The sweat test and cystic fibrosis

The sweat test is used to help diagnose cystic fibrosis (CF).

Download our sweat test factsheet

  • What is a sweat test?

    The sweat test measures the amount of salt (or chloride) in sweat. This is done by collecting a small amount of sweat from the arm, or sometimes the upper part of the leg, of a small baby.

  • Why is the sweat test used?

    Cystic fibrosis (CF) is caused by changes (variants) in a gene that controls the movement of chloride across cell membranes. This results in higher amounts of chloride (as salt) in sweat compared with those who do not have CF. The sweat test is used to help make, or rule out, a diagnosis of CF where there is a family history or a possibility of CF. Screening for CF has been part of the national newborn blood spot screening programme (also known as the heel prick test) since 2007. All newborns are offered this screening at five days old. This means that most children with CF are diagnosed shortly after birth. A positive screening result, or ‘CF suspected’, suggests that a baby may have CF. A sweat test is needed to confirm or exclude the diagnosis.

    Sometimes, the sweat test is used in children or adults without a family history of CF, to look for the cause of frequent chest infections, unexplained diarrhoea, and not putting on weight or growing normally. This is usually to exclude a diagnosis of CF. In adults, the sweat test is also helpful to investigate bronchiectasis (a lung condition), infertility (difficulty in getting pregnant), and pancreatitis (a condition where the pancreas becomes swollen).

  • How does the sweat test work?

    The test is relatively simple and usually takes about 30 minutes.

    A small area of skin on the arm or leg is cleaned with water, and two gels or special pads are attached. These gels/pads contain a substance called Pilocarpine, which will make the skin sweat.

    To get the Pilocarpine into the skin, the area is stimulated by a small current from a battery for about five minutes. This may produce a tingling sensation but is not harmful and does not hurt.

    The gels/pads are removed and the skin is cleaned. A small coil device or a piece of special paper is placed onto the arm/leg. Sweat is collected into the coil or on the paper for about 20–30 minutes. The sweat is then taken to the laboratory for analysis.

    The area of the arm or leg used for the test may stay red for a few hours afterwards, but this is normal and nothing to worry about. The test is very safe and the risk of any problems is extremely small.

    Occasionally it is necessary to repeat the test if not enough sweat has been collected or there was a problem with the test. This does not necessarily mean that you, or your child, are more likely to have CF. Sometimes, the test needs to be repeated because of a borderline chloride result (when the result does not clearly show a high or normal level of salt in the sweat).

  • The result of the sweat test

    You will usually receive the result of the test within a few days from the doctor who requested the test. The result can help your doctor to make a diagnosis, but they will also rely on symptoms and the results of other tests.

    If your baby is being tested because of a newborn screening test result, arrangements will be made for the sweat test result to be explained to you by a doctor in your CF clinic. This is part of the follow up from the screening results and will usually be within 24 hours.

    If you have any questions or concerns about why this test is being performed, you should ask your doctor. You should not telephone the laboratory for results: laboratory staff are not allowed to give out results on the telephone, as they may not know the background for a specific patient.


Our sweat test factsheet

This factsheet explains how the test works, why it is used and what the results mean.

Page last reviewed: January 2024

Next review due: January 2026

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