Sweat Test in Cystic Fibrosis

Cystic fibrosis is a genetic disease, which is characterized by defective exocrine glands, including those producing mucus and sweat. The working of the sweat glands include the secretion and reabsorption of sodium chloride. Those affected with cystic fibrosis have defective sweat glands, which do not absorb the chloride properly. This results in high concentration of chloride in sweat. The level of sodium is also found to be high, but the sweat test is aimed at measuring the amount of chloride in sweat. It is said that people affected by cystic fibrosis have 2 to 5 times of sodium and chloride content in their sweat, than that of the normal persons. Sweat test has an important role in confirming the diagnosis of cystic fibrosis. Nowadays, this test is part of the newborn screening program in many countries.

Procedure

The sweat test is usually done on the forearm, but in infants it can be performed on the thighs or the back. This test may be conducted on a two-day-old baby too, but it should be repeated, as newborn babies may not produce enough sweat, and normally, their sweat contain very low amount of sodium chloride. Sweat test includes the following steps:

• After cleaning the skin, two gauze pads are placed at two points on the forearm. One pad contains sweat-inducing drug called pilocarpine, and the other is soaked with salt water.
• Both pads are connected with electrodes and mild electric current is passed through the pads. This current helps the skin to absorb pilocarpine, which in turn stimulates the sweat glands to produce sweat.
• After ten minutes, the electrodes and pads are removed, the test site is cleaned and dried.
• A piece of previously weighed filter paper is placed on the test site to collect sweat (specialized devices are also used to collect sweat). This filter paper is covered with plastic or paraffin to prevent evaporation.
• Sweat is collected for a time span of thirty to forty minutes. The filter paper is removed and weighed to find out the amount of sweat collected (weight test depends on the mode of collection of sweat).

Various tests are conducted to figure out the level of sodium and chloride in the sweat.

Interpretation of Results

If the concentration of chloride is higher than 60 milli equivalents per liter, the result is positive. If it is less than 40, the result is negative and the range between 40 to 60 is considered as borderline. The above said range is applicable for adults and children. The only difference in case of infants, is the regarding normal range. A chloride concentration of less than 30 is considered normal for infants, but the rest are same. Those who are in the borderline are expected to undergo the test for a second time. Some other medical conditions like abnormalities of thyroid, pituitary glands, infection of pancreas, etc. can also cause positive sweat test, but they are easily identified by their clinical conditions. Moreover, molecular tests can be done to confirm cystic fibrosis.

It will take 30 minutes to one hour to conduct sweat test, and is considered to be 98% accurate. This test does not cause any pain, but some people may feel a tingling sensation due to the passage of electric current. It may cause redness of the test site and excessive sweating for some time, after the test. As such, there is no risk or complications associated with sweat test. In short, sweat test is a simple and painless procedure to diagnose cystic fibrosis. It is also the most common and reliable test for this disease.