What is being tested?
The sweat test measures the amount of sodium and chloride in sweat. Sodium and chloride are part of your body’s electrolyte balance, and combine to form the salt found in sweat. They help regulate the fluid balance in your tissues. 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.

Except for sperm cells in males and egg cells (oocytes) in females, every cell in the body has 46 chromosomes, organized in 23 pairs. A gene on chromosome 7 is responsible for the normal production of CFTR. Patients with CF have a mutated gene on each chromosome 7. With the mutation, 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 most frequently used: chloride concentration and conductivity measurement. The 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.