Right, "Jeana_Lynn". To qualify for SSD benefits with CF, you must either meet the Blue Book listing or be found eligible for benefits under a “medical vocational allowance”, which essentially shows that while your case doesn’t match the CF listing, you are nonetheless disabled according to SSA regulations.
The percent of lung function is not the sole requirement in getting disability. I was a teacher for twelve years and my last year teaching I was hospitalized repeatedly with my lung function consistently dropping until it was 39%. I was encouraged to take a medical retirement, which I did. 3 years later, my lung function during good times is 55%, but I can not return to teaching because of multiple hospitalizations every year particularly during the fall, winter and spring. So, if someone were to look at my best three months (summer) and highest fev1, they may decide that I can teach, but that is not the case. I would hate to make a judgement on someone based solely on fev1. I know it is easy to make snap judgements without seeing the whole picture.