A Pulmonary Function Test (PFT) is a non-invasive diagnostic procedure designed to measure how well your lungs are working. This test provides objective data on lung capacity, flow rates, and the efficiency of gas exchange, which is important for diagnosing and managing conditions like asthma, Chronic Obstructive Pulmonary Disease (COPD), and pulmonary fibrosis. To ensure the results accurately reflect your baseline lung health, strict preparation is required, and one of the most important instructions is to abstain from smoking for a specified period before the test. Smoking, even a few hours beforehand, introduces temporary changes that can profoundly skew the measurements, leading to potentially incorrect medical conclusions.
What a Pulmonary Function Test Measures
A PFT is a comprehensive set of tests that assess two fundamental aspects of lung function: ventilatory mechanics and gas exchange. Ventilatory mechanics involve the physical movement of air, primarily measured through spirometry. Spirometry assesses the volume of air you can forcefully exhale and how quickly you can do it, providing flow rates and lung volumes. These flow rates, such as Forced Expiratory Volume in one second (FEV1) and Forced Vital Capacity (FVC), indicate whether a patient has an obstructive or restrictive pattern. The second aspect is gas exchange, which measures how efficiently oxygen moves from the air sacs into the bloodstream, known as the diffusing capacity of the lung for carbon monoxide (DLCO).
Immediate Interference with Airway Flow
Smoking immediately compromises the measurements related to the movement of air by causing acute physiological changes in the airways. Cigarette smoke contains irritants and nicotine that trigger a rapid, temporary tightening of the bronchial tubes, a process known as bronchoconstriction. This narrowing artificially reduces the speed at which you can exhale air, directly lowering measurements like the FEV1 and Peak Expiratory Flow Rate (PEFR). The irritants in the smoke also stimulate the production of mucus and cause temporary inflammation in the small airways. This increased secretion further obstructs the flow of air, mimicking the measurements of chronic lung conditions like asthma or COPD, and yielding results significantly worse than the patient’s true, stable baseline.
Distortion of Diffusion Capacity
The second interference from smoking is the distortion of the DLCO measurement, which assesses gas exchange efficiency. The DLCO test requires the patient to inhale a gas mixture containing a trace amount of carbon monoxide (CO) and measures how much is absorbed by the blood during a brief breath-hold. Cigarette smoke contains a substantial amount of CO. When inhaled, this CO enters the bloodstream and binds to hemoglobin—the oxygen-carrying protein in red blood cells—with an affinity over 200 times greater than oxygen. Smoking before the test essentially pre-saturates the patient’s hemoglobin, significantly reducing the number of binding sites available for the test gas. The PFT machine interprets this reduced uptake as a failure of the lungs’ diffusion capacity, leading to a falsely low result that can mistakenly indicate a major physiological problem, such as emphysema or interstitial lung disease.
Medical and Practical Implications of Invalid Results
The primary consequence of smoking before a PFT is the generation of invalid and misleading data. If results are artificially lowered due to acute bronchoconstriction or carbon monoxide saturation, a physician may misinterpret the findings as a severe obstructive or restrictive lung disorder. This could lead to a diagnostic error, such as prematurely diagnosing a patient with COPD or pulmonary fibrosis they do not have at that severity. An inaccurate diagnosis can result in the prescription of incorrect or unnecessary medications, such as bronchodilators or steroids, which carry their own risks and side effects. Furthermore, a skewed result could mask a different, true underlying condition, delaying appropriate treatment and wasting medical resources by requiring a repeat test.