A sweat electrolyte test detects the amount of sodium and chloride in your sweat. It’s also called an iontophoretic sweat test or chloride sweat test. It’s used primarily for people who have symptoms of cystic fibrosis (CF).
The body’s natural chemistry requires the right balance of sodium and chloride. These chemicals help regulate fluid in the tissues. People with cystic fibrosis have a mutation on chromosome 7 that affects a protein called the “cystic fibrosis transmembrane conductance regulator (CFTR).” This protein regulates the movement of chloride and sodium through the body.
When the CFTR protein doesn’t work properly or doesn't exist, chloride isn’t able to move through the body the right way. This causes an abnormal amount of fluid in the lungs, small intestines, pancreatic ducts, bile ducts, and skin. People with CF have large amounts of chloride and sodium in their sweat. They can have two to five times more than other people.
Your doctor may order this test if you have symptoms of CF. These symptoms include:
- frequent respiratory infections
- chronic cough
- continual diarrhea
- infertility in some adult males
This test is usually performed on children with suspected symptoms of CF. Because this condition is hereditary, a child with a close relative with CF may also be tested.
You don’t need to do much to prepare for this test. Avoid applying any creams or lotions to the skin 24 hours before the test.
If you have a small child, it’s a good idea to bring along some activities or toys to keep them occupied during the test.
During the sweat electrolyte test, the clinician will place two electrodes on your upper arm. In infants, the electrodes are normally placed on the thigh. Each electrode is covered with a piece of gauze that’s been soaked in a drug called pilocarpine, which stimulates sweating.
Once the electrodes are attached, a small electrical current will flow to the site for five to 12 minutes. The clinician will then remove the electrodes, wash the arm or leg with distilled water, and place a paper disk over the test site.
Next, the disk is covered with wax to keep it sealed and keep the sweat from evaporating. After an hour, the clinician will remove the disk with the sweat and send it to a lab for analysis of the amount of sodium and chloride.
Overall, the electrode sweat should take 90 minutes.
There are no risks associated with this test. The electrolyte sweat test is not painful. You may feel a slight tingle as the electrodes pass a small current through the site where they are attached. The area may still sweat after the test is finished, and the testing area may be red for a brief period.
It may take one or two days to get test results from the electrolyte sweat test.
For infants 6 months and under, a chloride level of 29 mmol/L or less indicates CF is unlikely. A chloride level above 60 mmol/L means it is likely that the child has CF. If the chloride level is between 20 and 59 mmol/L, it means that CF is possible and the test could need to be repeated.
Children and Adults
For children and adults, a chloride level of 39 mmol/L or less indicates CF is unlikely. A chloride level above 60 mmol/L means it is likely that the child has CF. If the chloride level is between 40 and 59 mmol/L, it means that CF is possible and the test may need to be repeated.
The sweat electrolyte test is very reliable and accurate. It's the gold standard in diagnosing cystic fibrosis. Since cystic fibrosis can lead to other complications, it’s very important to detect it early.