The grading is based on how well tests are performed and how reproducible they are, meaning how consistent the attempts are in a session. An acceptable test is a test performed without errors, such as stopping exhaling before the lungs are completely empty.
A - EXCELLENT
At least 3 acceptable maneuvers.
For adults, top-two FVC and FEV1 from such maneuvers are within 150 mL.
For children aged 2-6, top-two FVC and FEV1 from such maneuvers are within 100 mL or 10% of the highest result (whichever is greater).
B - VERY GOOD
At least 2 acceptable maneuvers.
For adults, FVC and FEV1 from such maneuvers are within 150 mL.
For children aged 2-6, FVC and FEV1 from such maneuvers are within 100 mL or 10% of the highest result (whichever is greater).
C - GOOD
At least 2 acceptable maneuvers.
For adults, FVC and FEV1 from such maneuvers are within 200 mL.
For children aged 2-6, FVC and FEV1 from such maneuvers are within 150 mL or 10% of the highest result (whichever is greater).
D - POOR
At least 2 acceptable maneuvers.
For adults, FVC and FEV1 from such maneuvers are within 250 mL.
For children aged 2-6, FVC and FEV1 from such maneuvers are within 200 mL or 10% of the highest result (whichever is greater).
E - VERY POOR
One acceptable maneuver.
F - NOT ACCEPTABLE
No acceptable maneuver.
Reference: Recommendations for a Standardized Pulmonary Function Report. An Official American Thoracic Society Technical Statement.