A nurse is reinforcing teaching about diagnostic tests with the parents of a child who has suspected cystic fibrosis. Which of the following diagnostic tests should the nurse include as the most definitive when diagnosing cystic fibrosis?

Cystic fibrosis (CF) is caused by mutations in the CFTR gene, which encodes a chloride channel protein. In individuals with CF, defective CFTR leads to abnormally high chloride (and sodium) concentrations in sweat. The sweat chloride test measures the concentration of chloride in sweat collected after pilocarpine iontophoresis stimulation of the sweat glands. A result of 60 mEq/L or higher on two separate occasions is considered diagnostic of cystic fibrosis. This test is the gold standard and most definitive diagnostic method for CF. Stool fat content analysis, or fecal fat testing, can support the diagnosis by identifying fat malabsorption caused by pancreatic insufficiency, but it is not definitive and can be elevated in many other conditions. Sputum culture identifies organisms colonizing the airways (such as Pseudomonas aeruginosa, which is common in CF), but it does not diagnose the underlying genetic condition. Pulmonary function tests evaluate the degree of airway obstruction and monitor disease progression, but they too are not diagnostic of CF. Students should memorize that the sweat chloride test is the confirmatory test for cystic fibrosis, with genetic testing used as an adjunct when sweat test results are borderline.