Sweat chloride threshold for CF diagnosis?

• A. Greater than 40 mEq/L
• B. Greater than 60 mEq/L
• C. Greater than 20 mEq/L
• D. Greater than 80 mEq/L

Answer: (B)

Sweat chloride testing measures how much chloride is in sweat, which reflects CFTR function in the sweat glands. In cystic fibrosis, CFTR is defective, so chloride reabsorption in the sweat ducts is impaired and sweat becomes much more chloride-rich. When a child has a compatible clinical picture, a sweat chloride result greater than 60 mEq/L is diagnostic for CF. Values in the 40–59 mEq/L range are considered intermediate and typically prompt further testing (such as repeat sweat testing or CFTR genetic analysis), while values below about 40 mEq/L make CF unlikely. The test is usually performed using pilocarpine iontophoresis, a standard method to induce sweat collection.