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The sweat chloride test is used to diagnose a child who may have cystic fibrosis. People with cystic fibrosis have a gene mutation which causes secretion of excess mucus and other body fluids, leading to a range of symptoms which include increased susceptibility to lung infections. This genetic disease shortens the lifespans of affected individuals considerably, with an average survival age of a little less than 37 years. The sweat chloride test is a simple test which has been the main diagnostic test for cystic fibrosis for more than four decades. In most cases the test is carried out on children.
The cystic fibrosis sweat chloride test is used to easily diagnose this disease because people with cystic fibrosis have abnormal sweat glands. When normal sweat glands are active, salt and water are transferred from these glands to the skin. Most of the salt is then reabsorbed into the sweat glands, leaving behind only a small amount of salt, in the form of sodium ions and chloride ions. In people with cystic fibrosis, the mechanism which reabsorbs salt is defective. As a result, the amount of sodium and chloride remaining on the skin is much higher than normal.
During the test, the child being tested will have a sweat-inducing chemical applied to his or her skin, typically on the leg or forearm. Once it is applied, an electrode is placed on the area, and a small electrical current is applied through the electrode to stimulate sweating. This electrical current is minimal, and will only cause a mild tingling sensation or a feeling of warmth on the skin. After the electrical current has been applied, the child’s skin is swabbed with a filter paper to collect the sweat. The entire test takes between 30 and 60 minutes.
Sweat chloride test results are evaluated on the basis of the amount of chloride in the sweat. A normal sweat chloride level is between 10 and 35 milliequivalents per liter. To be diagnosed with cystic fibrosis, a child must have a sweat chloride level of more than 60 milliequivalents per liter in two tests carried out on two different days.
Children with a sweat chloride test result of between 35 and 60 milliequivalents per liter may have a milder variant of cystic fibrosis, or may not have the disease at all. In these cases the test is repeated, usually at least once, to ensure that false positive or false negative results have not been obtained.
Another reason repeat testing is important is that certain medical conditions can skew the results of sweat chloride tests. For example, children with cystic fibrosis who are malnourished may have a normal sweat chloride level when tested. In addition, a child who does not have cystic fibrosis may test positive for the disease if he or she has another condition such as a pancreatic infection or a disease of the thyroid gland or pituitary gland.