<b>pesmom,</b>
The sweat test *can* be very accurate -- when you fall pretty high and there isn't much of a question as to the results -- many people test out in say the 100+ number -- usually the docs will take that (along with CF symptoms) as a pretty clear diagnosis and will follow up with genetic testing to see what genes the person has, but they can pretty safely say "yes, you have CF"
The problem with sweat tests not being accurate is when they fall low -- say in the 30-60 range.... then docs can sometimes tell people they have "negative" or "normal" test results, when in fact the person does have two genes and should be tested to find them (many of the mild genes cause a lower sweat test number - but a lot of "regular" docs don't really know that)
If you have time, read the first entry on my blog page for more detailed info. (link in my signature line)