The sweat electrolyte test for cystic fibrosis (CF) involves testing the sweat for irregular salt levels. Learning about the procedure can help a parent or caregiver understand what to expect when a baby has a sweat test.

CF is a genetic condition that causes a buildup of fluid in multiple organs, including the lungs, skin, pancreas ducts, bile ducts, and small intestine. It occurs due to a mutation in the cystic fibrosis transmembrane regulator (CFTR) protein.

The CFTR protein helps regulate the transportation of chloride throughout the body. When a mutation occurs, the function no longer works properly, leading to an imbalance between sodium and chloride levels.

Scientists found that a person living with CF had atypical sweat and developed the sweat electrolyte test a few years later, in 1959. Since then, doctors have used the test primarily to check for cases of CF.

This article explores the sweat electrolyte test in more detail, including how it works, who may need it, and more.

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The primary purpose of a sweat electrolyte test is to check for and diagnose CF. The test looks for irregularities in salt levels in a person’s sweat.

The procedure is noninvasive and is effective for infants and children. A healthcare professional can administer the test on the majority of infants over 48 hours old, but they will typically schedule it to occur before the age of 4 weeks.

Early testing for CF can help doctors identify CF and advise on treatments as early as possible.

Learn about cystic fibrosis in children.

The sweat electrolyte test checks for irregularities in the amount of chloride, a component of salt, in a person’s sweat. A healthcare professional collects a sample of sweat from the baby to send to a lab for analysis.

The lab returns a value for the chloride levels found in the sweat. A low reading of less than 30 millimoles per liter (mmol/L) is a typical range. Levels higher than 60 mmol/L indicate CF. Anything between those values is borderline and requires additional testing.

A doctor may order a sweat electrolyte test for a few reasons. These include:

  • the infant or child presents with symptoms that could indicate CF
  • an infant had a positive CF screening test
  • parents test positive as a genetic carrier

When doctors suspect an infant may have CF, they will order the test to occur within the first 4 weeks of life. The current recommendation is to perform the testing after the child reaches 10 days old.

Symptoms that may indicate the need for testing can include:

Learn more about cystic fibrosis.

The sweat electrolyte test is noninvasive. It does not involve needles and should not cause discomfort or pain for the infant.

To perform the test, a healthcare professional will first apply pilocarpine, an odorless and colorless chemical, to the skin of the arm or leg. They will then apply a small amount of electrical stimulation. This stimulates the skin to produce sweat. This first part lasts about 5 minutes.

The next part of the test involves collecting the sweat using filter paper, a plastic coil, or gauze. This will last about 30 minutes.

Once the healthcare professional collects enough sweat, they will send the sample to a lab for analysis. The whole test typically takes about 1 hour. A parent or caregiver will often receive the results the same day, but it is best to check the timeframe with a doctor.

A parent or caregiver does not usually need to change much about the child’s routine before the sweat test. They can usually continue to eat, drink, and take medications as usual.

However, it is best to avoid applying creams or ointments to the infant’s skin for 24 hours before the test. These may interfere with the results.

A person can ask the doctor ordering the test if they have any special preparation instructions.

A doctor can use the results of the lab test to determine the next steps. The values returned fall into one of three categories:

  • chloride levels that are less than 30 mmol/L indicate that CF is not present
  • chloride levels between 30–60 mmol/L are inconclusive, and further testing is typically necessary
  • chloride levels over 60 mmol/L usually indicate CF

The sweat electrolyte test is painless and does not involve needles. The infant may feel a slight tingling sensation or warmth.

For safety, experts recommend healthcare professionals use battery-operated devices only. Otherwise, they consider the test safe and reliable in the diagnosis of CF.

Doctors use the sweat electrolyte test to check for and diagnose cystic fibrosis (CF). The test is typically painless and noninvasive.

A doctor can order the test for young infants or children. Reasons they may order the test include the person showing symptoms of CF, or a parent or infant has received positive screening results.