Background
Cystic fibrosis is caused by defects in a protein found in many tissues, including the airways and the sweat glands. As a result, these tissues do not work properly. Sweat testing takes advantage of the fact that cystic fibrosis patients have defective sweat glands.
Sweat glands produce sweat through a well understood process of secretion and reabsorption of sodium chloride (salt). Secretion entails the movement of salt and water from sweat gland cells into the sweat duct. Reabsorption occurs in the duct with the movement of salt from the sweat back into sweat duct cells. What remains is sweat, a salt solution with a relatively finely tuned concentration of sodium and chloride.
For normal salt reabsorption to occur, individual ions of sodium and chloride must be taken from the sweat and moved back into cells of the sweat duct. These ions are moved by transporters called ion channels. In the case of sodium, there is a sodium channel; for chloride, there is a chloride channel called CFTR. For sweat to be produced with the proper concentrations of sodium and chloride, sodium channels and chloride channels (CFTRs) must work properly.
In cystic fibrosis, the CFTR chloride channel is defective, and does not allow chloride to be reabsorbed into sweat duct cells. Consequently, more sodium stays in the duct, and more chloride remains in the sweat. The concentration of chloride in sweat is therefore elevated in individuals with cystic fibrosis.
The concentration of sodium in sweat is also elevated in cystic fibrosis. Unlike CFTR chloride channels, sodium channels behave perfectly normally in cystic fibrosis. However, in order for the secretion to be electrically neutral, positively charged sodium cations remain in the sweat along with the negatively charged chloride anions. In this way, the chloride anions are said to "trap" the sodium cations.
Read more about this topic: Sweat Test
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