What is the definitive test for diagnosing cystic fibrosis?

The definitive test for diagnosing cystic fibrosis is the sweat test. This test measures the concentration of chloride in sweat, as individuals with cystic fibrosis have elevated levels of chloride due to a defect in the CFTR gene, which affects ion transport across epithelial cells. During the sweat test, a small amount of sweat is collected from the patient after the application of a sweat-inducing agent (e.g., pilocarpine), and the chloride concentration is analyzed. If the chloride levels are above a certain threshold, this indicates a positive result for cystic fibrosis.

In contrast, other tests like sputum cultures, fecal fat tests, and pancreatic insufficiency evaluations serve supportive roles in diagnosis and management. Sputum cultures can help identify infections commonly associated with cystic fibrosis but are not definitive for the condition itself. The fecal fat test assesses fat malabsorption, which can occur in cystic fibrosis due to pancreatic insufficiency, but it also does not confirm the diagnosis. A Chymex test evaluates pancreatic function but again does not specifically diagnose cystic fibrosis. The sweat test remains the gold standard in diagnosing this genetic disorder.