What Is FEF 25-75 in a Pulmonary Function Test?

FEF 25-75 is the average speed of air flowing out of your lungs during the middle portion of a forced exhale, specifically between the 25% and 75% marks of your total exhaled breath. It appears on spirometry reports alongside more familiar numbers like FEV1 and FVC, and it is meant to reflect what is happening in your smaller airways, the narrow tubes deep inside your lungs that are harder to assess with standard measures.¹ Doctors have debated its clinical usefulness for decades, and it occupies an unusual position: widely reported on test results yet not formally recommended for diagnosis by major guidelines. That gap between its presence on your report and its contested status in practice is worth understanding.

How the Measurement Works

During a standard spirometry test, you take the deepest breath you can and then blow out as hard and fast as possible into a device that records airflow. The machine tracks the total volume of air you exhale (your forced vital capacity, or FVC) and how quickly that air comes out at every point during the exhale. FEF 25-75 captures the average flow rate during the middle half of that exhale, ignoring the first quarter and the last quarter. Think of it as the “cruising speed” of your breath after the initial blast and before the tail end when your lungs are nearly empty.

The reason this middle segment matters is anatomy. The early part of a forced exhale is dominated by your large airways, the trachea and major bronchi, which are relatively stiff and wide. By the time you are halfway through breathing out, the airflow increasingly depends on smaller, more peripheral airways. These small airways, generally those less than about 2 mm in diameter, have historically been called the “silent zone” of the lungs because significant damage can accumulate there before standard tests pick it up.² FEF 25-75 was designed to listen to that zone.

What a Low FEF 25-75 Suggests

A reduced FEF 25-75, usually reported as a percentage of the predicted value for someone of your age, sex, height, and ethnicity, points toward narrowing or obstruction in the small airways. This can show up even when the more commonly used numbers on your spirometry report, like FEV1 (the volume you exhale in the first second) and the FEV1/FVC ratio, still look normal. One study at a tertiary care center found that among patients whose FEV1/FVC ratio was completely normal, about one in ten still had a decreased FEF 25-75, suggesting isolated small airway problems that the standard measures missed.³

That pattern, a low FEF 25-75 sitting alongside a normal FEV1, is one of the main reasons the measure draws clinical interest. It raises the possibility that something is going wrong in the small airways before the disease progresses far enough to show up on the numbers doctors rely on most.

The Asthma Connection

Asthma is where FEF 25-75 has gotten the most attention. Research has found that a reduced FEF 25-75 is linked to asthma development and progression, and that it can be more sensitive than FEV1 at reflecting airway inflammation and hyperresponsiveness in asthma patients.⁴ In children specifically, a low FEF 25-75 in the setting of a normal FEV1 has been associated with greater asthma severity, more steroid use, and more frequent flare-ups.⁵

One study of children and adolescents with asthma who were sensitized to house dust mites illustrates this well. If researchers had ignored FEF 25-75 and relied only on standard spirometry of the large airways, roughly 62% of the sensitized participants would have been classified as normal despite having small airway obstruction.⁶ In other words, the standard numbers missed real disease that FEF 25-75 caught.

For pediatric asthma management, FEF 25-75 may also be useful in tracking treatment response. A post hoc analysis of clinical trial data in children and adolescents with symptomatic asthma found that FEF 25-75 was potentially more sensitive at detecting improvement from an inhaled bronchodilator than FEV1 was.⁷ Similarly, among children whose FEV1 and FEV1/FVC were both normal, about one in five showed a bronchodilator response in FEF 25-75, and those children were significantly more likely to test positive for airway hyperresponsiveness on challenge testing.⁸

Why Major Guidelines Are Cautious

Given all that, you might expect FEF 25-75 to be a standard part of every respiratory diagnosis. It is not. The latest European Respiratory Society and American Thoracic Society technical standards note that forced expiratory flows like FEF 25-75 are highly variable and strongly influenced by changes in FVC, which makes comparing pre- and post-bronchodilator readings unreliable.⁹ That variability is the core problem. If a patient’s total exhaled volume changes, FEF 25-75 shifts too, even if the small airways themselves haven’t changed. The measurement is tangled up in things that have nothing to do with what it is supposed to measure.

Effort also plays a role. Unlike FEV1, which stabilizes once the lungs are fully inflated before the blow, FEF 25-75 remains noticeably affected by the negative effects of expiratory effort regardless of lung volume.¹⁰ In plain terms, small differences in how hard a patient blows, or how repeatable their technique is between tries, can shift FEF 25-75 in ways that have nothing to do with their airway health. This makes it harder to use for tracking changes over time or comparing before-and-after bronchodilator results.

One proposed workaround has been to adjust FEF values for FVC, creating a ratio that strips out some of the volume dependence. Reference equations for these adjusted values have been developed, though they remain far from widespread clinical use.¹¹

FEF 25-75 and COPD Risk

Where FEF 25-75 has shown striking results is in predicting who might go on to develop chronic obstructive pulmonary disease. A ten-year follow-up study found that among patients whose lung function was otherwise normal at baseline, those with a low FEF 25-75 developed COPD at a dramatically higher rate than those whose FEF 25-75 was normal: roughly 42% versus 7%.¹² Low FEF 25-75 at baseline carried about a threefold increase in the risk of later COPD, independent of age and smoking status.

