An infant has a positive newborn screening test for cystic fibrosis. What cut off of sweat chloride confirms cystic fibrosis?
**Core Concept**
Cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene, leading to abnormal chloride transport across epithelial membranes, resulting in thickened secretions and various organ system complications. The sweat chloride test is a diagnostic tool used to screen for cystic fibrosis in newborns and infants.
**Why the Correct Answer is Right**
The sweat chloride test measures the concentration of chloride ions in sweat, which is elevated in individuals with cystic fibrosis due to the defective CFTR protein. In infants, a cut-off of 60 mmol/L is used to confirm the diagnosis of cystic fibrosis. This is because infants with cystic fibrosis have a higher concentration of chloride ions in their sweat compared to healthy infants.
**Why Each Wrong Option is Incorrect**
**Option A:** 30 mmol/L - This is the cut-off for the intermediate sweat chloride category, which requires further evaluation and testing. It is not diagnostic for cystic fibrosis in infants.
**Option B:** 40 mmol/L - This is below the cut-off for the diagnosis of cystic fibrosis in infants, and would likely be seen in healthy individuals or those with other conditions.
**Option C:** 80 mmol/L - While this is an elevated sweat chloride level, it is not the specific cut-off used to confirm cystic fibrosis in infants.
**Clinical Pearl / High-Yield Fact**
The sweat chloride test is a crucial diagnostic tool for cystic fibrosis in newborns and infants, and a cut-off of 60 mmol/L is used to confirm the diagnosis. It is essential to remember that this test should be interpreted in conjunction with clinical evaluation and other diagnostic tests.
**Correct Answer:** C. 60 mmol/L