FEF 25-75 does not have a single universally agreed-upon “normal” number in liters per second because the value depends heavily on your age, height, sex, and ethnicity. Instead, your result is compared to a predicted value calculated from reference equations, and the traditional threshold for normal has been at or above 65-80% of that predicted value. But that threshold is surprisingly controversial: major respiratory societies now caution that FEF 25-75 is highly variable and often does not change clinical decisions beyond what other spirometry numbers already tell you.
What FEF 25-75 Actually Measures
FEF 25-75 stands for the forced expiratory flow between 25% and 75% of your vital capacity. In practical terms, it captures the average speed of air leaving your lungs during the middle portion of a hard, fast exhale. It is expressed either as an absolute flow rate (liters per second) or, more commonly in clinical reports, as a percentage of the predicted value for someone of your age, sex, height, and ethnic background.1Chronic Obstructive Pulmonary Diseases: Journal of the COPD Foundation. Forced Expiratory Flow at 25%-75% Links COPD Physiology to Emphysema and Disease Severity in the SPIROMICS Cohort
The reason clinicians care about this middle slice of the exhale is that it reflects airflow through the smaller, more peripheral airways in the lungs. The larger airways dominate the beginning of a forced breath, so the early part of the maneuver is mostly captured by FEV1 (the total volume exhaled in the first second). By the time you’re between 25% and 75% of your total exhaled volume, the smaller airways are doing more of the work. That makes FEF 25-75 a tempting indicator of “small airway disease,” a concept that has driven decades of research and clinical debate.2BMJ Open Respiratory Research. Small airway function measured using forced expiratory flow between 25% and 75% of vital capacity and its relationship to airflow limitation in symptomatic ever-smokers: a cross-sectional study
How the “Normal Range” Is Determined
Because lung size and airflow vary enormously from person to person, your FEF 25-75 result is compared against a predicted value generated by reference equations. The most widely adopted set comes from the Global Lung Function Initiative (GLI-2012), which pooled spirometry data from tens of thousands of healthy, non-smoking individuals across multiple ethnic groups and age ranges.3PubMed Central. FEF 25-75% Values in Patients with Normal Lung Function Can Predict the Development of Chronic Obstructive Pulmonary Disease The GLI equations generate a predicted value and a z-score for each person. A z-score of zero means your result matches the average for someone of your age, sex, height, and ethnicity. A z-score below about -1.64 (the lower 5th percentile) is sometimes used to flag an abnormally low result, mirroring the approach used for FEV1.4PubMed. Measurement of FEF25–75% and FEF75% does not contribute to clinical decision making
In everyday clinical practice, though, you are more likely to see your result reported as “percent predicted.” A report that reads “FEF 25-75: 82% predicted” means your mid-expiratory flow is 82% of what would be expected for someone with your demographics. Older conventions flag anything below 80% of predicted as abnormal, while some pediatric references use a cutoff of 65% predicted. Neither threshold is carved in stone, and the choice of cutoff changes the clinical picture dramatically.
Why Fixed Cutoffs Are Unreliable
The biggest practical problem with FEF 25-75 is its enormous natural variability. Even among perfectly healthy never-smokers, the scatter around predicted values is wide. A landmark study examining this directly found that if you used the common cutoff of less than 80% of predicted to flag “abnormal” results, more than a quarter of never-smokers would be falsely labeled as having a problem. At the same time, among smokers with genuine airflow obstruction confirmed by other spirometry measures, 42% had FEF 25-75 values that looked normal.5Chest. Discriminating Measures and Normal Values for Expiratory Obstruction That is a staggering rate of both false positives and false negatives from a single measurement, and it is the core reason respiratory guidelines have grown cautious about leaning on it.
