A 6 year old boy presented with recurrent chest infections. Sweat chloride test was done and it shows raised values. What is the next BEST investigation to confirm the diagnosis?

CF, an autosomal recessive disease, results in a syndrome of chronic sinopulmonary infections, malabsorption, and nutritional abnormalities. Essentials of Diagnosis and Typical Features: Greasy, bulky, malodorous stools; failure to thrive Recurrent respiratory infections Digital clubbing on examination Bronchiectasis on chest imaging Sweat chloride > 60 mmol/L The diagnosis of CF rests on the combination of clinical criteria and abnormal CFTR function as documented by sweat tests, nasal PD measurements, and CFTR mutation analysis. Elevated sweat Cl- values are nearly pathognomonic for CF. The nasal PD measurement can document CFTR dysfunction if the sweat Cl- test is normal or borderline and two CF mutations are not identified. Ref: Federico M.J., Stillwell P., Deterding R.R., Baker C.D., Balasubramaniam V., Zemanick E.T., Sagel S.D., Halbower A., Burg C.J., Kerby G.S. (2012). Chapter 19. Respiratory Tract & Mediastinum. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.