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The sweat chloride test measures the concentration of chloride that is excreted in sweat.
It is used to screen for defects in a protein found in many tissues, including the airways and the sweat glands.
Sweat chloride testing makes use of the fact that cystic fibrosis patients have defective sweat glands.
Sweat glands produce sweat by a process of secretion and reabsorption of sodium chloride.
Sweat secretion entails the movement of salt and water from sweat gland cells into the sweat duct.
Reabsorption of salt occurs in the duct with the movement of salt from the sweat back into sweat duct cells.
The remaining 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 removed from the sweat and into cells of the sweat duct.
Transporters called ion channels move these ions.In the case of sodium.
There is a sodium channel; for chloride, and a chloride channel called CFTR.
CFTR-Cystic Fibrosis Transmembrane conductance Regulator.
Proper concentrations of sweat sodium and chloride are dependent sodium channels and chloride channels and (CFTRs) must work properly.
In cystic fibrosis, the CFTR chloride channel is defective.
The CFTR chloride channel does not allow chloride to be reabsorbed into sweat duct cells.
Therefore, 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.
Sodium channels behave normally in cystic fibrosis, unlike CFTR chloride channels,
Pilocarpine induces sweating by iontophoresis.
An electrode is placed over a test site with gauze containing pilocarpine and electrolyte solution that will not interfere with the sodium and chloride measurement.
At another test site a second electrode without pilocarpinea mild electric current will draw the pilocarpine into the skin where it stimulates the sweat glands.
A piece of preweighed filter paper is placed over the test site and collection devices may also be used.
Sweat is collected for 30 minutes, and the filter paper is retrieved and weighed to determine the weight of sweat collected.
For infants up to and including 6 months of age, a chloride level of:
Equal to or less than 29 mmol/L = CF is very unlikely
30 – 59 mmol/L = intermediate means that CF is possible
Greater than or equal to 60 mmol/L = CF is likely to be diagnosed
For people older than 6 months of age, a chloride level of:
Equal to or less than 39 mmol/L = CF is very unlikely
40 – 59 mmol/L = intermediate means that CF is possible
Greater than or equal to 60 mmol/L = CF is likely to be diagnosed
Two reliable positive results on two separate days is diagnostic for CF.
Because of the existence of milder variants, borderline or even near-borderline negative results may be used to diagnose CF.
Clinical presentation, family history and patient age must be considered to interpret the results in borderline results.
Technical errors, insufficient sample evaporation, contamination, dehydration, mineralocorticoid hormone therapy, and skin rash may produce incorrect results.
False positive test results may also be caused by malnutrition, adrenal insufficiency, glycogen storage diseases, hypothyroidism, hypoparathyroidism, nephrogenic diabetes insipidus, G6PD deficiency or ectodermal dysplasia.