The sweat test and cystic fibrosis

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.

In people with CF, there is a problem in moving chloride across cell membranes, which means higher concentrations of chloride (as salt) in sweat – compared with those who do not have cystic fibrosis.

If there is a family history or a possibility of CF, the sweat test (also known as the sweat chloride test) is able to detect abnormally high levels of chloride. Therefore, it is one of the special methods that helps to establish, or exclude, a diagnosis of cystic fibrosis.

The sweat test is used with babies suspected of having CF, as part of the follow-up to the screening process. As one of the investigations looking for possible causes of illness, sweat tests for children – and in some cases sweat tests for adults – will need to be done even when there’s no family history of cystic fibrosis.

The test is relatively simple and usually takes about 30 minutes. The person carrying out the test will clean a small area of skin on the arm or leg with water, and apply two gels or special pads containing a substance called Pilocarpine, which makes the skin sweat.

To absorb the Pilocarpine into the skin, the area is stimulated by a small current from a battery, for around five minutes. This may produce a tingling sensation, but it does no harm and does not hurt.

The person testing will then remove the gels or pads, clean the skin and will then place a small coil device or a piece of special paper onto the arm/leg. The sweat is collected into the coil or on the paper for about 20–30 minutes, before being 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.

Sometimes the test will need to be repeated if:

• not enough sweat has been collected;
• there has been some contamination; or
• if a borderline chloride result has been obtained.

This does not necessarily mean that your baby, child or an adult is more likely to have cystic fibrosis.

The sweat test result is usually available within a few days, from the doctor who requested the test. It can help your doctor to decide if it is CF, but they will also look at other symptoms and the results of other tests.

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