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What is the Sweat Test?

What is the sweat test?

The sweat test is called a “diagnostic test.” It is the most accurate way to tell whether or not your baby has CF. It is sometimes called the “gold standard” test for CF. The sweat test measures how much salt is in your baby’s sweat. Remember, most babies who need to have a sweat test because of an abnormal newborn screen do not have CF. But it is important to be sure that they do not.

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Some parents try to figure out whether their baby has CF by tasting their baby’s skin to see if it’s salty. This is not an accurate test. Most babies, regardless of whether they have CF, taste salty because everyone’s sweat contains some salt.
Do all babies who receive a positive (abnormal) screen for CF need to have a sweat test?

Yes. If your baby’s screening test was “abnormal” or “positive” for CF, the baby needs to have a sweat test to find out if she or he has CF.

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When I was pregnant, I had a blood test to see if I was a CF carrier and it was negative. Now my baby has a positive (abnormal) screen for CF. Does my baby still need a sweat test?

Yes. Any baby who had a positive (abnormal) newborn screen for CF should have a sweat test. Standard prenatal genetic screening tests look for only the most common CF gene mutations, not all of them.

You could carry a gene mutation that was not included in the test you had when you were pregnant. Remember, most babies who have a positive (abnormal) screen for CF do not have CF. But, it is important to be sure that they do not have CF. This is true regardless of whether your test for being a CF carrier was negative.

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What does the sweat test measure?

A technician collects sweat from your baby’s arms or legs and then measures the salt in the sweat. If the amount of salt is high (referred to as abnormal or positive results), then the baby has CF. If the amount is low (referred to as normal or negative results), the baby does not have CF. Most of the time test results are clearly low or high. However, sometimes results are in the middle and another sweat test and/or possibly other testing is needed.

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Where do we go for a sweat test?

The sweat test should be done at a CF Foundation-accredited care center. This test can usually be done within the first few weeks of your baby’s life. If you have questions about or need help scheduling the sweat test, talk with your baby’s doctor.

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How should we prepare our baby for the sweat test?

On the day of the sweat test, do not use any lotions, creams or moisturizing soaps on your baby’s arms or legs. Bring an extra blanket or sweater and hat to keep your baby warm so he or she will sweat during the test. Be sure your baby has been fed. Doing so may help your baby sweat more. Your baby also may be able to feed during the test.

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What does the test involve?

The sweat test does not hurt. You can stay with your baby throughout the procedure. A special gel is put on your baby’s skin on an arm or leg. Small patches with wires cover the gel for about 5 minutes. This causes your baby to sweat. The skin may feel warm and tingly while the patches are in place, but it is not painful. The sweat is collected on a gauze pad or disk. After about 30 minutes, the gauze or disk is removed and taken to a laboratory to measure the salt. The sweat test takes about one hour from start to finish.

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Who should go to the sweat test appointment?

It would be helpful if your spouse/partner comes with you to the appointment. Many CF care centers also offer parents genetic counseling at the time of the sweat test. This counseling can help you understand the genetics of CF and what it means for you and your partner as well as for your baby in the future.

If you have other young children, it might be best to leave them with a babysitter or bring someone to watch them at the appointment. This will allow you to talk with the genetic counselor or health care provider with fewer distractions.

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What should we do while waiting to get the sweat test?

We recognize that it can be very stressful for parents to wait for the sweat test. Therefore, staff at CF care centers make every effort to schedule sweat tests as quickly as possible. While even a few days' wait can add to your worry and anxiety, your baby is not in any immediate danger.  Call your baby's doctor if you have any concerns about your baby's health or well-being while you are waiting for the sweat test.

If you have questions or concerns while you are waiting for the sweat test appointment, we encourage you to call the CF Foundation-accredited care center and ask to speak with a nurse or genetic counselor who can answer your questions.

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How soon can we expect the results of the sweat test?

Results of the sweat test should be available to you and your doctor within a few days.

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What do the sweat test results mean?

The sweat test looks at how much salt is in your baby’s sweat. Salt is made up of two parts — sodium and chloride. The sweat test measures the amount of chloride (Cl-) in a person’s sweat.

There are four possible results of a sweat chloride test:

  • Negative result: This means that a normal amount of salt (less than 30 mEq/L Cl-) was found in your baby’s sweat. It is very rare for a person to have CF if the sweat test result is negative.

