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.

Last reviewed by a Cleveland Clinic medical professional on 02/14/2019.


  • Cystic Fibrosis Foundation. Sweat Test. Accessed 2/15/2019.
  • American Association for Clinical Chemistry/Lab Tests Online. Sweat Chloride Test. Accessed 2/15/2019.
  • National Heart, Lung, and Blood Institute. Cystic Fibrosis. Accessed 2/15/2019.

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