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Coming for a sweat test

This resource is for parents of children who have been referred for a sweat test. It tells you what you can expect, and how to get the results.

What is a sweat test?

A sweat test is done to find out how much salt (usually chloride) is in the sweat. This information tells you about how the sweat is collected. Once the sweat has been collected it will then be tested in the laboratory to measure the amount of salt in the sweat.

Why do we do this test?

This test is done on children or adults who are having frequent chest infections, people who have lots of unexplained diarrhoea, people who are not putting on weight or growing properly, or as part of the national newborn screening programme. There are also other reasons why your doctor may have asked for a sweat test to be done. A positive result may mean that your child has cystic fibrosis (CF), but a final diagnosis will also take into account other test results and clinical findings. It is important to diagnose CF as soon as possible in order to start the right treatment. People with cystic fibrosis have a high amount of salt in their sweat. A normal result can be helpful to rule out cystic fibrosis.

Who does the test?

Laboratory staff carry out the test. Unfortunately they can not give any medical information or advise, so please talk to your doctor about any questions you might have that are not about the process of the sweat test.

Does the test hurt?

The test is not painful, but some people do feel tingling on the arm or the leg where the sweat has been collected which some children may find unpleasant.

Information:

No needles are used in this test.

How long does the test take?

The test to collect sweat lasts for about 45 minutes. Afterwards, you will be asked to stay for 10 minutes to make sure that enough sweat has been collected to be able to do the analysis. You may need to book in for a repeat test if we are not able to collect enough sweat.

How is the test done?

Special pads are soaked in a chemical called pilocarpine, which makes you sweat more. These pads are put on the lower arm or leg. A small electrical current is sent through the pad from a battery box to make you sweat even more.

The pads are left on for about 5 minutes, and then taken off. There is usually a red mark where the chemical has worked on the skin which should fade within a few hours. The skin is then carefully washed with pure water and dried. A piece of filter paper is put over the test area of skin and secured in place. You will then be asked to wait for about 30 minutes for the sweat to soak into the filter paper.

While you wait, your child is free to play, read, or eat. They should not have any salty foods, like crisps, because this can contaminate the test.

The filter paper is then taken off and sent to the lab for analysis.

The results

Most of the time tests will clearly show either a normal level of salt in the sweat, or a high level of salt. Sometimes the results are not so clear and the test will need to be done again. The test might also need to be done again because it did not work properly, for example if there was not enough sweat collected. Other times, doctors might like to do a second sweat test to confirm a high salt level.

How long will it take to get the results?

Results are usually ready after 1 week.

Who will let me know the results?

Your doctor will give you the results of the sweat test. You should not telephone the laboratory for results as the laboratory staff are not allowed to give out results to patients.

Please note

The skin where the sweat is going to be collection from has to be free from lesions, for example severe eczema.

Please contact us and let us know if your child is currently taking topiramate, an anti-epileptic drug, or fludrocortisone, a type of steroid. This might mean that the sweat test needs to delayed.

Although sweat testing is a very safe and useful test, on an extremely small number of people it has been reported to cause blistering of the skin or an allergic skin reaction (urticaria or hives) around the area where the sweat was collected.

Further information

If you have any questions about the process of doing the sweat test please contact the Clinical Chemistry Laboratory (details below).

If you have further questions regarding why your child needs a sweat test please speak to the doctor who has referred you for this test as they can give you further information,

Further information on CF and the sweat tests can be found on the Cystic Fibrosis Trust website

Contact us

Clinical Chemistry
Email: scn-tr.clinicalchemistry.sch@nhs.net
Telephone: 0114 271 7307

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Disclaimer

Please note: this is a generic information sheet relating to care at Sheffield Children’s NHS FT. These details may not reflect treatment at other hospitals. This information is not intended as a substitute for professional medical care. Always follow your healthcare professionals’ instructions. If this resource relates to medicines, please read it alongside the medicine manufacturer’s patient information leaflet. If this information has been translated into another language from English, efforts have been made to maintain accuracy, but there may still be some translation errors. If you are unsure about any of the guidance in this resource or have specific questions about how it relates to your child, always ask your healthcare professional for further advice.

Resource number: CLC1

Resource Type: Article

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