Sweat tests analyse the amount of salt left on the skin when a person sweats. This can help to diagnose cystic fibrosis.
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 poo (faeces).
- 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. This means that you are born with it and it is passed on through families through special codes inside cells called genes. 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 salt (sodium and chloride ions). There are many different abnormalities (mutations) of the cystic fibrosis gene. They all stop the cell from dealing with sodium and chloride properly.
When people who have this faulty gene sweat, the pores of the skin can't soak up the right amount of sodium and chloride. This means that there is more sodium and chloride left on the skin than with a person who doesn't have this condition. The sweat test looks for excessive amounts of sodium chloride left on the skin following sweating.
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. A small electrical current is passed through the pad from a battery box to stimulate the sweating process further. Some people experience 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. 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. Some 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. You may be advised not to use creams or lotions on the skin for a day before the test.
Are there any side-effects or complications from a sweat test?
The test is not harmful. Some people feel a small tingling sensation. There may be a small red area where the test has been carried out. This settles very quickly.
Further reading and references
Cystic fibrosis: diagnosis and management; NICE Guideline (Oct 2017)
Standards of care; Cystic Fibrosis Trust
Farrell PM, White TB, Ren CL, et al; Diagnosis of Cystic Fibrosis: Consensus Guidelines from the Cystic Fibrosis Foundation. J Pediatr. 2017 Feb181S:S4-S15.e1. doi: 10.1016/j.jpeds.2016.09.064.
Guidelines for the Performance of the Sweat Test for the Investigation of Cystic Fibrosis in the UK; Association for Clinical Biochemistry & Laboratory Medicine (ACB) March 2014. Endorsed by the Royal College of Paediatrics and Child Health (RCPCH)