Classification and grading of ventilatory impairments based on spirometry^[1,2]

FEV₁: forced expiratory volume in one second; FVC: forced vital capacity; LLN: lower limit of normal, the 5th percentile.

* Low refers to levels below the 5th percentile, or a z-score <–1.645; absolute values are not used due to changes in spirometry with age and other factors.

¶ A reduced FVC does not prove a restrictive process. Confirmation of restriction requires evaluation of lung volumes in a pulmonary function laboratory (ie, total lung capacity z-score <–1.645 or below fifth percentile).

Δ A reduced FVC with normal FEV₁/FVC and lung-volumes is a "nonspecific" pattern that may be followed over time. One-third of patients with nonspecific patterns develop obstructive or restrictive disease in the next three years.

◊ Many patients with reduced FEV₁/FVC and low FVC have simple obstruction with air-trapping or failure to complete exhalation.

§ The severity of obstructive and mixed obstructive/restrictive ventilatory impairments are physiologically graded by decrement in FEV₁. Patients with restriction should have restrictive impairment confirmed and graded based on total lung capacity, but may be monitored by changes in FEV₁. FEV₁ may also be used as an alternative method to grade severity of confirmed restriction when only spirometry or % predicted values are available.

¥ Z-score is the preferred method for grading severity based on 2022 European Respiratory Society/American Thoracic Society (ERS/ATS) guidelines because it reduces bias due to age, sex, and other factors. Some spirometry software continues to report percent predicted, so we also include categorization based on this reporting method. The percent predicted severity classification has been adapted from earlier guidelines and modernized by reducing the number of distinct categories.