In a suggestive context, normal sweat chloride values (<60 mmol/L) do not always suffice to exclude the diagnosis of CF.
Case-report: A 19-year-old female presented with a diagnosis of bronchiectasis. Her past medical history was noteworthy for the onset of respiratory symptoms in the infancy, colonization of the respiratory tract by Pseudomonas aeruginosa for three years and previous treatment for allergic bronchopulmonary aspergillosis. She was heterozygote for the DeltaF 508 mutation of the CFTR gene. Sweat chloride values were repeatedly normal, ranging from 25 to 46 mmol/L. The diagnosis of CF was confirmed by the identification of a second CFTR mutation (D1152H) and the demonstration of typical nasal potential.
Conclusion: It is now estimated that approximately 2% of CF patients will present an "atypical" phenotype with sweat chloride values<60 mmol/L. For these patients, the diagnosis can be confirmed by the identification of a CF-causing mutation in each CFTR allele or in vivo demonstration of CFTR dysfunction by nasal potential difference study.