Part of the variability comes from how tightly FEF 25-75 is linked to total lung volume. If you exhale a slightly different total volume on two consecutive blows (which is common), the “25% to 75%” window shifts, and the calculated flow rate changes with it. Reference equations for FEF 25-75 also explain a smaller fraction of the overall variation compared to equations for FEV1 or FVC. One analysis of adjusted flow measurements found that even after accounting for age, height, and weight, the best models explained only about 30-46% of the variation in mid-to-late expiratory flow, leaving the rest unexplained.6European Respiratory Society. Predicted values for the forced expiratory flow adjusted for forced vital capacity, a descriptive study When the “normal” range is that broad, distinguishing real disease from statistical noise becomes difficult.
What Major Guidelines Say
The 2022 ERS/ATS technical standard on interpreting lung function tests addressed FEF 25-75 directly and was notably unenthusiastic. The document states that mid-range flow measurements during a forced exhalation are highly variable, poorly reproducible, and not specific for small airway disease in individuals. It adds that these measurements usually do not contribute to clinical decision making beyond what FEV1, FVC, and the FEV1/FVC ratio already provide, and that there is insufficient evidence to support using spirometry alone to identify small airway dysfunction.7PubMed. ERS/ATS technical standard on interpretive strategies for routine lung function tests
That is a strong statement from the two leading respiratory societies, and it has shaped how many pulmonologists interpret spirometry reports. In practice, a low FEF 25-75 on an otherwise normal report rarely triggers a change in diagnosis or treatment by itself. It may prompt further testing or closer follow-up in certain contexts, but it is not treated as a standalone alarm the way a low FEV1/FVC ratio would be.
Where FEF 25-75 Still Gets Clinical Attention
Despite the official caution, FEF 25-75 has not disappeared from clinical practice or research. Several scenarios keep it relevant, especially when the rest of the spirometry looks unremarkable.
Asthma with “Normal” Spirometry
Some people with asthma have normal FEV1 and a normal FEV1/FVC ratio but still report symptoms like cough, chest tightness, or wheeze. In that group, a reduced FEF 25-75 can be an early clue that the smaller airways are already affected. One study of outpatients with asthma found that about 20% of those with normal FEV1 and FEV1/FVC had an impaired FEF 25-75, and that impairment was associated with roughly double the odds of having active asthma symptoms.8PubMed Central. Role of FEF25–75 in characterizing outpatients with asthma in clinical practice Another study found that an FEF 25-75 below 50% of predicted was associated with more than twice the risk of airway hyperresponsiveness (the hallmark of asthma) compared to values above 70%.9Allergy, Asthma & Immunology Research. Small Airway Impairment and Bronchial Hyperresponsiveness in Asthma Onset
These findings suggest FEF 25-75 may flag people whose small airways are already narrowing before the usual spirometry numbers catch up. Whether acting on that information improves outcomes is less clear, which is partly why the guidelines remain cautious.
Early COPD Detection in Smokers
A similar logic applies in people who smoke. FEV1 and FEV1/FVC can remain in the normal range for years while the small airways accumulate damage. Some researchers have proposed using FEF 25-75 as an early screening tool for smokers who have not yet developed overt COPD.10European Respiratory Journal. Low mid-expiratory airflow on spirometry is an early indicator of impaired lung function in asymptomatic adult smokers A prospective study of patients who initially had normal pulmonary function tests found that those whose FEF 25-75 z-scores fell below about -0.84 were significantly more likely to go on to develop COPD, with sensitivity around 70% and specificity near 79%.3PubMed Central. FEF 25-75% Values in Patients with Normal Lung Function Can Predict the Development of Chronic Obstructive Pulmonary Disease That performance is not strong enough to be diagnostic on its own, but in the context of a person with a heavy smoking history and borderline results, it adds a piece to the puzzle.
