Spirometry is a common pulmonary function test used to measure how effectively air moves into and out of the lungs. The procedure requires the patient to inhale deeply and then forcefully exhale into a spirometer. This maneuver captures measurements of airflow and lung volume, which are compared to established standards. Interpreting the resulting data allows healthcare professionals to identify and assess various breathing conditions, making the results a fundamental component of respiratory diagnosis.
Understanding the Foundational Measurements
The interpretation of a spirometry report relies on three core measurements that quantify a person’s ability to move air rapidly. The first is the Forced Vital Capacity, or FVC, which represents the total volume of air a person can forcefully exhale after taking a maximal breath. This measurement reflects the overall size and capacity of the lungs and is a measure of volume. A low FVC suggests a problem with the total amount of air the lungs can hold.
The Forced Expiratory Volume in 1 Second, or FEV1, measures the volume of air expelled during the first second of the FVC maneuver. This measurement indicates the speed and force with which air can be pushed out. It is a sensitive indicator of airway narrowing or obstruction, particularly sensitive to conditions that affect the smaller airways.
The third calculation is the FEV1/FVC ratio, which is the FEV1 expressed as a percentage of the FVC. This ratio compares the rate of airflow to the total lung volume, providing a clear picture of whether air is being trapped in the lungs. If the FVC is large but the FEV1 is disproportionately small, the resulting low ratio suggests airflow obstruction.
Determining Predicted Values and Normal Ranges
To determine if a patient’s measured values are considered typical, the results must be compared against a standard known as the “Predicted Value.” This predicted value is calculated using reference equations derived from large populations of healthy, non-smoking individuals. These equations account for the natural variation in lung size and function that is influenced by demographic factors, specifically age, height, sex, and ethnicity.
A patient’s measured FEV1 and FVC are then reported as a percentage of this predicted value, indicating how their performance compares to an average person with similar characteristics. For example, an FEV1 of 100% of predicted means the patient’s result matches the expected mean for their demographic group. However, using a single, fixed threshold, such as 80% of predicted, to define abnormality is statistically inaccurate, especially in younger and older individuals.
The current standard for defining a result as abnormal uses the Lower Limit of Normal (LLN), which is a statistically derived threshold that accounts for the normal variability in the population. The LLN is equivalent to the 5th percentile of the healthy reference population, meaning only 5% of healthy individuals would naturally fall below this limit. A measured value is labeled abnormal if it falls below the LLN.
The LLN is applied to the FEV1, FVC, and the FEV1/FVC ratio, providing a more precise and individualized definition of normal lung function. Using this method helps prevent the misclassification of healthy older adults as abnormal and avoids missing early disease in younger individuals. The LLN establishes the necessary benchmark against which the specific patterns of lung disease are classified.
Recognizing Obstructive and Restrictive Patterns
Obstructive Pattern
The primary step in interpreting spirometry is to examine the FEV1/FVC ratio to determine the presence of an obstructive pattern. An obstructive pattern signifies that the airways are narrowed, making it difficult to exhale air quickly, which is reflected by a significantly reduced ratio. This reduction occurs because the FEV1 drops much more sharply than the FVC, indicating that air is being trapped in the lungs.
Airflow obstruction is diagnosed when the FEV1/FVC ratio falls below the Lower Limit of Normal (LLN) for the individual. A common fixed value used as a simple screening rule for adults is an FEV1/FVC ratio below 0.70. Conditions such as Chronic Obstructive Pulmonary Disease (COPD) and asthma are typically classified as obstructive disorders, with the severity of the obstruction graded based on the measured FEV1 percentage of predicted.
Restrictive Pattern
If the FEV1/FVC ratio is within the normal range, the next step is to evaluate the FVC itself to check for a restrictive pattern. A restrictive pattern is characterized by a reduction in the total lung volume, which means the FVC is low. In this case, the FEV1 and FVC are reduced proportionally, resulting in a normal or even elevated FEV1/FVC ratio.
A diagnosis of a restrictive ventilatory defect is suggested when the FVC is below the LLN while the FEV1/FVC ratio remains normal. Restrictive conditions are often caused by issues that prevent the lungs from fully expanding, such as pulmonary fibrosis, sarcoidosis, or chest wall disorders. However, spirometry alone can only suggest restriction, as confirmation requires further testing to measure the Total Lung Capacity (TLC).
Mixed and Normal Patterns
A mixed pattern is identified when a patient shows evidence of both obstruction and reduced lung volume. This occurs when both the FEV1/FVC ratio and the FVC are below their respective LLNs. This scenario can represent two separate disease processes or a single severe obstructive disease that causes significant air trapping, which secondarily reduces the FVC.
A truly normal spirometry result is one where the FEV1/FVC ratio, the FEV1, and the FVC all fall at or above their individual LLNs. This indicates that both the speed of airflow and the total lung capacity are within the expected range for the patient’s demographic.