To confirm CF after a positive newborn screen, which test is performed?

• A. Chest CT
• B. Sputum culture
• C. Complete blood count
• D. Sweat chloride test

Answer: (D)

When a newborn screen suggests cystic fibrosis, the test used to confirm the diagnosis is the sweat chloride test. This test directly assesses CFTR function by measuring how much chloride is in the sweat. In people with CF, the CFTR channel doesn’t reabsorb chloride efficiently in the sweat ducts, so the collected sweat has a high chloride concentration. A result well above the normal range (commonly around or above 60 mmol/L in infants, with exact thresholds varying by age and lab) confirms CF. A normal result makes CF unlikely, though genetic testing for CFTR mutations might be used if the sweat result is inconclusive or if more detail about the mutations is needed. Other options like chest imaging, sputum culture, or a complete blood count do not diagnose CF; they’re useful for assessing infections or complications, not for confirming the disease.