Sweat Test for Cystic Fibrosis (CF)

What is cystic fibrosis?

Cystic fibrosis (CF) is a genetic (inherited) disease that causes thick, sticky mucus to build up in organs, including the lungs and the pancreas. Other organs that are affected by CF include the liver, sinuses, intestines and sex organs.

In a healthy person, mucus that lines organs and body cavities, such as the lungs and the sinuses, is slippery and watery. In people with CF, the mucus becomes thick and clogs the airways, which makes it difficult to breathe.

CF is a chronic (long-lasting) and progressive (gets worse over time) condition. Children with CF may have one or more of the following symptoms:

• Failure to thrive (inability to gain weight despite having a good appetite and taking in enough calories)
• Loose or oily stools
• Trouble breathing
• Persistent wheezing
• Frequent lung infections (recurrent pneumonia or bronchitis)
• Repeated sinus infections
• A nagging cough
• Slow growth

What is the sweat test for cystic fibrosis?

Doctors diagnose CF by a thorough evaluation and with different tests, including genetic tests, chest and sinus X-rays, and lung function tests.

The most conclusive testing for CF is the sweat test, which measures the amount of chloride in the body’s sweat. (Salt is made up of chloride and sodium.) People who have CF have higher levels of chloride in their sweat because chloride is unable to move in or out of the body’s cells, which is what causes it to build up.

A sweat test is usually done if a child may be at risk for inheriting CF, or if he or she is showing symptoms. The test might also be ordered for the child’s siblings to see if they also have inherited the gene and might be at risk for passing the gene along to their children.

How is a sweat test done?

People of any age can have a sweat test, although newborn babies may not be able to make enough sweat to complete the test in the first days of life. Therefore, the sweat test in babies is usually done when the child is between two and four weeks old.

In the test, a healthcare provider spreads a chemical called pilocarpine on the skin of child’s arm or leg. The provider then applies a small amount of electric stimulation so that the sweat glands will produce sweat. The test does not use a needle and is not painful, though the patient may feel a slight tingling in the arm or leg being tested.

Next, the provider collects the sweat in a plastic coil or on a piece of filter paper or gauze. If filter paper is used, it is weighed before and after the test to measure the amount of sweat collected. The sample is then sent to a lab to measure the amount of chloride. This part of the test lasts approximately 30 minutes.

What do the results of the sweat test mean?

If the child has CF, the sweat test will show that he or she has a higher-than-normal level of chloride. In terms of measurement, if the child’s chloride level is:

• Above 60 mmol/L (millimoles per liter), he or she probably has CF.
• Below 40 mmol/L, he or she does not have CF.
• Between 40 and 60 mmol/L, the test may have to be repeated and additional tests may be necessary to rule out atypical forms of CF.

How do I prepare my child for a sweat test?

You do not have to limit your child’s diet, activities or medications before a sweat test. Don’t put any creams or lotions on your child’s skin before the test.