The most specific test for diagnosing cystic fibrosis is which test?

• A. Blood sugar test.
• B. Chest X-ray.
• C. Sweat chloride test.
• D. Sputum culture.

Answer: (C)

In cystic fibrosis, the defective CFTR protein leads to abnormal salt (chloride) transport in sweat glands. The sweat chloride test directly measures how much chloride is in sweat, so a high chloride level is a strong, specific marker of CF. Because this test targets the underlying transporter defect rather than just symptoms or complications, it provides the most reliable confirmation of the diagnosis when the collection is done correctly. Other tests described—like a blood sugar test, chest X-ray, or sputum culture—assess unrelated aspects (metabolic status, structural lung changes, or infection) and can be abnormal for reasons other than CF, so they’re not diagnostic on their own.