Spirometry is a common pulmonary function test that measures how well your lungs take in and expel air. This test requires you to blow forcefully into a device called a spirometer, which records the speed and volume of your breath. The results help healthcare professionals assess overall lung health and diagnose conditions such as asthma or Chronic Obstructive Pulmonary Disease (COPD). Understanding the report can seem complicated, but the numbers are simply measurements of your lung mechanics. The goal of reading a spirometry report is to interpret what the combination of values means for your respiratory function.
Defining the Core Spirometry Values
A spirometry report focuses on three specific measurements that quantify the capacity of your lungs and the rate of airflow. The first is Forced Vital Capacity (FVC), which represents the total amount of air you can forcibly exhale from your lungs after taking the deepest breath possible. This value is a measure of your total lung volume available for a maximal breath, and it is reported in liters.
The second primary measurement is the Forced Expiratory Volume in 1 Second (FEV1). This figure measures the volume of air you can forcefully push out of your lungs during the first second of the exhalation maneuver. The FEV1 is important because it reflects the flow rate through the airways, indicating how quickly air can be moved out of the lungs.
The third and most telling figure is the FEV1/FVC Ratio, calculated by dividing the FEV1 value by the FVC value and expressing the result as a percentage. This ratio shows the proportion of your total lung capacity that you can expel in the initial second. While the FVC indicates the size of your lungs, the FEV1/FVC ratio is the primary indicator of whether an obstruction is present in the airways.
Understanding How Your Results Are Graded
The raw values for FVC and FEV1 are not interpreted in isolation but are compared against a calculated standard known as the “Predicted Value.” This predicted baseline is determined using large population studies and is customized to your specific physical characteristics. Factors like your age, height, sex, and ethnicity are factored into the mathematical model to establish the statistically expected lung capacity for someone with your profile.
The spirometry report will then display your actual measured FEV1 and FVC as a “Percentage of Predicted.” This calculation translates the raw liters into a meaningful context, showing how your lung performance compares to the expected healthy standard. For most healthy individuals, a result that is 80% or more of the predicted value is considered within the typical range.
The percentage of predicted value serves as a universal benchmark, allowing for a standardized interpretation of lung function. Reporting the data as a percentage of predicted eliminates ambiguity, providing a consistent scale for grading severity.
Distinguishing Between Obstructive and Restrictive Patterns
The combination of FVC, FEV1, and their resulting ratio is used to identify one of three patterns: normal, obstructive, or restrictive. A normal result is characterized by both FEV1 and FVC being 80% or greater of the predicted value, alongside a FEV1/FVC ratio that is within the normal range for your age. This pattern confirms that both lung capacity and airflow are performing as expected.
An obstructive pattern is diagnosed primarily by a low FEV1/FVC ratio, typically below 70%. In this scenario, the total lung volume (FVC) may be near normal, but the ability to exhale that air quickly (FEV1) is significantly diminished. This is conceptually similar to trying to empty a balloon through a kinked hose, where the total air volume is present but the flow is blocked. Conditions that cause narrowing or inflammation of the airways, such as COPD, asthma, and emphysema, lead to this obstructive flow limitation.
A restrictive pattern, conversely, is indicated by a low FVC, often less than 80% of the predicted value. In this case, the FEV1 is also low, but the FEV1/FVC ratio remains normal or even slightly high. This result suggests that the total volume of air the lungs can hold is reduced, which is like having a small balloon that cannot inflate fully. The rate of exhalation is appropriate for the reduced lung size, meaning the air is expelled quickly and efficiently. Restrictive defects are associated with conditions that reduce lung volume or stiffen lung tissue, such as pulmonary fibrosis, sarcoidosis, or even severe obesity.