What is the key diagnostic test for cystic fibrosis?

The key diagnostic test for cystic fibrosis is the elevated sweat chloride test. This test measures the concentration of chloride in sweat, which is typically higher in individuals with cystic fibrosis due to a defective cystic fibrosis transmembrane conductance regulator (CFTR) protein. This protein is crucial for the regulation of chloride and sodium ions across epithelial cell membranes. In patients with cystic fibrosis, the impaired function of CFTR leads to excessive chloride in the sweat. Therefore, an elevated sweat chloride level (greater than 60 mEq/L) is indicative of cystic fibrosis and is considered the gold standard for diagnosis.

In contrast, while genetic testing can confirm the diagnosis by identifying mutations in the CFTR gene, it is often used as a secondary test or for carrier screening rather than as the primary diagnostic tool. Similarly, pulmonary function tests, although valuable in assessing lung function and disease progression in patients already diagnosed with cystic fibrosis, do not diagnose the condition. A CT scan plays a role in evaluating lung pathology but is not a primary diagnostic test for cystic fibrosis itself. Thus, the elevated sweat chloride test remains the cornerstone for diagnosing this condition.