hi shamrock
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i can kinda see what you mean. predicted values of fev1 are based on height, sex and AGE. In this sense, at 24 with a height of 5'1 my predicted fev1 is 2.86, so with a fev1 of 1.95 my percentage is 68%. However, even if my actual lung function of 1.95 didn't change by the time i was 30 it would be 71% (as the predicted would now be 2.73, based on height, sex and AGE.) So even though i hadn't actually gained any lung function at all, my percentage is up ( a bit false wouldn't you say?).
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However it is still a function against predicted. So, yes by the time you're 90 you have lost a lot of lung function granted. But so has everyone else, so the predicted of that is a lot less, 1.55. So if you had dropped to 0.22l, your lung function would be 14% of predicted. So if i've made any sense - that;s the point. The percentages are based on your predicted values for people your age, height and sex. As the population decreases naturally in lung function as they get older, if yours is decreasing faster - that's bad.
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incidently i prefer to look at my actual numbers than base it against a theoretical population statistic. I know if my lung function drops to 1.75 or lower i feel lousy. nuff said.
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xxxx