The Test Sample
What is being tested?
The sweat test measures the amount of sodium and chloride in sweat. Sodium and chloride are part of the body's electrolyte balance and combine to form the salt found in sweat. They help regulate tissue fluid balance. 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, the cystic fibrosis transmembrane conductance regulator (CFTR), that 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 (CF) is 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. Since CFTR levels are usually highest in the epithelial cells lining the internal surfaces of the pancreas, sweat glands, salivary glands, intestine, and reproductive organs, these are the areas most affected by CF. Dysfunctional or absent CFTR causes the cells to be impermeable to chloride conductance and results in increased sodium and chloride concentrations in sweat.
Two methods of sweat analysis are frequently used: sweat chloride concentration and sweat conductivity measurement. Sweat chloride analysis is recommended as the diagnostic test for CF. Sweat conductivity 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. A tiny amount of a sweat-stimulating liquid is applied to a small patch of skin on the arm or leg. An electrode is then placed over the site and a weak electrical current stimulates the area. This is a painless procedure that may create a tingling or warm sensation. After several minutes, the area is cleaned and sweat is collected for about thirty minutes, either into a plastic coil of tubing or onto a piece of gauze or filter paper. 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 the skin 24 hours before the test.