• Cystic fibrosis

Cystic fibrosis (CF) is a well-described genetic disease with characteristic defects in ion transport in disease-affected tissues. CF results from dysfunction of the cystic fibrosis transmembrane conductance regulator protein (CFTR) which is an ATP binding cassette protein that, in addition to chloride channel function, regulates other ion transport pathways such as sodium channels, other chloride channels and bicarbonate transport.1 Diagnosing CF is generally straightforward in patients with classic disease and builds upon these basic ion transport features, with well-defined clinical manifestations combined with elevated sweat chloride values, nasal ion transport abnormalities and/or common CFTR mutations.2 Newborn screening algorithms have added elevated serum immunoreactive trypsinogen levels to the diagnostic pathway (typically prior to symptoms), and together allow healthcare providers to confidently provide diagnostic and prognostic information to the majority of families and patients with CF.3

Unfortunately, there is a spectrum of disorders that have been linked to CFTR dysfunction which may not fulfil the diagnostic criteria for CF. In general, these milder manifestations of CFTR dysfunction can present in numerous ways such as recurrent upper and lower airway respiratory symptoms, pancreatic disease, male infertility, liver disease and vague gastrointestinal symptoms.3–5 Standard CF diagnostic testing may provide information that is conflicting or sits squarely in the ‘grey zone’, with intermediate sweat chloride values (above the normal range but below the CF diagnostic cut-off), nasal potential difference measurements with both CF and non-CF features, inconclusive genetic testing and additional (less specific) clinical measurements that may support …