How does a chloride sweat test for cystic fibrosis work?

The chloride sweat test, also known as the sweat test, is the oldest and most reliable way to test for cystic fibrosis. Typically, a sweat test is required following a positive newborn screening result for cystic fibrosis. The test is very simple; it measures the amount of chloride in an infant's sweat. High chloride levels indicate cystic fibrosis. Children with cystic fibrosis have two to five times the normal amount of chloride in their sweat [source: Kids Health]. Chloride levels above 60 millimoles per liter indicate the presence of cystic fibrosis. Chloride levels between 30 and 59 mmol/L are borderline, and indicate the possibility of cystic fibrosis. If your child receives a borderline result, the sweat test is repeated.

The test is painless and involves no needles. A chemical that helps induce sweating is put on your child's arm or leg. An electrode is then put on this spot. A small electric current produces sweat on the skin. Some children feel a mild tingle during this process. The electrodes must remain in place for about five minutes, and the sweat is subsequently removed from the area and sent to a lab for testing.


Typically, the sweat test is only done once a child is two weeks old, as newborn babies do not produce enough sweat for this test. The results of a sweat test do not change with age, body temperature or illness. The sweat test does not involve any preparation beforehand; you should, however, avoid applying any cream on your child's body 24 hours prior to the test. Some children will have red skin or excessive sweating following the test, and, in rare cases, the affected area may appear sunburned.