Sweat test

Read our guide below to help you understand about sweat tests.

You can also download a PDF version of this patient information by following the link on the right.

What is a sweat test?

It measures the amount of salt (chloride) that’s in the sweat. It doesn’t involve any needles and doesn’t hurt but it can feel a little strange.

Staff in the paediatric department do the test and send the sweat sample to the biochemistry department. The collection process usually takes about 1 hour.

Why do a sweat test?

We usually do them for children who have:

• recurrent chest infections
• problems gaining weight
• growing problems
• a family history of cystic fibrosis.

We also do sweat tests as a follow up to the newborn bloodspot screening programme.

There are other rarer reasons for a sweat test that your doctor will explain to you.

How do you do the sweat test?

We’ll clean an area of your child’s arm or leg with alcohol and water to remove any oil or residual salts from the skin.

We then put two discs containing a chemical called Pilocarpine that stimulates sweat production on the area and secure the discs in place with straps.

These discs connect to a device which passes a small electric current through the discs and skin to further stimulate the sweat production.

Is the test painful?

No although a tingling sensation may happen at this point. This part of the test takes 5 minutes then we remove the dics. There should be a red mark where the chemical Pilocarpine stimulates the skin. This is normal and will fade within a few hours.

What happens next?

We wash the skin again with sterile water and carefully dry it.  We then put a plastic collecting disc over over the stimulated area and secure it with a strap. The disc contains a
small, coiled tube and has a dot of blue dye on the back which helps us to see the sweat.

How long does it take to collect a sweat sample?

As soon as we see sweat forming, we set a timer 30 minutes. We cover the disc with a light bandage.
Your child can then play and eat and drink, although they must avoid salty foods such as crisps to minimise any risk of contamination.

After 30 minutes we measure the amount of sweat collected and remove the tubing from the disc. We’ll then send the sample in a container to the lab for analysis.

Are there any side effects?

Some people get a tingling sensation in the skin around the discs. The discs need to be fastened firmly so the straps feel quite tight, but this is necessary. Any marks left will disappear within a few hours.

The hand or foot below the test site can sometimes look a bit dusky due to the straps but this will go as soon as we remove the discs and straps.

Very rarely (1 in 25,000 cases),patients have reported minor skin burns. Where this happened, the patients showed no sign of pain or discomfort during the test and only discovered the burn
when the discs were removed from the skin.

The burns heal completely within 1 to 2 weeks with little or no scarring.

It’s highly unlikely that your child will get a burn during the test.

When will I get the results?

Normally within 24 hours of the test. Someone from the paediatric department will contact you with the result.

The doctor who referred your child for the test will then discuss implications and next steps. Sometimes we may need to re-do the test if  it’s inconclusive or we didn’t collect enough sweat.

Reducing associated healthcare infections

Find out how you can help to reduce healthcare associated infections when visiting hospital