Figure 2 Algorithm for the diagnosis of CF starting with the CFTR DNA test. When entering the algorithm it is advised to continue the diagnostic work up if symptoms in a patient persist as well as when symptoms have resolved but are highly suspicious for CF such as pancreatitis or Pseudomonas aeruginosa lung disease. Wherever the algorithm ends with “CF unlikely” it is advised to investigate for alternative diagnoses such as primary ciliary dyskinesia, humoral immunodeficiency, Shwachman syndrome. For patients with CFTR dysfunction, the physician needs to decide the most appropriate diagnostic label (non‐classic CF or an item from the WHO diagnostic list shown in table 1 in patients with very limited symptoms). Patients with a borderline sweat test, only one CFTR mutation identified, and an inconclusive nasal potential difference (PD) can not at present be classified correctly. They are at least CF carriers. In the presence of persistent symptoms they need structured follow up at an appropriate facility (for some patients this may be the CF centre) and symptomatic treatment. Genetic counselling is important in these patients and their families. CFTR DNA test: screening test to search for the most frequent mutations in the population from which the patient originates. Mutation scanning of CFTR gene: this test is only necessary in some patients in whom the diagnosis cannot be supported by other means. The tests in the grey area are optional because detection of two disease causing mutations is sufficient to support the diagnosis of CF in a compatible clinical setting. However, in many CF centres a sweat test will also be performed to further confirm this lifelong diagnosis. Consult genetic lab: in patients with two mutations detected it would be unusual to have a sweat chloride value below 60 mmol/l unless one or both mutations are classified as mild. One should not, however, forget the possibility of mislabelling a sample or errors in sweat test or DNA test; in patients with an elevated sweat chloride level it would be unusual but not impossible not to find any mutation. In case of doubt about the diagnosis, a mutation scanning of the complete gene can be done. A falsely positive sweat test and the possibility of CF heterogeneity also need to be considered.