Which test is most specific for diagnosing cystic fibrosis?

Which test is most specific for diagnosing cystic fibrosis?

• A. Sweat chloride test.
• B. Genetic testing for CFTR mutations.
• C. Chest X-ray.
• D. Sputum culture.

Answer: (A)

The test that is most specific for diagnosing cystic fibrosis relies on measuring chloride in sweat. In CF, the CFTR protein doesn’t function properly in sweat glands, so the ducts can’t reabsorb chloride efficiently, leading to very high chloride levels in sweat. When a patient with compatible symptoms has a sweat chloride value above the diagnostic threshold (typically greater than about 60 mmol/L), this finding is highly indicative of CF and can establish the diagnosis in the right clinical context. This direct link between CFTR dysfunction and the abnormal sweat result makes it highly specific. Genetic testing for CFTR mutations can support the diagnosis, but it isn’t perfectly specific because many mutations exist and panels may miss rare or unknown variants; a negative panel doesn’t completely exclude CF, especially if the clinical picture fits. Chest X-ray and sputum culture don’t diagnose CF themselves—they reflect lung disease or infection that can occur in many conditions—so they aren’t diagnostic tests for CF.

The test that is most specific for diagnosing cystic fibrosis relies on measuring chloride in sweat. In CF, the CFTR protein doesn’t function properly in sweat glands, so the ducts can’t reabsorb chloride efficiently, leading to very high chloride levels in sweat. When a patient with compatible symptoms has a sweat chloride value above the diagnostic threshold (typically greater than about 60 mmol/L), this finding is highly indicative of CF and can establish the diagnosis in the right clinical context. This direct link between CFTR dysfunction and the abnormal sweat result makes it highly specific.

Genetic testing for CFTR mutations can support the diagnosis, but it isn’t perfectly specific because many mutations exist and panels may miss rare or unknown variants; a negative panel doesn’t completely exclude CF, especially if the clinical picture fits. Chest X-ray and sputum culture don’t diagnose CF themselves—they reflect lung disease or infection that can occur in many conditions—so they aren’t diagnostic tests for CF.