Spirometry is a common pulmonary function test that provides objective measurements of how well your lungs are working. The test requires a person to take a deep breath and then exhale forcefully and quickly into a device called a spirometer. This procedure yields data about the volume of air moved and the speed of that movement. Interpreting these results involves understanding specific metrics and comparing them to established standards.
Defining the Core Measurements
The spirometry report focuses on three primary values that quantify lung mechanics. The first is Forced Vital Capacity (FVC), which represents the total volume of air you can forcefully exhale after taking a full, deep breath. FVC measures the overall capacity that the lungs can move out during the maneuver.
The second measurement is the Forced Expiratory Volume in 1 second (FEV1), which is the specific volume of air expelled during the very first second of that forced exhalation. This metric is a measure of how quickly air can be pushed out, reflecting the flow dynamics of the airways.
Finally, the FEV1/FVC Ratio is calculated by dividing the FEV1 value by the FVC value. This ratio is expressed as a percentage and is the most immediate tool for pattern recognition. It indicates the proportion of the total capacity that can be expelled in one second.
Establishing the Baseline for Normalcy
Raw spirometry numbers alone do not determine lung health, as lung size and function vary significantly between individuals. To make clinical sense of the measured volumes, they must be compared against established “predicted values”. These reference standards are statistically generated from data collected from large populations of healthy non-smokers across multiple centers and ethnicities.
The calculation of an individual’s predicted value takes into account personal demographics, including age, height, sex, and often ethnicity, using complex predictive equations like those developed by the Global Lung Function Initiative (GLI). The reference equations help define a statistically normal range for a person with that specific profile.
The spirometry report then takes the actual FEV1 and FVC measurements and expresses them as a percentage of this calculated predicted value. This percentage of predicted figure provides context, showing how an individual’s lung function compares to a healthy person with similar physical characteristics.
In general, values at or above 80% of predicted are considered within the expected range of normal for both FVC and FEV1. Interpretation relies heavily on understanding this percentage relative to the predicted baseline, rather than the raw liters of air exhaled.
Identifying Obstructive and Restrictive Patterns
The FEV1/FVC ratio is the primary determinant used to distinguish between two broad categories of lung impairment. This ratio helps determine whether the issue is related to airflow speed or overall lung volume. In healthy adults, this ratio typically falls between 70% and 80%.
An obstructive pattern is identified when the FEV1/FVC ratio is below the lower limit of normal, often defined as less than 70%. This pattern suggests difficulty in exhaling air quickly due to narrowed or blocked airways. The FEV1 is disproportionately low compared to the FVC, meaning a person has a relatively full volume of air but cannot empty it rapidly.
Conditions like Chronic Obstructive Pulmonary Disease (COPD) and asthma are associated with this obstructive pattern. The narrowing of the bronchial tubes increases resistance, which slows the rate at which air can be forced out of the lungs.
Conversely, a restrictive pattern is suggested when the FVC measurement is low, but the FEV1/FVC ratio remains normal or even elevated. In this scenario, both the FEV1 and FVC are reduced proportionally, indicating the lungs cannot fully inflate, or the chest wall movement is restricted. The difficulty is in getting air in, rather than getting it out quickly.
The total volume of the lungs is reduced in restrictive diseases, which may be caused by conditions such as pulmonary fibrosis, sarcoidosis, or chest wall stiffness. While spirometry suggests restriction, confirmation often requires additional tests, such as measuring Total Lung Capacity (TLC). The normal FEV1/FVC ratio differentiates this pattern because the airways are functionally open, but the overall container size is smaller.
Grading the Severity of Lung Impairment
Once an obstructive pattern is confirmed by a low FEV1/FVC ratio, the next step in interpretation is to quantify the degree of lung impairment. Severity is graded using the FEV1 expressed as a percentage of the predicted value. A lower FEV1 percentage correlates directly with a more advanced stage of disease.
For obstructive diseases like COPD, standardized guidelines use FEV1 percentages to categorize severity:
- Mild obstruction: FEV1 of 80% or higher of predicted.
- Moderate obstruction: FEV1 generally falls between 50% and 79% of predicted.
- Severe obstruction: FEV1 is categorized between 30% and 49% of predicted.
- Very severe obstruction: FEV1 percentage is below 30% of predicted.
These categories help guide treatment decisions and prognosis for patients with airflow limitation. Final diagnosis and management require a healthcare provider to integrate these findings with the patient’s symptoms and medical history.