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The Test Sample
What is being tested?
The sweat chloride test measures the amount of chloride in sweat. Chloride is part of the body's electrolyte balance and combines with sodium to form the salt found in sweat. People with cystic fibrosis (CF) have an elevated level of chloride in their sweat.
Normally, chloride travels in and out of the body's cells, helping to maintain electrical neutrality and water balance. This movement occurs through a protein called the cystic fibrosis transmembrane conductance regulator (CFTR). It serves as a channel, letting chloride out of cells and into the surrounding fluid and also reducing sodium absorption. Sodium levels thus usually mirror those of chloride.
Cystic fibrosis is an inherited disease caused by a mutation in each of the two copies of the CFTR gene (one copy from each parent). With two mutations, the CFTR protein may be dysfunctional or totally absent. With dysfunctional or absent CFTR, chloride does not move out of the ducts into surrounding fluid, resulting in the production of thick, sticky mucus. Since CFTR levels are usually highest in the epithelial cells lining the internal surfaces of the bronchi of the lungs, pancreas, sweat glands, salivary glands, intestine, and reproductive organs, these are the areas most affected by CF. (Read the article on Cystic Fibrosis to learn more.)
Dysfunctional or absent CFTR results in increased chloride in sweat, which can be measured with a sweat test. Two methods of sweat analysis are frequently used:
- Sweat chloride concentration is recommended as the diagnostic test for CF.
- Sweat conductivity testing may be used to screen for CF.
How is the sample collected for testing?
A sweat sample is collected using a special sweat stimulation procedure. Below is one example (Courtesy: Jim Faix, MD):
- The patient's skin (usually the forearm) is washed with water and patted dry.
- A pad soaked in pilocarpine (chemical that stimulates sweat glands) is placed on the skin. An electrode creates a mild current that moves the chemical through the skin into the area where the sweat glands are. This procedure may create a tingling or warm sensation and takes about five minutes.
- The electrode and pad are removed. The area of the skin that was stimulated has started to sweat.
- A piece of filter paper that was previously weighed is placed over the area.
- The area is covered with plastic and tightly bandaged. Sweat is collected for 30 minutes.
- The piece of filter paper is now filled with sweat. It will be removed, weighed (to determine how much sweat was obtained), and placed into buffer so that the concentration of chloride can be determined.
For other, similar procedures, sweat may be collected into a plastic coil of tubing or onto a piece of gauze. The sweat obtained is then analyzed.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No special preparation is needed. However, you may be instructed to avoid applying creams or lotions to your skin 24 hours before the test. It is also important to be well-hydrated before undergoing the procedure.