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.
Transepithelial nasal potential difference
DNA analysis F 508 mutation
CT chest
72-hour fecal fat estimation
Disorders of the Respiratory Tract