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 to be between 40-60 mmol/l on two separate occasions. Which of the following test should be performed next to suppo the diagnosis of Cystic fibrosis?

Cystic Fibrosis is diagnosed based on the combination of clinical criteria and abnormal CFTR function indicated by sweat tests, nasal potential difference measurement and CFTR mutation analysis. Nasal Potential Difference is a sensitive test of electrolyte transpo and is used to document CFTR dysfunction when the sweat Cl- test is normal or borderline and two cystic fibrosis mutations are not identified. So in this patient with borderline sweat chloride level, nasal potential difference detection is the next most appropriate investigation. Elevated sweat Cl- values are nearly pathognomonic for CF. Its level varies with age, in adults a Cl- concentration of >70 meq/L indicate the diagnosis. 1-2% patients who have normal sweat Cl- level will have raised nasal transepithelial potential difference. Ref: Current Medical Diagnosis and Treatment, 2012, Chapter 9 ; Harrison's Internal Medicine, 18th Edition, Chapter 259