A Pulmonary Function Test (PFT) is a non-invasive procedure that measures how effectively the lungs are working. This diagnostic tool assesses lung capacity, volume, and the rate of airflow, helping to diagnose or monitor conditions like asthma and Chronic Obstructive Pulmonary Disease (COPD). Because the test relies on maximum effort and accurate measurements, proper preparation is necessary to ensure reliable results. Failing to follow preparation instructions can lead to skewed data.
Dietary and Beverage Restrictions
While eating is generally permitted before a PFT, avoid consuming large or heavy meals within one to two hours of the appointment. A full stomach expands and exerts upward pressure on the diaphragm, the primary muscle responsible for breathing. This physical restriction limits the diaphragm’s full downward motion, reducing total lung capacity. This makes it difficult to perform the required deep breaths and forceful exhalations accurately.
Avoid common stimulants like caffeine, found in coffee, tea, soda, and chocolate. Caffeine affects the nervous system and can temporarily alter breathing patterns or heart rate, skewing baseline measurements. Alcohol consumption should also be avoided for at least four hours before the test, as it can interfere with the central nervous system’s control over breathing.
A light snack, such as fruit or yogurt, may be acceptable to maintain comfort or manage blood sugar. Avoid heavy meals that require significant digestion. High-fat or high-carbohydrate meals are discouraged within two to three hours, as their digestion increases oxygen demand and stimulates changes in breathing patterns. Hydration with water is encouraged to keep airways moist, but limit excessive fluid intake to avoid discomfort during forceful breathing maneuvers.
Medications, Smoking, and Physical Activity
A primary preparation concern involves bronchodilator medications, such as inhalers, which temporarily open the airways. To assess lung function without medication, patients are typically instructed to withhold short-acting bronchodilators for four to eight hours and long-acting bronchodilators for 12 to 36 hours. These are general guidelines, and patients must follow the specific withholding instructions provided by their ordering physician.
Smoking must be avoided for a period typically ranging from four to eight hours before the test. Inhaling smoke causes immediate airway constriction and introduces irritants, impairing lung function and leading to inaccurate readings. Strenuous physical activity should also be avoided for at least 30 minutes to a few hours before the appointment. Intense exercise temporarily changes lung capacity and breathing mechanics, which interferes with obtaining accurate baseline measurements.
What to Expect During the Test
When arriving for the PFT, wear loose-fitting clothing that does not restrict movement in the chest or abdomen. The testing occurs in a specialized laboratory where the patient will be seated comfortably in a chair. A respiratory therapist or trained technician will guide the patient through the process, explaining each step.
A soft clip is placed on the nose so all breathing occurs solely through the mouth. The mouth is then sealed tightly around a disposable mouthpiece connected to a spirometer. The patient will be instructed to perform various breathing maneuvers, often including taking a maximum inhalation and then exhaling as hard and fast as possible until the lungs are completely empty.
The technician will coach the patient throughout the process, which may be repeated multiple times to ensure consistent and high-quality data. The test is non-invasive and generally painless, though the forceful breathing can sometimes cause temporary lightheadedness or fatigue.