Is there a chance that results of a sweat test are incorrect? We had the test done at a reputable center, but I am still concerned, there have been a couple new signs that my son could have cf, such as mucous in his stool, which has smelled kind of fishy, and was a green-grey color, which was a mustard yellow a week ago. He is definitely gaining weight, and does not taste salty. Any input would be appreciated