The sweat test is one of the most important methods for diagnosing cystic fibrosis. This test measures the level of chloride in sweat.
Cystic fibrosis is a genetic disease, which is characterized by defective exocrine glands, including those producing mucus and sweat. The function of the sweat glands include secretion and reabsorption of sodium chloride. Those affected with cystic fibrosis have defective sweat glands, which do not absorb 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. When compared to normal, healthy people, those with cystic fibrosis have higher levels of sodium and chloride content in their sweat. When it comes to diagnosis of cystic fibrosis, the sweat test is highly valued. Nowadays, this test is part of the newborn screening program in many countries.
The sweat test for cystic fibrosis is usually performed on the forearm. In case of infants, the site for the test can be the thighs or even the back. This test can be conducted on a two-day-old baby too, but it should be repeated, as newborn babies may not produce enough sweat. Apart from that, their sweat contains very low amount of sodium chloride. Go through the following points for a better understanding about the sweat test procedure.
- 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 30 to 40 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). The level of sodium and chloride in the sweat, is also determined.
Interpretation of Results
Adults and Kids: If the concentration of chloride is higher than or equal to 60 mmol/L, the result is positive. If it is less than 40 mmol/L , the result is negative, and the range between 40 to 60 mmol/L is considered borderline.
Infants (0 to 6 months): A chloride concentration of less than 30 mmol/L is considered normal for infants. If the concentration is greater than or equal to 60 mmol/L, the result is indicative of cystic fibrosis. The borderline range is 30 to 60 mmol/L.
Those who are in the borderline are expected to undergo the test for a second time. Some other medical conditions like abnormalities of thyroid and pituitary glands, and infection of the pancreas can also cause a positive sweat test, but they can be easily identified by their clinical conditions. Moreover, molecular tests can be done to confirm cystic fibrosis.
It will take 30 minutes to an hour to conduct the sweat test, which is considered 98% accurate. This test does not cause any pain, but some people may feel a tingling sensation due to passage of electric current. The test site may remain red and continue sweating for some time, after the test. As such, there is no risk or complications associated with this 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.
Disclaimer: This article is for informational purposes only and should not be used as a replacement for expert medical advice.