  • Positive result: A positive sweat test means that there is too much salt (greater than 60 mEq/L Cl-) in the sweat and your baby most likely has CF. A second sweat test may be done and an appointment should be made at a CF Foundation-accredited care center that specializes in treating people with CF. Your doctor can refer you to the closest CF center or you can find a care center on the CF Foundation’s website.

  • Intermediate (borderline) result: Sometimes the sweat test result falls between positive and negative (greater than 29 but less than 60 mEq/L Cl-). This means that your baby should be seen at a CF Foundation-accredited care center to have another sweat test in a few weeks and again after six months of age. Additionally, genetic testing may be done at this time.

Babies with a sweat test that is in this range may be diagnosed with CF after a repeat sweat test and genetic testing. It is also possible that the repeat sweat test and genetic testing show that it is unlikely for the baby to have CF. In many cases, however, the repeat sweat testing remains in this range, and the genetic test results are also uncertain.

In this situation, the baby’s condition is described as CFTR-related metabolic syndrome (CRMS). Babies with CRMS may never develop any symptoms of CF, or may have one or two symptoms of CF that appear later in life. Newborns with CRMS should be brought to a CF Foundation-accredited care center for follow-up testing at least once a year in addition to their routine pediatric care.

  • Quantity Not Sufficient (QNS): This means that there was not enough sweat in the gauze or disk to measure the amount of salt. Your baby will need to come back on another day to repeat the sweat test

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The lab didn’t get enough sweat (QNS) – now what?

This sometimes happens because young infants have immature sweat glands. It has nothing to do with whether or not your baby has CF. You will need to talk to your baby’s health care provider to see what next step he or she recommends.

Depending on the results of the newborn screen, your provider might say it’s okay to wait to do the sweat test later.

Or, your provider might suggest other tests such as DNA or genetic testing. It can be very frustrating for parents when the first tests do not answer the question of whether their baby has CF.

It is important for you to know that the amount of sweat a baby produces has no bearing on whether or not the child has CF.

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Can any other test diagnose CF?

The sweat test is recommended as the first diagnostic test for CF. But sometimes a sweat test cannot be done successfully or in a timely manner. Reasons include an infant’s premature birth; a hospital or clinic that does not routinely perform sweat tests; not enough sweat collected during the sweat test; or intermediate (borderline) results from the sweat test.

In these cases, the baby’s doctor or the CF care center staff might suggest a genetic test instead of, or in addition to, the sweat test. Genetic testing can be done by collecting a small amount of blood from the baby and sending it to a special lab to check for CF gene mutations. It can take several weeks before the results are available.

If your health insurance does not cover genetic testing, a member of the CF care center can talk with you about your options. For more on the guidelines for diagnosing CF, see the CF Foundation’s consensus report, “Guidelines for Diagnosis of Cystic Fibrosis in Newborns through Older Adults.”

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If my baby doesn’t have CF, why was the newborn screen positive (abnormal)?

If your baby’s newborn screen for CF was abnormal but the baby’s diagnostic test (sweat test) was normal, your child does not have CF. We call this situation a “false positive” newborn screen. Reasons for false positives can include prematurity and a lot of stress during delivery. Babies who have one CF gene mutation may also have a positive newborn screen but not have cystic fibrosis. Anyone with one CF gene mutation is said to be a "carrier of CF."

It is important to remember that newborn screening is done to make sure that all babies with CF are found as soon as possible. It also finds babies who do not have CF. It is a good thing that your baby doesn't have CF, but a positive newborn screen also causes a lot of stress for many parents.

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What if the sweat test shows my baby doesn’t have CF but the NBS results showed that he or she is a carrier or has one CF gene mutation?

Some babies with an abnormal newborn screening result do not have CF but are carriers of one CF gene mutation. People who carry only one CF gene mutation do not have CF. They are fine and usually do not know they are CF carriers.

If your baby is a carrier of CF, it also means that at least one parent is a CF carrier and both parents could be carriers. If you are both CF carriers and you plan to have more children, there is a chance that you could have a child with CF in the future. In time, your child should learn about what it means to be a CF carrier (especially in terms of childbearing).

To learn more about being a CF carrier and what you can do, see the "What does it mean to be a carrier?" section.

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Additional Resources

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Updated 1/27/14

The Cystic Fibrosis Foundation is an accredited charity of the Better Business Bureau.