The sweat chloride test diagnoses cystic fibrosis, a genetic disorder that causes excess mucus and bodily fluids, leading to lung infections and a shortened lifespan. The test measures the amount of chloride in sweat, with a level above 60 milliequivalents per liter indicating cystic fibrosis. Retesting is important to avoid false positives or negatives.
The sweat chloride test is used to diagnose a child who may have cystic fibrosis. People with cystic fibrosis have a genetic mutation that causes them to secrete excess mucus and other bodily fluids, leading to a range of symptoms that include an increased susceptibility to lung infections. This genetic disorder significantly shortens the life span of affected individuals, with the median survival age just under 37 years. The sweat chloride test is a simple test that has been the primary diagnostic test for cystic fibrosis for more than four decades. In most cases the test is performed 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 only a small amount of salt, in the form of sodium ions and chloride ions. In people with cystic fibrosis, the mechanism that reabsorbs salt is faulty. As a result, the amount of sodium and chloride that remains on the skin is much higher than normal.
During the test, the child being tested will have a sweat-inducing chemical applied to their skin, typically on their leg or forearm. Once applied, an electrode is placed on the area and a small electric current is applied through the electrode to stimulate sweating. This electrical current is minimal and will only cause a slight tingling sensation or a warm sensation on the skin. After the electric current is applied, the baby’s skin is swabbed with a filter paper to collect perspiration. The entire test takes 30 to 60 minutes.
The results of the sweat chloride test are evaluated based on the amount of chloride in the sweat. A normal level of chloride in sweat is between 10 and 35 milliequivalents per litre. To be diagnosed with cystic fibrosis, a child must have a sweat chloride level above 60 milliequivalents per liter in two tests done 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 not have the disease at all. In these cases the test is repeated, usually at least once, to ensure that no false positive or false negative results have been obtained.
Another reason retesting is important is that certain medical conditions can alter the results of sweat chloride tests. For example, children with cystic fibrosis who are malnourished may have a normal level of chloride in their sweat when tested. Also, a child who does not have cystic fibrosis may test positive for the disease if they have another condition such as a pancreatic infection or thyroid or pituitary disease.
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