FEF 25-75 in Children
Pediatric asthma is one area where FEF 25-75 gets more attention than the guidelines might suggest. Children’s lungs are still growing, and the small airways mature at a different pace than the larger ones, so early signs of obstruction may show up in mid-expiratory flows before FEV1 drops. A study of newly diagnosed asthmatic children found that about 19% had impaired FEF 25-75 (defined as below 65% predicted), and those children were far more likely to have uncontrolled asthma and overt bronchial obstruction than children with normal FEF 25-75 values.11PubMed Central. Role of FEF 25-75 in managing children with newly-diagnosed asthma in clinical practice
In children with asthma whose FEV1 was normal, a low FEF 25-75 was still linked to increased asthma severity, more frequent use of systemic steroids, and more exacerbations.12PubMed Central. The utility of forced expiratory flow between 25% and 75% of vital capacity in predicting childhood asthma morbidity and severity There is also evidence that FEF 25-75 may be more sensitive than FEV1 at detecting a treatment response in pediatric patients, at least for certain add-on medications like tiotropium.13PubMed Central. Forced Expiratory Flow (FEF(25-75%)) as a Clinical Endpoint in Children and Adolescents with Symptomatic Asthma Receiving Tiotropium: A Post Hoc Analysis For that reason, some pediatric pulmonologists track FEF 25-75 more closely than their adult-medicine counterparts, treating it as one more data point rather than the decisive number.
Bronchodilator Response and FEF 25-75
When you perform spirometry before and after inhaling a bronchodilator, the change in FEF 25-75 can sometimes reveal reversible airway narrowing that FEV1 misses. One study looking at bronchodilator response criteria found that an increase in FEF 25-75 of 10% or more was the single most sensitive criterion for detecting a positive response, flagging about 63% of tested subjects, while conventional FEV1-based criteria detected fewer.14European Respiratory Journal. Characterization of bronchodilator response by spirometry and plethismography A separate comparison between asthma and COPD patients found that an increase in FEF 25-75 of 20% or more was among the criteria best at detecting airway reversibility in both groups.15Eurasian Journal of Pulmonology. Bronchodilator reversibility: What are the differences between asthma and chronic obstructive pulmonary disease?
The catch is that the high natural variability of FEF 25-75 applies here too. A change that looks like a real bronchodilator response could partly be noise from slightly different effort or volume on the second blow. Clinicians who use bronchodilator change in FEF 25-75 tend to interpret it alongside the rest of the picture rather than as proof of reversibility by itself.
Newer Ways to Assess Small Airways
Part of the reason FEF 25-75 remains in clinical reports despite mixed reviews is that, until recently, there was no good alternative for assessing small airway function during a routine office visit. That is changing with impulse oscillometry (IOS), a technique where the patient breathes normally through a mouthpiece while the machine sends small pressure pulses into the airways and measures how the airways respond. IOS does not require a forced exhalation, which eliminates many of the effort-dependent variability issues that plague FEF 25-75.
Research comparing the two approaches has produced interesting and somewhat humbling results for FEF 25-75. One study of symptomatic patients with normal spirometry found that IOS-derived measures (specifically the resonant frequency, or Fres) had greater sensitivity to detect small airway dysfunction than FEF 25-75, and patients identified by IOS were more likely to have symptoms like wheeze or sputum production.16The Journal of Allergy and Clinical Immunology: In Practice. Small Airway Dysfunction by Impulse Oscillometry in Symptomatic Patients with Preserved Pulmonary Function Another study found no meaningful correlation between FEF 25-75 and the IOS parameter commonly used for peripheral airways (R5-R20), suggesting the two measurements may be picking up different aspects of airway physiology rather than measuring the same thing in different ways.17Respiratory Medicine. Prevalence, clinical characterization, and impact on asthma control of impulse oscillometry-defined small airway dysfunction in asthmatic patients with normal spirometry
A broader analysis did find significant correlations between some IOS parameters and FEF metrics, with the reactance area parameter (AX) showing the strongest association, regardless of how impaired the patient’s other spirometry numbers were.18PubMed Central. Analysis of the correlations and inconsistencies between spirometry and impulse oscillometry in the diagnosis of small-airway dysfunction The picture that emerges is complicated: FEF 25-75 and IOS overlap somewhat but are not interchangeable, and IOS appears to catch cases that FEF 25-75 misses. IOS equipment is still relatively uncommon outside academic centers, though, so FEF 25-75 remains the only small-airway proxy available on most standard spirometry reports.
