Lung function above 70% is sometimes considered mildly diseased. 40% to 70% would be moderately diseased and lung function below 40% is sometimes considered severely diseased. But this doesn’t really tell the whole story.
Lung function is truly measured in volume. FEV1 is the volume (typically liters) that a person can forcefully exhale in 1 second. FVC is the amount of air volume forcefully exhaled after a full inspiration. To give these volumes context, the labs are reported as a comparison against the population of healthy individuals with similar gender, age, and size. This is reported as FEV1% or FVC%. So a person that blows an FEV1% of 100% is at the predicted volume for their gender, age, and size. But like any average some people are naturally above and some are naturally below.
What seems to be more important for an adult with CF than just basic percentage, is where lung function is compared against recent tests (like the past 3-4 PFTs). It is also important to consider where they are at relative to their personal baseline. Baseline is the highest or best test ever (with the caveat that many CF folks have significant lung damage prior to reaching their mature size). For example my baseline FEV1% is 108 which I blew in my mid 20s. But I’ve dropped 14% since then. This drop indicates that my disease is slowly progressing. Another CF-person might have a baseline of 90% but in a decade might only drop to 85% therefore their disease is progressing slower than mine despite the fact that I have better lung function percentage. The key to CF health and the goal of our lung-centric treatments is to minimize lung function decline.
