A seven year old child with recurrent chest infections and exocrine pancreatic insufficiency is suspected of having cystic fibrosis Sweat chloride levels have been observed between 40-60 mmo1/1 on two separate occasions. Which of the following test should be performed next to suppo the diagnosis of Cystic fibrosis:
Repeat Sweat chloride levels on a different day Demonstrate an abnormal nasal potential difference Demonstrate an abnormal F508 mutation by DNA analysis Demonstrate an abnormal 72 hour fecal fat