Air Pollution and FEF 25-75
Outside of the clinic, FEF 25-75 has found a useful niche in environmental health research precisely because it may capture subtle small-airway effects that FEV1 does not. Declines in FEF 25-75 have been associated with short-term exposure to fine particulate matter (PM2.5), sulfur dioxide, and carbon monoxide in studies of community-dwelling adults.19Journal of Exposure Science & Environmental Epidemiology. Associations between short-term exposure to ambient air pollution and lung function in adults Longer-term residential exposure to air pollution, even at levels below European Union limits, has been linked to impairment in small airway function measured by FEF 25-75.20Environmental Research. Residential exposure to outdoor air pollution and adult lung function, with focus on small airway obstruction
Interestingly, the relationship runs the other way too: when air pollution drops, FEF 25-75 can improve. A study tracking the effects of declining particulate matter over time found that a reduction in PM10 of 10 micrograms per cubic meter was associated with a measurable slowing of the normal age-related decline in FEF 25-75, though the magnitude of that benefit varied by genetic background.21PubMed. HMOX1 and GST variants modify attenuation of FEF25–75% decline due to PM10 reduction For epidemiologists studying the lung effects of environmental exposures at a population level, FEF 25-75 offers sensitivity to early airway changes that broader spirometry measures may not pick up, even if the measurement is too noisy for individual clinical decisions.
Post-COVID Small Airway Changes
The COVID-19 pandemic renewed interest in small airway assessments, including FEF 25-75. SARS-CoV-2 has a particular affinity for the small airways, and researchers wanted to know whether recovered patients showed lingering damage there even when their standard spirometry normalized. A cross-sectional study of healthcare workers who had recovered from COVID-19 found that about 19% had evidence of small airway obstruction, even though the group’s average FEF 25-75 was 96% of predicted, which is well within the normal range.22PubMed Central. Factors associated with small airway obstruction in COVID-19 survivors: A cross-sectional study among health-care providers In a smaller comparison of post-COVID patients grouped by initial disease severity, a minority in both the mild-moderate and severe-critical groups had FEF 25-75 below normal, with no statistically significant difference between severity groups.23Jurnal Respirasi. Comparison of Forced Expiratory Flow (FEF) 25-75% between Post-COVID-19 Patients with Different Severity at Universitas Gadjah Mada Academic Hospital
These findings suggest that COVID can leave a footprint in the small airways in some people, detectable by FEF 25-75, but the effect is not universal and does not appear to track neatly with how sick someone was during the acute infection. For people dealing with persistent breathing difficulties after COVID, a low FEF 25-75 on an otherwise normal spirometry report might help explain ongoing symptoms, though it would not, on its own, dictate a specific treatment.
Exercise-Induced Asthma in Athletes
One population where FEF 25-75 (and its close cousin FEF50, the flow at 50% of vital capacity) has been specifically tested and found wanting is elite athletes being evaluated for exercise-induced bronchoconstriction. The diagnosis typically relies on a drop in FEV1 after an exercise challenge. Adding mid-expiratory flows to the diagnostic criteria did not improve diagnostic accuracy in a study of elite athletes and actually reduced sensitivity, meaning more true cases were missed.24PubMed. Mid-expiratory flow versus FEV1 measurements in the diagnosis of exercise induced asthma in elite athletes If you are an athlete undergoing exercise-challenge testing, the FEV1 change is the number that matters for the diagnosis. Mid-expiratory flow alone is not sensitive enough to catch exercise-induced airway narrowing reliably in that setting.