The spirometry test is a foundational tool in respiratory medicine, providing a detailed assessment of how well the lungs are functioning. The measurement known as FEV1, or Forced Expiratory Volume in 1 second, quantifies the volume of air a person can forcefully expel from their lungs during the first second of a maximum effort exhalation. This non-invasive measurement offers significant insight into a patient’s airflow dynamics.
Understanding the Spirometry Procedure and Measurements
Spirometry is a straightforward test that requires patient cooperation to ensure accurate results. The patient inhales as deeply as possible, filling their lungs completely with air. They must then exhale as rapidly and forcefully as they can into a mouthpiece connected to the spirometer machine, which records the volume and rate of air expelled.
Two primary values are recorded during this maneuver: the FEV1 and the Forced Vital Capacity (FVC). FEV1 measures the volume of air pushed out during the initial one-second burst. FVC represents the total volume of air the patient can forcibly exhale after taking the deepest possible breath. These two measurements are analyzed together to calculate a ratio, which helps diagnose the type of potential lung impairment.
Defining a Normal FEV1 Result
Determining a “normal” FEV1 result is not based on a single, fixed number, but rather on a comparison to a predicted value for that individual. This predicted value represents the statistical average volume of air expected from a healthy person sharing similar physical and demographic characteristics. The calculation takes into account several key demographic factors: the patient’s age, biological sex, height, and sometimes ethnicity.
For instance, a taller person is expected to have a higher lung capacity than a shorter person, and lung function naturally declines with increasing age. The measured FEV1 is then expressed as a percentage of this calculated predicted value. A result is conventionally considered within the normal range if the measured FEV1 is 80% or greater than the predicted value. This 80% threshold serves as a general cutoff point for acceptable lung function.
A result that is 100% of the predicted value means the patient’s lung function aligns perfectly with the statistical mean for their demographic group. Some healthy young adults may even register values exceeding 100%, sometimes up to 120% of the predicted value. The use of the predicted value allows clinicians to account for the wide natural variation in lung size and function across the population. Newer guidelines increasingly recommend using the lower limit of normal (LLN), defined statistically, instead of a fixed 80% threshold, to improve diagnostic accuracy.
Interpreting Results: What Low Numbers Mean
When the measured FEV1 falls below the normal threshold, it suggests a potential issue with lung function or airflow. To interpret this finding, clinicians look closely at the FEV1/FVC ratio, which is the percentage of the total air volume (FVC) exhaled in the first second (FEV1). A low FEV1 combined with a low FEV1/FVC ratio indicates an obstructive ventilatory pattern.
This obstructive pattern is defined by a ratio below the lower limit of normal or less than a fixed threshold of 70% in adults. It suggests that the airways are narrowed or blocked, making it difficult to push air out quickly, a characteristic of diseases like Chronic Obstructive Pulmonary Disease (COPD) or asthma. Conversely, if the FEV1 is low but the FEV1/FVC ratio remains normal or high, this suggests a restrictive ventilatory pattern.
In a restrictive pattern, the total lung volume (FVC) is reduced, meaning the person cannot take a full, deep breath, but the air they inhale is expelled at a normal rate. This pattern is often associated with conditions that stiffen the lung tissue, such as pulmonary fibrosis, or restrict chest wall movement. The relationship between FEV1 and FVC provides the necessary context to determine the specific nature of any detected lung impairment.