Thankyou for changing the colour, much easier :D
Sweat tests are meant to be the most accurate of tests. 98% accurate. Whether they actually are or not, I'm not sure. The bordeline grey area is very confusing! I think the reason they usually do the sweat test is because the genetics test is harder and not as accurate.
With the genetics test they are looking for 2 copies of the CFTR protein, sitting on Choromosome 7. Unfortuantely though, they cannot test for all mutations of the gene. So they may spot one of the copies, but that doesn't indicate definately not CF, as the other one could be a gene they cannot yet test for.
With the sweat test, they are testing for problems with the chloride channels, which is what CF is all about, the doors for chloride don't work causing a build up of mucous everywhere. Infact I believe one of the only places CF doesn't effect is the eyes. People with CF sweat 100% more salt than the average person, so really the test should be extremely accurate. But then there are also other genes that vary from person too person, which override some of the effects the CF gene has.
As I say 1 in 25 of us are carriers. There is a 1 in 4 chance of 2 carriers having a child with cf, a 1 in 4 chance of their child not even being a carrier and 2 in 4 chance that their children will be carriers. Genetics are very confusing though. However CF is the UK's most common life-threatening illness. I don't know whether this is true for the US too. But it is a very common illness everywhere I think.
Sorry my post jumps around from one thing too another. Hope it's helpful too you though!