4 year old boy presented with recurrent chest infections. Sweat chloride test was done, showed values of 36 and 42. What is the next best investigation to confirm the diagnosis ?
72 hour fecal fat estimation
CT chest
Transepithelial nasal potential difference
DNA analysis of delta F 508 mutation
Diagnosis of cystic fibrosis 
1.   Sweat chloride testing

The sweat test is the standard approach to diagnosis.
The diagnosis is made by elevated sodium and chloride level in the sweat > 60 meq/1.
Two test 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.

2.   Nasal potential difference

Measurement of nasal transepithelial potential difference in vivo can be useful adjunct in the diagnosis of cystic fibrosis.
Individuals 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.

3.   Genetic analysis

Cystic fibrosis is an autosomal recessive disorder.
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 associated with cystic fibrosis syndrome.
The most prevalent mutation of CFTR is the deletion of single phenylalanine residue at amino acid A508.
This mutation is responsible for high incidence of cystic fibrosis in northern European populations.
Approximately 50% of individuals with CF who are of northern European ancestry are homozygous for A508 and >70% carry at least one A508 gene. The remainder of patients has an extensive array of mutation, none of which has prevalance of more than several percent.
Testing for cystic fibrosis mutation was not possible because of the large no. of mutations associated with the disease.
Now days commercial laboratories test for 30-80 of the most common CFTR mutations.
This testing identifies >90% individuals who carry 2CF mutations.
No where it is mentioned in the texts that testing only for A508 is enough for diagnosis. Detection of atleast 2 CF mutations are necessary for making the diagnosis of cystic fibrosis.