Imanov,
Have you considered talking to your child's physician? If you don't believe that your child has CF, I'm certain your doctor can perform a genetic test to check. False negative certainly are daunting, but the sweat test is used because it's the test that least invades your child's privacy. If the sweat test is negative, doctors can almost definitely rule out CF, which is extremely important.
Good luck on your next sweat test, and be sure to have a chat with your pediatrician as soon as possible!