A Pulmonary Function Test (PFT) measures lung capacity and the speed of airflow, providing a snapshot of how well the lungs are working. A person with asthma can pass this test, but the result depends entirely on the current state of their disease. When asthma is well-controlled, lung function can be within the normal range, leading to a passing PFT result. Conversely, active or uncontrolled disease reveals an abnormality reflecting underlying airway obstruction. The PFT is instrumental in diagnosing and monitoring asthma because results vary widely based on disease stability.
Understanding Key Pulmonary Function Test Metrics
The primary measurements taken during the standard PFT, called spirometry, focus on the volume and flow rate of air during a forced exhalation. Two fundamental values are the Forced Expiratory Volume in 1 second (FEV1) and the Forced Vital Capacity (FVC). The FEV1 measures the amount of air a person can forcefully blow out in the first second, indicating airflow speed and the presence of airway narrowing.
The FVC represents the total amount of air a person can forcefully exhale after taking the deepest possible breath. Measured values are compared against “predicted values,” which are statistical estimates of what a healthy person of the same age, sex, height, and ethnicity should achieve.
The most informative metric for obstructive diseases like asthma is the FEV1/FVC ratio. This ratio is the percentage of the total air volume (FVC) pushed out in the first second (FEV1). A healthy adult typically has an FEV1/FVC ratio greater than 70% or 75%, making it the clearest indicator of whether airflow is blocked or slowed.
How Asthma Control Determines the PFT Result
Asthma involves chronic airway inflammation and hyperresponsiveness, leading to bronchoconstriction, or the tightening of airway muscles. This inflammation and constriction create an obstruction that significantly slows the rate at which air can be expelled. This obstruction directly impacts the FEV1, causing it to decrease disproportionately more than the FVC.
A person with uncontrolled asthma exhibits an obstructive pattern on their PFT, meaning the FEV1/FVC ratio falls below the lower limit of normal (often less than 70% or 80% of the predicted value). This abnormal result shows the patient cannot empty their lungs quickly enough due to narrowed airways. The reduction in FEV1, expressed as a percentage of the predicted value, classifies the severity of the obstruction.
Conversely, an individual with well-controlled asthma, whose airways are not inflamed or constricted, may have a PFT result entirely within normal limits. In this case, the FEV1 and FEV1/FVC ratio will be at or above 80% of their predicted values, yielding a passing result. The goal of asthma management is to maintain this normal or near-normal lung function.
However, some patients with severe, long-standing asthma may develop airway remodeling, causing a fixed, or irreversible, component of obstruction. Even with aggressive treatment, these individuals may never achieve a completely normal PFT result, though their post-treatment values reflect the best possible lung function. The PFT is a dynamic measure that reflects the variable, day-to-day control of the disease for many asthmatics.
The Diagnostic Significance of Bronchodilator Reversibility Testing
When an obstructive pattern is observed in the initial spirometry, the healthcare provider proceeds with bronchodilator reversibility testing to help confirm the asthma diagnosis. This procedure involves administering a short-acting bronchodilator medication, such as albuterol, and repeating the spirometry measurements 10 to 15 minutes later. The aim is to see if the medication, which relaxes the airway muscles, can quickly reverse the obstruction.
A significant response to the bronchodilator is a hallmark feature of asthma, demonstrating the characteristic reversibility of the airway narrowing. The criteria for a significant response is an increase in FEV1 of at least 12% and an absolute increase of 200 milliliters (mL) compared to the pre-bronchodilator measurement. This threshold indicates that the airflow limitation is due to reversible smooth muscle constriction.
The degree of reversibility serves as a diagnostic tool and monitors the effectiveness of ongoing asthma treatment. Improvement in FEV1 confirms that a significant portion of the patient’s airflow limitation is responsive to rescue medication. This reversibility helps distinguish asthma from other chronic obstructive lung diseases, which show less or no improvement after medication. However, a lack of significant reversibility on a single test does not completely rule out asthma.