What is a sweat test for cystic fibrosis?
A sweat test, also called a sweat chloride test, is used to diagnose cystic fibrosis (CF). The test measures the amount of salt in a person’s sweat and is considered the best method for diagnosing CF. The test is for all ages, but it’s often done on babies and young children.
- A sweat test follows up on a screening test done on all babies at birth: A common reason for needing a sweat test is to follow up after a newborn screen (a blood test done on all babies at birth) flags “out of range” for cystic fibrosis. An out-of-range result means a diagnosis of CF is possible, but we need more information. Most babies with an out-of-range result do not have CF. To get more information, a sweat test is recommended before a child is 4 weeks old.
- A sweat test might be recommended for other indications, including CF symptoms: If your child is older, a healthcare provider may recommend a sweat test because of symptoms that are connected to CF, including frequent lung and sinus infections, chronic cough, and coughing up blood.
What happens during a sweat test?
The sweat test takes about one hour. It is performed by placing a small disc on a child’s forearm and stimulating sweat. The sweat is then collected in a plastic wristband. Your child may feel a tingling in their arm, but the test is not painful. There are no serious risks. The testing equipment does not pierce skin. In some cases, if too little sweat is collected, the test may need to be repeated.
Preparing your child for a sweat test
- There are no restrictions on activity or diet before the test.
- For a newborn baby: Continue to feed your child on their regular schedule. Please bring a bottle to offer. We recommend also bringing a blanket because keeping your baby warm gives us the best chance to collect enough sweat.
- For an older child: Make sure your child is drinking plenty of fluids the morning of the test. Please bring warm clothes. If your child is a toddler or older, it may be helpful to bring an electronic tablet or age-appropriate toy to distract them during the test.
- Please, do not apply creams or lotions to the skin 24 hours before the test.
- All regular medications may be continued and will have no effect on test results.
- If your child has a fever 24 hours before the test, please contact our lab.
Understanding sweat test results
People with CF have an increased amount of chloride (salt) in their sweat. There are four possible outcomes of the sweat test:
- Negative, which is not suggestive of CF
- Indeterminate, which is a borderline result
- Positive, which is consistent with a CF diagnosis
- Quantity not sufficient (QNS), which means there was not enough sweat collected to produce a result
Sweat test following an out-of-range newborn screen for CF: If the sweat test took place after an out-of-range newborn screen, you will receive the result of the test that day. Results will also be reviewed at a later date with a genetic counselor through a telehealth appointment. If the result is quantity not sufficient (QNS) or indeterminate, it will need to be repeated in four to six weeks.
Sweat test for other indications, including CF symptoms: If the sweat test was ordered because of specific symptoms, the results will be sent to the healthcare provider who ordered the test. If the result is quantity not sufficient (QNS) or indeterminate, it may need to be repeated.
How to schedule a sweat test at Boston Children’s:
An order is needed from your healthcare provider to schedule a test. Please have your child’s healthcare provider fax a sweat test order to our pulmonary lab at 617-730-6224.
You or your child’s provider may call our team to schedule a sweat test. Please call our lab at 617-355-7510 and ask to schedule a sweat test.
Sweat test following an out-of-range newborn screen for CF: Your child’s primary care provider will make you aware that a sweat test is recommended. Please let us know if the reason for the sweat test is for an out-of-range newborn screen.
For providers
Please fax all sweat test orders to 617-730-6224. Include the patient’s demographic information and an active phone number.
Newborn screen for CF
If you have questions about a follow-up sweat test for an out-of-range newborn screen, please call our pulmonary lab at 617-355-7510 and let them know you have questions about a newborn screen sweat test. We are happy to help.
Key facts
- We recommend that sweat tests following a newborn screen occur within the first four weeks after birth. This allows early treatment, if needed, and is consistent with current recommendations from the Cystic Fibrosis Foundation.
- Babies of all races and ethnicities can have CF. An “out-of-range” newborn screen should prompt a timely referral for a sweat test in every single infant to determine if a child has CF. Delays in referral may result in healthcare disparities.
- An out-of-range newborn screen for CF is not a diagnosis of CF. It indicates a child is either a carrier or has CF. Most babies who flag out of range on a newborn screen are carriers. A sweat test is needed for further information.
About the process
- The New England Newborn Screen program (NENSP) contacts the identified primary care provider for an infant with an out-of-range newborn screen for CF.
- The primary care provider will update the family with the result.
- Either the primary care provider or the family will call to schedule the sweat test.
- All newborn babies with an out-of-range newborn screen for CF will also be scheduled for a visit with a genetic counselor to review results of the sweat test.
- If a patient misses a sweat test appointment, our center will call the family to reschedule and update the referring primary care provider.
