I've never heard of the 'SIN' thing before so I don't know what that stands for. That wasn't mentioned on any of the kids' sweat tests here and we've had positive (Emily, twice), borderline (Shawn, twice) and negative (Tim) results here.
I know it's often been posted but just wanted to point out that many people with cf have sweat test results lower than 60. Borderline results (which go either way) are between 40-60, and esp with babies some drs feel the cutoff should be even lower. Results always have to be considered combined with results from genetic testing and symptoms when making a dx.