I really don't think there is a set schedule you should follow. I had my first one 26 years ago and was diagnosed as severe and didn't have another one for about 15 years. It really depends on your condition. If your condition stays stable you probably wont need one until something unusual happens. I am so bad now I have a short PFT every year just to check my FEV1%, I also have an ABG every year to be sure I'm not retaining. Your infections are something else. Either the Dr. is not clearing it up or you need a Corticosteroid to help control it. You need to get it under control.
My last one(last year) was 5 years from the last and I had to mention it since I was starting to feel more SOB. I think he was sort of shocked that it had been that long but he is not a big tester unless something different is going on since he believes it is more about how you feel than what the numbers say.
My experience is that if you are stable, you could go for a very long time without one. Most docs will do one if something changes; very often they will do them every three or six months after you've had something that caused a major change in your symptoms or something that put you in the hospital. It's simply one measuring stick among many others; the best measuring stick is how you feel and what you can do comfortably.
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