Once it has been determined that airway obstruction is present its severity is determined by the percent predicted FEV1:
| FEV1 percent predicted: | Obstruction is: |
| ≥70% | Mild |
| ≥60%, <70% | Moderate |
| ≥50%, <60% | Moderately Severe |
| ≥35%, <50% | Severe |
| <35% | Very Severe |
This presumes however, that only obstruction is present. When restriction and obstruction are present at the same there are no particular guidelines for assigning severity because the decrease in FEV1 is due at least in part to restriction. One possible approach for this circumstance is to use the percent predicted of the FEV1/FVC ratio instead:
| FEV1/FVC ratio: | Obstruction is: |
| ≥85%, <95% | Mild |
| ≥65%, <85% | Moderate |
| <65% | Severe |
This approach dates from the ITS Intermountain conference in the early 1970’s however, and is not in the ATS/ERS interpretation guidelines.
The ATS/ERS statements do not provide any guidelines to assessing the severity of a reduced FVC. A widely used approach for grading an FVC, however would be:
| FVC %Predicted: | |
| Normal | ≥LLN or 80% |
| Mild | ≥60% and <LLN or 80% |
| Moderate | ≥40% and <60% |
| Severe | <40% |
A more nuanced version of this would be:
| FVC %Predicted: | |
| Normal | ≥LLN or 80% |
| Mild | ≥65% and <LLN or 80% |
| Mild to moderate | ≥60% and <65% |
| Moderate | ≥45% and <60% |
| Moderate to severe | ≥40% and <45% |
| Severe | ≥20% and <40% |
| Very Severe | <20% |
These are optional however, and although the ATS/ERS guidelines do not explicitly recommend grading the FVC, they do not forbid doing so either.
PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License