In a child with respiratory distress, failure to thrive. His sweat chloride levels were estimated 35 mEq/Land 41 mEq/L. What is next best test to do cystic fibrosis for diagnosis?

Ans. a. Transepithelial nasal potential difference (Ref: Nelson 18/e 1804, 1803]Nasal potential difference is a sensitive test of electrolyte transport (CFTR) function that can be used to support or refute a diagnosis of cystic fibrosis.Diagnosis of cystic fibrosisSweat chloride testingNasal potential differenceGenetic analysis* The sweat test is the standard approach to diagnosis.* The diagnosis is made by elevated sodium and chloride level in the sweat >60 mEq/l.* Two tests on different days are required for accurate diagnosis.* A normal sweat chloride dose not exclude the diagnosis.* Genotyping and other tests such as measurement of nasal membrane potential difference, pancreatic function should be done if there is high clinical suspicion of cystic fibrosis.* Measurement of nasal transepithelial potential difference in vivo can be useful adjunct in the diagnosis of cystic fibrosis.* Individual with cystic fibrosis demonstrate a significantly more negative baseline nasal potential difference, with the topical application of amiloride there is loss of this potential difference.Nasal potential difference is a sensitive test of electrolyte transport (CFTR) function that can be used to support or refute a diagnosis of cystic fibrosis.* It is caused due to defect in CFTR (Cystic fibrosis transmembrane conductance regulator) protein.* Cystic fibrosis is associated with large number of mutations.* More than 1500 CFTR polymorphisms are associate with cystic fibrosis syndrome. * The most prevalent mutation of CFTR is the deletion of single phenylalanine residue at amino acid D508This mutation is responsibly for high incidence of cystic fibrosis in northern European populations.* Now days commercial laboratories test for 30- 80 of the most common CFTR mutations. Detection of atleast 2 CF mutations are necessary for making the diagnosis of cystic fibrosis. Diagnostic Criteria for Cystic FibrosisPresence of typical clinical features(respiratory, gastrointestinal or genitourinary)orA history of CF in a siblingorA positive newborn screening testplusLaboratory evidence for CFTR dysfunctionTwo eleveated sweat chloride concentrations obtained on separate daysorIdentification of two CF mutationsorAn abnormal nasal potential difference.