A Pulmonary Function Test (PFT) is a non-invasive assessment that measures the mechanical performance of the lungs. By having a person breathe into specialized equipment, the test translates lung and airway mechanics into measurable numbers. These results are used to diagnose respiratory conditions, such as asthma or Chronic Obstructive Pulmonary Disease (COPD), and to monitor known lung diseases. Understanding the report requires recognizing these numbers and what they represent about the volume and flow of air moving in and out of the body.
The Core Metrics: Spirometry Values
The foundation of any PFT report is spirometry, which measures how much air is moved and how quickly. The first metric, Forced Vital Capacity (FVC), represents the total amount of air a person can forcefully exhale after taking the deepest possible breath. This volume measurement, typically expressed in liters, provides a measure of overall lung capacity.
A person’s actual FVC is compared against a “Predicted Value,” which is a statistical estimate based on their age, sex, height, and ethnicity. The resulting “Percent of Predicted” indicates how the person’s performance compares to a healthy peer. A value of 80% or higher for FVC is generally considered within the normal range.
The second primary metric is the Forced Expiratory Volume in 1 second (\(\text{FEV}_1\)). This measures the volume of air forcefully exhaled during the first second of the FVC maneuver. Since \(\text{FEV}_1\) reflects the speed of airflow, it is particularly sensitive to obstruction or narrowing within the airways. It is expected to be reduced when conditions like asthma or emphysema are present.
The most informative single value is the \(\text{FEV}_1/\text{FVC}\) ratio, calculated by dividing the \(\text{FEV}_1\) by the FVC and expressing the result as a percentage. The ratio helps determine if reduced lung function is due to air trapping or a limitation in total lung volume. A normal \(\text{FEV}_1/\text{FVC}\) ratio is typically greater than 70% in adults.
Classifying Impairment: Obstructive Versus Restrictive Patterns
Interpreting the core spirometry values allows for the classification of lung impairment into two patterns: obstructive or restrictive. The \(\text{FEV}_1/\text{FVC}\) ratio is the primary tool used to establish this distinction.
A low ratio signifies an obstructive pattern, meaning the person has difficulty getting air out quickly. In this pattern, the \(\text{FEV}_1\) is disproportionately low compared to the FVC, causing the ratio to fall below the normal range. This is consistent with diseases like COPD or asthma, where narrowed airways hinder the rapid expulsion of air. The \(\text{FEV}_1\) percentage of predicted is then used to grade the severity of the obstruction.
A restrictive pattern is suspected when the \(\text{FEV}_1/\text{FVC}\) ratio is normal or elevated, but the FVC itself is low. This suggests the total volume of air the lungs can hold is reduced. However, the person can still push out the inhaled air at a normal rate. Conditions like pulmonary fibrosis or issues with the chest wall (e.g., severe scoliosis) can cause this pattern.
If spirometry shows a low FVC but a normal ratio, it suggests a restrictive process. A true diagnosis requires further testing to confirm the reduced lung volume. Sometimes, a person may exhibit a mixed defect, characterized by both a low \(\text{FEV}_1/\text{FVC}\) ratio and a significantly reduced FVC. This combined pattern indicates the presence of both flow and volume limitations.
Beyond Spirometry: Lung Volumes and Gas Exchange
A comprehensive PFT often includes measurements beyond simple flow rates to confirm restrictive patterns and assess gas transfer.
Total Lung Capacity (TLC)
Total Lung Capacity (TLC) is the volume of air in the lungs after a maximal inhalation. It is the definitive measurement for confirming a restrictive defect. If the TLC is below 80% of the predicted value, it confirms that the lungs cannot fully expand, establishing a true restrictive disease.
Residual Volume (RV)
Residual Volume (RV) is the air remaining in the lungs after the most forceful exhalation possible. In obstructive diseases, the RV is often elevated due to air trapping, a phenomenon called hyperinflation. Measuring the RV provides additional context for the severity of air trapping in these conditions.
Diffusing Capacity (\(\text{DLCO}\))
The diffusing capacity of the lung for carbon monoxide (\(\text{DLCO}\)) measures how efficiently oxygen moves from the air sacs into the bloodstream. Carbon monoxide is used as a surrogate for oxygen in this measurement. A low \(\text{DLCO}\) indicates impaired gas exchange, often suggesting damage to the thin walls of the air sacs or the surrounding capillaries. A reduced \(\text{DLCO}\) is common in conditions like emphysema or interstitial lung disease, where the membrane barrier is thickened or destroyed. This metric provides physiological insight into the functional capacity of the lungs, complementing the mechanical information from flow and volume measurements.