Spirometry is performed by deeply inhaling and forcefully exhaling into a spirometer (the device that records the various measurements of lung function). There are two measurements that are crucial in the interpretation of spirometry results. The first is called the forced vital capacity (FVC). This is a measurement of lung size (in liters) and represents the volume of air in the lungs that can be exhaled following a deep inhalation. The second is the forced expiratory volume-one second (FEV1). This is a measure of how much air can be exhaled in one second following a deep inhalation. You will also see another number on the spirometry test results — the FEV1/ FVC ratio. This number represents the percent of the lung size (FVC) that can be exhaled in one second. For example, if the FEV1 is 4 and the FVC is 5, then the FEV1/ FVC ratio would be 4/5 or 80%. This means the individual can breath out 80% of the inhaled air in the lungs in one second.
The three key spirometry measurements (the FVC, FEV1 and FEV1/FVC ratio) for a given individual are compared to reference values. The reference value is based on healthy individuals with normal lung function and it tells the doctor the values that would be expected for someone of the same sex, age and height. To find the reference value on your spirometry report, look for the column marked “reference” or “predicted” value.
Interpretations of spirometry results require comparison between an individual’s measured value and the reference value. If the FVC and the FEV1 are within 80% of the reference value, the results are considered normal. The normal value for the FEV1/FVC ratio is 70% (and 65% in persons older than age 65). When compared to the reference value, a lower measured value corresponds to a more severe lung abnormality. (See table below.)Understanding Your Breathing Test Results – Worker Health Protection Program