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?: (A) 72 hour fecal fat estimation, (B) CT chest, (C) Nasal electrode potential difference, (D) DNA analysis of delta F-508 mutation
Answer is C. 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.