The Most Reliable Test

The sweat test is considered the most reliable for diagnosing cystic fibrosis. Sweat tests should be done at a CF Foundation-accredited care center, where guidelines are used to help ensure accurate results. The sweat test is performed by a trained technician and the results are evaluated in an experienced and reliable laboratory.

The sweat test can be done for anyone older than 48 hours. However, some infants may not make enough sweat to do the test. If an infant does not produce enough sweat the first time, the test should be repeated.

If your baby had a positive newborn screening (NBS) or you received a positive prenatal genetic test, it’s important to schedule a sweat test  once your newborn is 48 hours old. At the latest, babies with a positive NBS or prenatal genetic test should have a sweat test performed by the age of 4 weeks to ensure that any health issues or changes can be found early and treated quickly.

What to Expect During a Sweat Test

The sweat test measures the amount of chloride (a component of salt) in the sweat. There are no needles involved in this test. In the first part of the test, a colorless, odorless chemical (pilocarpine) and a little electrical stimulation is applied to a small area of the arm or leg to encourage the sweat glands to produce sweat. A person may feel tingling in the area, or a feeling of warmth. This part of the test lasts about five minutes.

The sweat is then collected on a piece of filter paper or gauze or in a plastic coil. This step lasts for 30 minutes. The collected sweat is then sent to a hospital laboratory to measure how much chloride is in the sweat — usually later the same day. The sweat test usually takes about an hour, but it may take longer. When you schedule the test, ask how long it will take and when you can expect to learn the results.

Preparing for a Sweat Test

There is no activity limit or special diet needed before the sweat test. However, you should not apply creams or lotions to the skin 24 hours before the test. You can continue all regular medications. These will have no effect on the test results.

Babies should be fed their usual amount at their usual times.

Read the CF Foundation's guidelines for sweat testing.

Understanding Sweat Test Results

People with CF have more chloride in their sweat than people who do not have CF.

For a child who has CF, the sweat chloride test results will confirm the diagnosis by showing a high chloride level. A baby has to sweat enough to do the test. Full-term babies usually produce enough sweat by 2 weeks of age. The test should be done as soon as possible between 10 days and, at the latest, 4 weeks of age for babies who have had a positive NBS or prenatal genetic test. 

Typically, sweat chloride values do not change from positive to negative or negative to positive, as a person grows older. Sweat test results also do not vary when a person has a cold or other brief illness. If a sweat test is done correctly, then results that are positive will show a high chloride level.

To understand what the sweat test results mean, a chloride level of:

• Less than or equal to 29 mmol/L = CF is unlikely regardless of age*.
• Between 30 - 59 mmol/L = CF is possible and additional testing is needed.
• Greater than or equal to 60 mmol/L = CF is likely.

*Although a sweat test result of less than 29 mmol/L makes CF unlikely, there are CFTR mutations associated with sweat test results of less than 29 mmol/L. 

Some conditions and factors are associated with a false positive result. A clinician who specializes in CF can meet with you to interpret your child’s results, discuss what they mean, and explain next steps.

When sweat chloride test results fall between the range of 30-59 mmol/L, the sweat test is usually repeated.  

If your child had a positive NBS and has a sweat test of 30-59, you should consult with an CF clinician who can explain the results and recommend further testing. Further testing may be recommended for those whose sweat test results fall into the intermediate range and whose genetic analysis determines unknown mutations or that their CFTR genotype is undefined. If further testing is unavailable or found to be inconclusive, the diagnosis cannot be resolved, and they may be considered to have CFTR-related metabolic syndrome. 

When the Sweat Test Confirms a CF Diagnosis

First of all, remember that you are not alone. There is a strong community of people with CF, other parents and families, and multidisciplinary teams of health care professionals who make up the CF Foundation’s care center network to combine clinical research with medical care best practices to help you and your baby. Thanks to this network — and to aggressive research and new therapies — people with CF are living longer and healthier lives.

There are also new medications called cystic fibrosis transmembrane conductance regulator (CFTR) modulators designed to treat the underlying cause of CF in people with certain CF gene mutations, and similar therapies are in development for more people living with CF.

Thanks to the community of people living with CF and their families who participate in CF Foundation-supported research and work with care teams at CF Foundation-accredited care centers across the country, there are more options available to treat CF than ever before. These include drugs to help clear CF mucus out of the airways, reduce inflammation, fight germs, and help your child grow.

A diagnosis of CF can lead to many different emotions. Understanding how to cope with these emotions is the first step in learning how to raise a happy, healthy child.

• The American Academy of Pediatrics has information on a variety of children's health topics.
• Baby's First Test has information about newborn screening at the local, state and national level.
• NewSTEPs is a resource for state newborn screening programs.
• The March of Dimes has information on newborn screening tests.
• KidsHealth has information about health, behavior and development from before birth through the teen year.
• View a collection of newborn screening videos from the Genetic Alliance.

You should contact a CF Foundation-accredited care center as soon as possible. The health care professionals at these centers will help you learn more about CF and become your partners in creating a plan to keep your baby healthy.

What Can I Do To Keep My Baby Healthy? 

The most important thing is to work closely with the CF health care professionals at your care center. They will work with you to develop a plan for your baby's health — including avoiding germs, eating smart, taking digestive enzymes, and keeping lungs clear of mucus.

Your baby will need proper nutrition to remain as healthy as possible and follow a daily treatment routine to stay healthy and maintain optimal lung function.