A separate multinational longitudinal study confirmed that isolated small airway obstruction detected by FEF 25-75 was associated with nearly triple the odds of developing chronic airflow obstruction at follow-up.¹³ In people who already had established airflow limitation, low FEF 25-75 was nearly universal: one cross-sectional study of symptomatic ever-smokers found it in 99.9% of patients with airflow limitation. But it was also present in about half of those who did not yet meet the criteria for airflow limitation, suggesting it captures a stage of disease that the standard diagnostic threshold misses.¹⁴

Lifetime Trajectories and Early-Life Origins

Some of the most compelling recent work tracks FEF 25-75 not as a snapshot but as a trajectory across a person’s life. A prospective cohort study following individuals from age 7 to 53 identified distinct FEF 25-75 trajectories and found that three impaired patterns were associated with sharply elevated COPD risk by middle age. The most worrying was a persistently low trajectory, which carried odds of COPD over 100 times higher than the average trajectory. An early-normal trajectory with reduced growth and rapid decline carried about 41 times the odds, and an early-below-average trajectory with reduced growth carried about three times the odds.¹⁵

The implication is that small airway function established in childhood can set the stage for disease decades later. This gives FEF 25-75 a potential role not just in diagnosing current problems but in identifying children and young adults whose lung development puts them on a riskier path, well before any symptoms appear.

Transplant Monitoring

One clinical setting where FEF 25-75 has proven especially valuable is in monitoring patients after lung or bone marrow transplants. A feared complication of lung transplantation is bronchiolitis obliterans syndrome (BOS), a form of chronic rejection that progressively scars the small airways. Studies have found that a decline in FEF 25-75 can precede the decline in FEV1 that officially defines BOS by an average of several months, providing an earlier warning.¹⁶

In pediatric lung transplant recipients, that early-warning capacity was even more striking. A 20% drop in FEF 25-75 had 100% sensitivity and 90% specificity for detecting BOS.¹⁷ After bone marrow transplants, where BOS is also a risk, FEF 25-75 measured around 80 days post-transplant predicted later BOS development, and it added significant predictive value beyond what FEV1 alone could provide. Once clinicians had the FEF 25-75 value, knowing the FEV1 value added little additional information about BOS risk.¹⁸

Transplant monitoring is arguably the strongest use case for FEF 25-75 right now. The patient population is well-defined, the condition being monitored originates specifically in the small airways, and earlier intervention makes a genuine difference in outcomes.

Occupational and Environmental Exposures

FEF 25-75 also turns up in studies of workplace hazards. In a study of the French general population, occupational exposure to crystalline silica dust was associated with significantly lower FEF 25-75 scores and a fourfold increase in the odds of having impaired FEF 25-75, after adjusting for smoking and other factors.¹⁹ Research in steel workers exposed to high levels of particulate matter found a similar pattern, with FEF 25-75 and other spirometric values all significantly lower in exposed workers compared to controls.²⁰

In these settings, FEF 25-75 can flag damage to the small airways that might not yet show up as a reduced FEV1, which matters for occupational health screening. Workers exposed to dust and fumes may benefit from earlier intervention if their small airway function is deteriorating, even when their overall spirometry still looks acceptable.

When FEV1/FVC Ratio Does the Job Better

It is worth noting that FEF 25-75 does not always outperform the standard numbers. In children with cystic fibrosis or asthma, one study found that the FEV1/FVC ratio was actually more likely to be abnormal than FEF 25-75 in early or mild disease, suggesting that the ratio remains the more reliable first-line screening tool.²¹ This is a useful counterpoint to the enthusiasm around FEF 25-75 and a reminder that its added value is clearest in specific situations, such as when the standard numbers are normal but clinical suspicion remains high, or in the transplant monitoring scenarios described above.

Newer Tests for Small Airways

FEF 25-75 is not the only way to assess the small airways, and newer methods are gaining ground. Impulse oscillometry (IOS), which measures airway resistance using gentle sound waves sent into the lungs during normal breathing rather than a forced exhale, correlates well with FEF 25-75 but has shown greater sensitivity in detecting small airway dysfunction in patients whose standard spirometry is preserved.²² Because IOS does not require a forceful blow, it sidesteps the effort-dependence issue that dogs FEF 25-75, and it is easier to perform in children and elderly patients who struggle with the spirometry maneuver.

Multiple-breath washout testing, which measures how evenly air distributes through the lungs, is another alternative. In adults with bronchiectasis, the lung clearance index derived from washout testing was more sensitive and specific than FEF 25-75 at distinguishing patients from healthy controls.²³ These tests are not yet widely available outside specialized centers, but they represent the direction the field is heading for small airway assessment. If your pulmonologist mentions oscillometry or washout testing, they are trying to get the kind of information FEF 25-75 was designed to provide, just with tools that are less variable and less dependent on a perfect effort from the patient.

What to Do If Your Report Shows a Low FEF 25-75

If you see a low FEF 25-75 on your spirometry results, the first thing to know is that it is not, by itself, enough to diagnose any specific condition. The high variability means a single low reading could reflect technique, effort, or genuine small airway narrowing. Context matters enormously. A low FEF 25-75 in someone with asthma symptoms, a history of dust exposure, or a transplant is interpreted very differently from the same number in someone with no symptoms and an otherwise clean bill of health.

Ask your doctor whether the rest of your spirometry was normal. If your FEV1 and FEV1/FVC ratio are both fine and the only abnormality is FEF 25-75, you may be looking at early small airway involvement, or you may be looking at a noisy measurement on a single test. The sensible next steps might include repeating the test to check reproducibility, performing a bronchodilator challenge to see whether the flow rates improve with an inhaled medication, or pursuing additional testing like oscillometry if it is available. A low FEF 25-75 is a conversation starter with your clinician, not a diagnosis on its own.