneuver. For them, impulse oscillometry (IOS) offers an alternative. You sit and breathe normally into a mouthpiece while the device sends gentle sound-wave pulses into your airways. Those pulses bounce back differently depending on how narrow or stiff your airways are, giving clinicians information about airway resistance and lung compliance without requiring any special breathing technique.18PubMed Central. Impulse oscillometry: The state-of-art for lung function testing
IOS has gained ground as a clinical tool over the past couple of decades, particularly in pediatrics and geriatrics. It can detect small-airway dysfunction that spirometry sometimes misses, and because it uses tidal breathing, it is feasible even in preschool-aged children.19PubMed Central. Lung Function Assessment by Impulse Oscillometry in Adults The main limitation is that reference values are less well established than those for spirometry, and not every pulmonary function lab has the equipment.
Getting Good Results: Why Technique Matters
PFT quality depends heavily on how you perform the test. Spirometry in particular is effort-dependent: if you do not blast the air out hard enough or stop exhaling too early, the numbers will underestimate your true lung function. To address this, the American Thoracic Society and European Respiratory Society have published detailed criteria for what counts as an acceptable maneuver. The top two FEV1 values from your session need to be within 150 milliliters of each other (or within 100 milliliters or 10% for children aged six and under), and the same repeatability standard applies to FVC.20PubMed Central. Standardization of Spirometry 2019 Update. An Official American Thoracic Society and European Respiratory Society Technical Statement
In practice, you will typically perform at least three acceptable blows, and the technician will coach you through each one. A good lab technician makes a real difference; their encouragement and instruction during the forced maneuver directly affect whether the results are usable. Even so, the guidelines acknowledge that some patients cannot meet all technical criteria on a given day, and results that fall short of full acceptability may still be clinically useful if they represent the best the patient can do.
Safety Considerations
PFTs are generally safe, but the forced breathing maneuvers do temporarily raise pressure inside your chest and abdomen. The highest-risk contraindications involve cardiovascular issues like a recent heart attack, pulmonary embolism, or an ascending aortic aneurysm, where that spike in pressure could be dangerous. Recent major surgery on the chest, abdomen, or head also warrants caution, since forceful exhalation can stress surgical sites.21Thorax. An update on contraindications for lung function testing For most people, though, the biggest discomfort is mild lightheadedness from the repeated deep breaths, which passes within seconds.
Occupational Exposures and Long-Term Tracking
PFTs are not only diagnostic tools; they are used to monitor lung health over time in workers exposed to dust, fumes, and other respiratory hazards. A study of two large general-population cohorts found that long-term occupational exposure to biological dust, mineral dust, and metals each accelerated the decline of FEV1 and the FEV1/FVC ratio by amounts comparable in magnitude to the effects of long-term smoking.22PubMed Central. Adult Pulmonary Cumulative Occupational Exposures and Lung-Function Decline in Two Large General-Population Cohorts That finding underscores why workplace lung-surveillance programs exist: serial spirometry over months and years can catch gradual declines before symptoms appear, giving workers and employers a chance to intervene with protective measures.
Home Spirometry and Remote Monitoring
Lab-based PFTs require a trip to a hospital or clinic, which limits how frequently they can be done. Portable smart spirometers paired with smartphone apps are starting to change that. These pocket-sized devices let patients perform spirometry at home and transmit results to their clinical team in real time. A study evaluating a remote monitoring program for patients with COPD and interstitial lung disease found weekly adherence rates of 84% and 94% respectively, figures that compare favorably with earlier remote spirometry programs.23PubMed Central. Evaluating a Remote Monitoring Program for Respiratory Diseases: Prospective Observational Study The appeal is obvious: frequent measurements can catch a flare-up or a medication failure days or weeks before the patient would have had a scheduled office visit. Home devices do not replace the full PFT suite (they cannot measure lung volumes or diffusing capacity), but for tracking FEV1 and FVC trends, they offer a practical complement to periodic lab testing.