Having a sweat test

A sweat test involves collecting a small volume of sweat and measuring the amount of salts (chloride).

This test helps us to assess whether your child might have cystic fibrosis (CF). CF is a genetic condition that affects the lungs and the digestive system.

A sweat test is used because people with CF have a higher amount of salt in their sweat than usual. A sweat test may still be needed if the newborn screening heel prick blood test was negative for CF.

We may use a sweat tests for babies or children who:

• Have had a CF suspected result on the newborn screening bloodspot test for CF. These babies may not have any symptoms associated with CF.
• Get a lot of chest infections.
• Have frequent and unexplained loose and pale poo.
• Are having problems gaining weight or growing properly.

These symptoms can sometimes indicate a child has CF. It is important to diagnose CF as soon as possible so that treatment can be started.

A member of staff from the biochemistry laboratory, who is experienced in performing sweat tests with babies and children.

1. A small area of skin on the arm or leg is cleaned with water.
2. Special pads are placed on the clean skin and secured in place. These pads are soaked in a chemical called pilocarpine to stimulate sweat production.
3. A small painless electric current is passed through the pads from a battery box. The current is applied for five minutes and then the pads are removed.
4. The skin is washed with pure water and carefully dried.
5. A sweat collector is placed over the stimulated area and secured in place. The sweat collection takes 30 minutes. During this time, your child can play and eat normally, but should avoid salty foods such as crisps to minimise any risk of contamination.
6. After 30 minutes, the sweat collector is removed and sent to the laboratory for analysis.
7. This process is then repeated on the other arm or leg.

The whole procedure usually takes between 1½ to 2 hours.

Your child will be able carry on as normal. There will be a red mark where the pilocarpine has stimulated the skin. This is normal and usually fades within about 24 hours.

To get an accurate result, enough sweat must be collected, so sometimes the test needs to be repeated.

No, although some children say that they have a tingling sensation on the arm or leg where the sweat is collected. You will be able to stay with your child throughout the test.

We carry out lots of sweat tests each year and our team are very experienced. However, as with all procedures, there are risks which, although unlikely, you should understand:

• There is a small risk of minor skin burns.
• There is a small risk of sensitivity to the chemical used (pilocarpine). This can cause reddening of the skin where the pad is placed. In some cases, one or more blister-like welts may also form. These are often mistaken as burns, but they are a reaction to pilocarpine. These ‘blisters’ usually disappear within 2 to 3 hours and leave no after-effects.

If your child is an inpatient at GOSH when they have the sweat test or if they are known to the CF team, then the result should be available the following day. If the result has to be sent to your local consultant or GP then it may take up to a week for you to be told the result.

If the levels of chloride salts are in the normal range then CF is unlikely. If the levels are above the normal range then your referring doctor/medical team will explain what these results mean. Your child may also need to have a blood test to check for genetics that cause CF.

Sometimes, even with enough sweat, the result can be hard to interpret and the sweat test has to be repeated.

Please speak with your child’s medical team if you have any questions or concerns.

If you would like to read more about cystic fibrosis, the Cystic Fibrosis Trust’s website has lots of information and resources- Cystic Fibrosis Trust website