Sweat Test (Cystic Fibrosis)
Definition
Measures the amount of chloride in sweat. In a healthy person, a protein called CFTR regulates the movement of chloride in cells; in cystic fibrosis, because CFTR does not work properly, chloride builds up abnormally and thick, sticky mucus forms that damages organs.
Purpose
Diagnosis of cystic fibrosis. Most often ordered in infants with a positive newborn screening test; it can also be done in children and adults who show symptoms or have a family history of cystic fibrosis.
The test is best performed when the infant is 10 days to 4 weeks old.
Results
Chloride level is measured in mmol/L:
| Chloride | Meaning |
|---|---|
| ≥ 60 mmol/L | Cystic fibrosis is likely |
| 30 to 59 mmol/L | Borderline, further testing is needed |
| < 29 mmol/L | Normal, cystic fibrosis is unlikely |
Risks and Limitations
A normal result does not rule out cystic fibrosis, some people with cystic fibrosis have a normal sweat test.
Rare conditions such as hypothyroidism can raise chloride. The procedure takes about an hour: pilocarpine is applied to the skin, sweating is stimulated with a weak electric current, and sweat is collected for 30 minutes.