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 and 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 back into sweat duct cells. Consequently, more chloride 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. The elevation in sodium concentration comes from the electrical charge of retained chloride trapping sodium in the sweat duct, preventing its reabsorption. Chloride carries a negative charge, while sodium's charge is positive; the opposing charges ensure that sodium and chloride attract one another. The extra chloride ions retained in the sweat duct produce a negative charge inside the duct and therefore attract sodium ions. The final result is that both sodium and chloride concentrations are elevated in individuals with cystic fibrosis.
The test site is carefully cleaned and dried, then a piece of preweighed filter paper is placed over the test site and covered with paraffin to prevent evaporation. Specialized collection devices may also be used. Sweat is collected for 30 minutes. The filter paper is retrieved and weighed to determine the weight of sweat collected. Several laboratory methods are then used to determine the sodium and chloride concentrations.