Diagnosis

Diagnostic investigations in cystic fibrosis include:

• immunoreactive trypsin or heel prick screening in neonates
• sweat test
• nasal potential difference testing.

Note that the genotype is so variable that it is of limited use in diagnosis; however it is becoming increasingly acceptable as an investigation in the neonate suspected of having cystic fibrosis.

Genotype is of most use in diagnosis of children of known carrier parents.

According to the NICE guideline, cystic fibrosis can be diagnosed based on (1):

• positive test results in people with no symptoms, for example infant screening (blood spot immunoreactive trypsin test) followed by sweat and gene tests for confirmation or
  
• clinical manifestations, supported by sweat or gene test results for confirmation or
  
• clinical manifestations alone, in the rare case of people with symptoms who have normal sweat or gene test results.

Reference:

• (1) NICE (October 2017) Cystic Fibrosis. Available at https://www.nice.org.uk/guidance/ng78. (Accessed on 05 June 2022)
• (2) NHS (2021) Cystic Fibrosis. Available at https://www.nhs.uk/conditions/cystic-fibrosis/. (Accessed on 05 June 2022)