Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
What is a sweat test?
A sweat test measures the amount of salt (made up of sodium chloride) that is in sweat produced during the test. A special chemical which is known to cause sweating is put on the skin. The amount of salt in the sweat is then analysed in a laboratory.
What is a sweat test used for?
This test is most commonly done to check whether a person has cystic fibrosis. The test may be carried out on children or adults who are having symptoms which might suggest they have cystic fibrosis such as:
- Recurrent chest infections.
- Frequent and unexplained pale stools.
- Problems gaining weight or growing properly.
- As part of a screening programme.
Rarely, a sweat test may be carried out for other reasons.
How does a sweat test work?
Cystic fibrosis is a genetic disorder. If you have cystic fibrosis, a particular pair of genes (on chromosome 7) does not work properly. This pair of genes helps to control the way the cells handle sodium and chloride ions (salt). There are many different abnormalities (mutations) of the cystic fibrosis gene but, basically, they all stop the cell from handling sodium and chloride properly.
As a result, cells in affected organs have a fault in the way sodium and chloride travel in and out of the cells. When you sweat, a certain amount of sodium and chloride is left on the skin. A certain amount is also reabsorbed into the pores of the skin.
In cystic fibrosis, the pores of the skin can’t reabsorb the right amount of sodium and chloride. This means that when people with cystic fibrosis sweat there is more sodium and chloride left on the skin. A person who doesn’t have cystic fibrosis has less sodium and chloride left on the skin when they sweat. (For more information on cystic fibrosis see separate leaflet called ‘Cystic Fibrosis’.)
In a sweat test, a chemical called pilocarpine is placed on the skin. This causes the skin to sweat. An electrode is placed on the skin at two points. A very small electrical current is passed between the electrodes. This helps to draw the pilocarpine into the skin, and produce sweating. After a while the area of skin is cleaned and an absorbent piece of paper is placed on the skin. This soaks up the sweat. After 30 minutes the paper is removed. It is sent to the laboratory where the amount of sodium and chloride is measured.
The results of the test are then sent to the doctor who requested the test.
What happens during a sweat test?
Special pads are soaked in pilocarpine and placed on the lower arm or leg. These are secured in place and a small electric current is passed through the pad from a battery box to stimulate the sweating process further. Some people find there is a tingling sensation at this point of the test, but it should not be painful. No needles are involved. The pads are left in place for about five minutes and then removed. There will usually be a red mark where the pilocarpine has stimulated the skin. This is normal and should fade within a few hours. The skin is then carefully washed with pure water and dried. A piece of filter paper or sometimes a plastic coil is placed over the stimulated area and secured.
You will then be asked to wait for about 30 minutes for the sweat to be absorbed into the filter paper or coil device. During that time you (or your child) are free to read, play or eat, although salty foods, such as crisps, should be avoided to minimise any risk of contamination. The filter paper or coil is then removed and sent to the laboratory for analysis.
What will the results show?
In most cases the results will clearly show either a high (abnormal) or normal salt level in the sweat. Sometimes the results can be borderline and the test will need to be repeated. In a few cases the test may need to be repeated for technical reasons, such as not enough sweat having been collected. Many doctors like to confirm an abnormal sweat test with a second sweat test.
What should I do to prepare for a sweat test?
There is usually very little you need to do to prepare for a sweat test. Your local hospital should advise you if there is anything they require you to do.
Are there any side-effects or complications from a sweat test?
Other than a small number of people feeling the tingling sensation and a small red area where the test has been carried out, the test carries very little risk.
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