What test confirms cystic fibrosis in suspected patients?

• A. Elevated sweat chloride (>60 mEq/L) with CFTR mutation testing.
• B. Sputum Gram stain.
• C. Chest X-ray.
• D. Blood glucose level.

Answer: (A)

Cystic fibrosis is diagnosed by a sweat test reflecting abnormal CFTR function, usually confirmed with genetic testing. A sweat chloride concentration greater than 60 mEq/L after pilocarpine-induced sweating is strongly diagnostic of CF, and identifying disease-causing CFTR mutations provides confirmation. The combination of a high sweat chloride result plus CFTR mutations leaves little doubt about the diagnosis. Other tests—like sputum Gram stain, chest X-ray, or random blood glucose—do not establish CF on their own: they may reveal infections, lung changes, or diabetes that can occur with CF, but they’re not diagnostic of the disease.