As a long time asthmatic (now diagnosed as COPD) I know the peak flow of old,but have never had a lot of faith in what it tells me. The only time I used it in the past was when I felt dangerously breathless and if it was less than 100 (sometimes 50) I got to the hospital. This year, on my honeymoon in New Zealand, I had a flare up and was admitted to hospital for 5 days. Excellent care, they wanted me to actually keep a record of my peak flow, so I started doing that. It was explained to me that if you do it regularly it will alert you when trouble is on the way. I now do it every night and morning when I take my meds and have been very interested to find it rarely changes from 150. Now I know that isn't a good score, but it is fine for me! Couple of weeks ago had a very wheezy time but the peak flow remaioned doggedly at 150 - so I assume there was no infection? Saw a doc (in Swanage - I just should not leave home...) who gave me anti-Bs - just in case - and a week of prednisolone, which cleared it up very fast.
So, people, I am now converted to the use of my peak flow, long may I blow 150!!