Pulmonary Function Tests (PFTs) are breathing measurements used to assess how well the lungs are working. The outcome of a PFT depends on the severity of the asthma, the current level of disease control, and the patient’s preparation before the test. For many individuals with well-managed asthma, a normal result is achievable.
Understanding Pulmonary Function Tests
A PFT typically involves spirometry, which measures the volume of air a person can breathe in and out and the speed of that airflow. The patient breathes into a mouthpiece connected to a spirometer. The two main measurements derived from this test are the Forced Vital Capacity (FVC) and the Forced Expiratory Volume in 1 second (FEV1).
The FVC represents the total amount of air that can be forcefully exhaled after taking the deepest possible breath. The FEV1 is the volume of air forcefully exhaled during the first second of the maneuver. These values are then compared against predicted normal values, which are calculated based on a person’s age, height, sex, and ethnicity.
The most informative measurement for diagnosing obstructive diseases like asthma is the ratio of FEV1 to FVC (FEV1/FVC). A result is considered normal when the FEV1 and FVC are both at or above 80% of the predicted value. A normal FEV1/FVC ratio is generally considered to be above 0.70.
How Asthma Obstructs Airflow
Asthma is characterized by chronic inflammation and hyper-responsiveness of the airways. This inflammation causes the airways to become swollen and sensitive to various triggers. When a trigger is encountered, the muscles surrounding the bronchial tubes tighten, a process known as bronchospasm.
This combination of inflammation and muscle constriction leads to a narrowing of the airways, which limits the speed at which air can be exhaled. This restriction of airflow creates the classic obstructive pattern seen on spirometry. Because it is difficult to push air out quickly, the FEV1 drops significantly, often more than the FVC.
The resulting decrease in the FEV1/FVC ratio is the hallmark sign of an obstructive ventilatory defect. This mechanical limitation explains why the lung function of an individual with uncontrolled asthma is often measurably impaired during a PFT.
Factors Influencing Test Outcomes
It is possible for a person with a known asthma diagnosis to achieve normal PFT results. This outcome occurs when the asthma is mild or exceptionally well-controlled through consistent treatment. An individual with mild intermittent asthma who is in a period of remission or has few current symptoms may demonstrate lung function within the normal predicted range.
The severity of the underlying condition strongly influences the likelihood of an abnormal result. Patients with severe persistent asthma, characterized by frequent symptoms and exacerbations, are more likely to show a significant reduction in their FEV1 and FEV1/FVC ratio. In contrast, individuals with mild, well-managed disease often maintain near-normal baseline lung function.
The timing of medication use before the test is a strong factor that can normalize a PFT result. Patients are often instructed to withhold their bronchodilator inhalers for several hours prior to the test. If a patient uses a bronchodilator before the test, the medication will temporarily relax the airway muscles, improving the FEV1 and FEV1/FVC ratio.
This improvement following medication is known as bronchodilator reversibility, a key feature of asthma. However, it means the test is not measuring the person’s baseline, unmedicated lung function. A PFT performed during a time of low or no symptoms is more likely to be normal than one performed during an acute flare-up or exacerbation.
What Abnormal Results Indicate
When a PFT reveals an abnormal result, specifically a reduced FEV1/FVC ratio, it signals the presence of airflow obstruction. This finding helps confirm the diagnosis of asthma, especially if the subsequent administration of a bronchodilator demonstrates significant reversibility. A positive reversibility test is defined by an increase in FEV1 of at least 12% and 200 milliliters following the medication.
The degree of reduction in the FEV1 is then used to grade the severity of the airflow limitation (mild, moderate, or severe). This objective measure guides the decisions on which maintenance medications and doses are most appropriate for the patient.
Abnormal results also prompt the consideration of other potential diagnoses that cause airway obstruction, such as Chronic Obstructive Pulmonary Disease (COPD). If asthma is suspected but the initial PFT is normal, a physician may order a bronchial challenge test, like a methacholine challenge, to provoke a temporary episode of bronchospasm and confirm airway hyper-responsiveness. Regular monitoring of PFT results over time assesses the stability of the asthma and the effectiveness of the current treatment regimen.