At a Glance
Why Get Tested?
To diagnose cystic fibrosis (CF)
When to Get Tested?
When a newborn or infant has symptoms that suggest CF, such as frequent respiratory infections and chronic cough, persistent diarrhoea, foul-smelling bulky greasy stools and malnutrition; and as a follow-up, confirmatory test to help diagnose CF
A sweat sample collected using a special sweat stimulation procedure
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. This abnormality causes mucous secretions to become thick and sticky and the sweat much saltier than normal. This elevation in salt in the sweat can be measured, as a sweat chloride test.
How is the sample collected for testing?
A sweat sample is collected using a special sweat stimulation procedure. A tiny amount of a clear, 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 sent to the laboratory for analysis.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.