Cystic fibrois in children.?

hi i am a single first time mom of twin boys that are 14 month old and the doctor wants to have us run some test on him for cf. I dont know much about it but I do know that he has some of the symptoms from what i have read about it online. He was born prematurely at 26 weeks and was only 1lb14 oz. He was in the hospital for a few months came home on o2 and heart monitors and just stays with really bad congestion coughs he has runny pop with a bad strong smell he hasn't gained alot of weight for as much as he eats. when they did the new born test they never said anything about it coming back pos. but how often does it not come back pos and it really is there? please any information on this would be greatly appreciated! thanks!