Sweat test

The sweat test measures the concentration of chloride and sodium that is excreted in sweat. It is used to diagnose cystic fibrosis (CF).

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.cite journal |author=Rowe SM, Miller S, Sorscher EJ |title=Cystic fibrosis |journal=N. Engl. J. Med. |volume=352 |issue=19 |pages=1992–2001 |year=2005 |month=May |pmid=15888700 |doi=10.1056/NEJMra043184 |url=http://content.nejm.org/cgi/pmidlookup?view=short&pmid=15888700&promo=ONFLNS19]

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.

Method

Sweating is induced by pilocarpine iontophoresis. At the test site, an electrode is placed over gauze containing pilocarpine and electrolyte solution that will not interfere with the sodium and chloride measurement. A second electrode (without pilocarpine) will be placed at another site and a mild electrical current will draw the pilocarpine into the skin where it stimulates the sweat glands.

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.

Results

Reference ranges

If the concentration of chloride is >60 mEq/L, the test is positive; 40-60 mEq/L is borderline; <40 mEq/L is negative. The reference range for sodium is <70-90 mEq/L. The minimum sample weight varies with the collection method.

Interpretation

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. Highly discordant sodium and chloride values may indicate technical errors.

ources of error

Technical errors, insufficient sample, evaporation, contamination, dehydration, mineralocorticoid hormone therapy, and skin rash on the tested area may produce incorrect results.Positive test results may also be caused by malnutrition, adrenal insufficiency, glycogen storage diseases, hypothyroidism, hypoparathyroidism, nephrogenic diabetes insipidus, G6PD deficiency or ectodermal dysplasia (source: http://www.eMRCP.com)

References

External links

* [http://www.cff.org/AboutCF/Testing/SweatTest/ Cystic Fibrosis Foundation Sweat Test Information Page]


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