Normal spirometry values vary, but they generally are considered normal when the forced expiratory volume, or FEV1, is less than 80 percent of the predicted value. They are also considered normal when the forced vital capacity, or FVC, to FEV1 ratio is 0.7 or less, says Patient.
Diagnostically essential values provided by spirometry include the forced vital capacity and the forced expiratory volume, notes Mayo Clinic. The first is a measurement of the most air one can exhale forcefully after taking in as much air as possible. The FEV1 reading indicates the volume of air one can force out of the lungs in a second.
An FVC reading that is lower than normal suggests restricted breathing, while a lower-than-normal FEV1 value indicates obstruction that is of greater significance, according to Mayo Clinic.
Normal spirometry values vary based on a person's size, age and sex, explains Patient. These are values that fall within a normal range for readings considered when making a diagnosis about lung function. Not all measurements recorded by spirometers are necessarily used in diagnosing lung issues.