Pulmonary Function Tests (PFTs) are non-invasive tests that measure how well the lungs take in and release air, and how efficiently oxygen is transferred into the bloodstream. These assessments are performed using a spirometer, which calculates lung volumes, capacities, and rates of flow. Achieving a successful PFT, often described as “passing,” means producing the most accurate and reproducible results possible for your current physiological state. This requires meticulous preparation and full cooperation during the testing maneuvers. The validity of the results depends entirely on adherence to pre-test instructions and the quality of effort provided.
Preparing Your Body and Medications Before the Test
Proper preparation before the test prevents external factors from skewing the measurements. Patients should wear loose-fitting clothing that does not restrict the chest or abdomen, ensuring full lung expansion. Tight belts or restrictive clothing can physically limit the maximum volume of air inhaled, potentially leading to a falsely restrictive result.
Dietary choices immediately before the appointment also require attention. A heavy meal can push against the diaphragm, interfering with its movement and maximum inspiration. Avoid eating a large or fatty meal for at least two to three hours before the test. Likewise, stimulants should be avoided on the day of the test because they can influence the respiratory system.
Caffeine possesses mild bronchodilator properties and can temporarily open the airways, which may mask underlying conditions. Therefore, abstain from caffeinated beverages like coffee, tea, and sodas, as well as smoking or using tobacco products, for a minimum of six hours before the scheduled test time. Vigorous exercise should also be avoided before the test, as it can alter normal breathing patterns and lung mechanics.
Consulting a physician about medication adjustments is an important preparatory step, especially for those who use inhaled bronchodilators. Short-acting bronchodilators, such as albuterol, are often withheld for four to eight hours before the test. Long-acting medications may require withholding for 12 to 24 hours to measure baseline lung function without the drug’s influence. Always follow the specific instructions provided by your healthcare provider regarding the temporary cessation of any prescribed medication.
Executing the Required Breathing Maneuvers
The most common part of the PFT is the Forced Vital Capacity (FVC) maneuver, which demands maximal inspiration followed by a forceful, sustained exhalation. The procedure begins with the patient sitting upright and taking the deepest breath possible to achieve maximal inspiration. This deep breath is immediately followed by forming a tight seal around the mouthpiece of the spirometer, often while wearing a nose clip to prevent air from escaping.
The critical phase is the exhalation, which must start with an explosive “blast” to measure the forced expiratory volume in the first second (\(\text{FEV}_1\)). The technician will coach the patient to blow out as hard and as fast as possible, emphasizing the need for full effort from the very beginning. This initial peak flow measures the airflow mechanics in the large airways and is dependent on patient effort.
Following the initial forceful expulsion, the patient must continue to exhale steadily for as long as possible, typically for a minimum of six seconds, until a volume plateau is reached. This sustained effort ensures that the maximum amount of air, known as the Forced Vital Capacity (\(\text{FVC}\)), has been measured. A complete maneuver means the exhalation volume-time curve has flattened, indicating that no more air is being expelled.
The technician provides continuous encouragement throughout the maneuver to elicit the greatest possible effort. A series of three to eight acceptable maneuvers will be performed to meet the technical standards for a valid result. For the test to be considered acceptable, the two largest values for both \(\text{FEV}_1\) and \(\text{FVC}\) must be consistent, typically within 150 milliliters of each other.
Avoiding Common Errors That Invalidate Results
Certain actions during the test compromise the quality and validity of the measurements, necessitating a repeat of the maneuver. One common error is hesitation or a slow start to the forced exhalation after the deep breath, which artificially lowers the measured \(\text{FEV}_1\) value. The exhalation must be an immediate and explosive “blast” to accurately capture the peak flow rate.
Maintaining a tight seal around the mouthpiece is paramount, as air leaking out through the side of the mouth or around the nose clip falsely reduces the measured volume and flow. Air leaks require the maneuver to be aborted and repeated. Similarly, a cough during the first second of the forced exhalation can spike the flow rate, leading to a falsely elevated \(\text{FEV}_1\).
Another frequent error is terminating the exhalation too soon, before the full six seconds or before the volume curve plateaus. Early termination results in a falsely low \(\text{FVC}\) measurement. This can lead to misinterpretation as a restrictive pattern, suggesting a problem with lung volume rather than effort.
The goal is not only to perform an acceptable maneuver but also to ensure reproducibility between attempts. Reproducibility means the measured values are consistent across multiple trials, confirming that the patient gave a maximal and sustained effort each time. Failure to meet these acceptability and reproducibility criteria means the test is technically invalid, potentially leading to misdiagnosis or the need for a retest.