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Cystic Fibrosis Facts
Cystic fibrosis (CF) is a serious genetic disorder affecting about 1 in 4000 children born in the U.S. In CF, certain of the glands that produce secretions do not work properly. The best known symptom of CF is breathing difficulties, including persistent coughing and frequent lung infections such as bacterial pneumonia. CF sufferers also have difficulty obtaining nutrients from food, and tend to have poor weight gain despite a normal appetite.
Another, less serious sign of CF is salty skin. The excess salt results from sweat glands that do not work properly. A common form of cystic fibrosis testing involves analyzing a patient's sweat. Doctors may diagnose CF when the amount of salt in the sweat is greater than normal.
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In a sweat chloride test, the technician applies a liquid that stimulates sweating to a portion of skin on the arm or leg. A painless electric current from a pair of electrodes further stimulates the sweat glands. Sweat is collected for about half an hour. The sweat chloride test is non-invasive and painless, although the electric current may tingle or feel warm.
The collected sweat is analyzed for both sodium and chloride ions. Patients with cystic fibrosis will have elevated levels of these two ions. The sweat test is usually part of several diagnostic tests used to confirm CF. Further testing, such as lung function and pancreatic function tests, are needed to tell doctors how severe the patient's symptoms are.
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Sweat Chloride Test Rationale
Sweat is secreted by sweat glands in the skin. It consists of water and sodium chloride (table salt). Like all salts, sodium chloride dissolves in water into positive and negatively charged ions—in this case, sodium (positive) and chloride (negative).
In a person with normal skin glands, much of the sodium and chloride ions in sweat are reabsorbed by cell structures called ion channels—one type for sodium and another for chloride. The chloride channels require a protein called CFTR to function properly.
In cystic fibrosis, a person carries two defective copies of the gene for the CFTR protein. As a result, the body produces either defective CFTR or none at all. Without fully functional CFTR, chloride cannot be reabsorbed properly from sweat. As a result, excessive amounts of chloride remain on the skin.
The chloride channels also indirectly regulate sodium absorption. Positively-charged sodium ions are attracted to negatively charged chloride ions, so they tend to move in the same direction. When excess chloride remains on the skin, excess sodium remains there as well.
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Limitations of the Test
Sweat chloride analysis does not tell doctors how severe the patient's symptoms will be. In addition, it cannot be used to detect whether a healthy person carries the gene for CF, which has recessive inheritance. A person who is a carrier has normal sweat glands.
Certain other conditions can cause a positive result, so the sweat chloride test is usually used along with other tests in patients suspected of having CF.