Pulmonary function tests (PFTs) are non-invasive procedures used to measure how well the lungs are working. These tests are performed by having a person breathe into a spirometer. The results provide objective data on lung volumes and airflow speeds. The Forced Expiratory Flow 25-75% (FEF 25-75%) is a specific measurement that offers unique insight into the flow of air deep within the lungs, and can be an early indicator of developing lung issues.
Defining Forced Expiratory Flow 25-75
The Forced Expiratory Flow 25-75% is an average flow rate measurement taken during the middle portion of a complete exhalation. The test begins with the patient taking the deepest breath possible (total lung capacity). They then exhale as hard and fast as they can into the spirometer, producing the Forced Vital Capacity (FVC) maneuver.
The “25-75” refers to the percentage of the FVC that has been exhaled, capturing the flow rate after the first 25% and stopping once 75% has been expelled. This middle segment is less dependent on the patient’s maximum effort. Instead, it reflects the passive recoil properties of the lung tissue and the diameter of the smaller airways.
What the Measurement Reflects
The FEF 25-75% provides specific information about the condition of the small airways, often referred to as the peripheral airways or bronchioles. These small airways have an internal diameter of two millimeters or less. They are responsible for a significant portion of the resistance to airflow during exhalation.
Large airways, such as the trachea, are supported by cartilage, which helps keep them open during forced exhalation. Conversely, small airways lack this rigid support, making them highly susceptible to early narrowing or collapse due to inflammation or disease. This makes the FEF 25-75% a sensitive indicator of pathology that might be missed by broader measurements.
The most common spirometry measurement, Forced Expiratory Volume in 1 second (FEV1), primarily reflects airflow through the larger airways. The FEF 25-75% acts as a complement, offering a window into the smaller, more fragile structures deep within the lungs. A reduction in this flow rate suggests that the resistance within these peripheral airways is abnormally high.
Interpreting Abnormal Results
A result for the FEF 25-75% is considered abnormal when it falls below a predicted normal range, calculated based on a person’s age, height, sex, and ethnicity. When this value is low, it strongly suggests a problem with the small airways. This can occur even if standard measurements like FEV1 and the FEV1/FVC ratio remain within the normal range. This isolated reduction is often interpreted as evidence of small airway dysfunction (SAD).
A low FEF 25-75% can be an early warning sign of obstructive lung diseases like asthma or Chronic Obstructive Pulmonary Disease (COPD). In patients who are symptomatic but have otherwise normal spirometry, a reduced FEF 25-75% indicates physiological evidence of lung damage. A low baseline FEF 25-75% has been shown to be a predictor for the later development of COPD in at-risk individuals, such as smokers.
In asthma, a low FEF 25-75% suggests that the inflammatory process is causing obstruction in the peripheral airways. Children with asthma, in particular, may have a low FEF 25-75% alongside a normal FEV1, indicating that the smaller structures are affected before the larger ones. This specific finding can correlate with increased disease severity, airway hyperresponsiveness, and inflammation.
Management and Follow-Up
An abnormal FEF 25-75% result, especially when other spirometry values are normal, warrants further clinical investigation. The healthcare provider may order follow-up tests, such as a bronchodilator challenge, to see if the flow rate improves after the patient inhales a fast-acting rescue inhaler.
A significant improvement in FEF 25-75% after bronchodilator use suggests reversible airflow obstruction, which is characteristic of asthma. Other advanced tests, such as lung volume measurements or chest imaging, may be necessary to rule out other conditions. Management may involve initial treatment with inhaled medications, such as inhaled corticosteroids, to address the underlying small airway inflammation.