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My Son's Sweat Test for Cystic Fibrosis

The author's son undergoes a sweat test to rule out cystic fibrosis.

The author's son undergoes a sweat test to rule out cystic fibrosis.

What Is a Sweat Test?

A sweat test is a standard method of determining whether or not an individual has a condition known as cystic fibrosis. Individuals with this condition excrete a high level of chloride ions in their sweat, and the disorder is diagnosed when the level of chloride exceeds normal levels.

This test was developed in the 1950s when Paul di Sant’Agnese noticed an increase in salt levels for people with cystic fibrosis (CF). The first test was slightly dangerous, as children with CF are prone to excessive salt loss—children were overheated to gather the sweat required for analysis. In 1959, Gibson and Cooke developed a novel method of performing the sweat test. With the use of a chemical called pilocarpine, the sweat glands could be dilated and sweat could be induced using a mild electrical current. This type of testing is called pilocarpine iontophoresis, and provides a safe, consistent method of determining the concentration of chloride ions in sweat.

Manifestations of Cystic Fibrosis

Cystic fibrosis affects many different body systems.

Cystic fibrosis affects many different body systems.

When Is a Sweat Test Ordered?

The vast majority of sweat tests are ordered after a baby's newborn screening test shows a high Immunoreactive Trypsinogen (IRT) level, a genetic screen shows one or more genes for cystic fibrosis. Infants who do not pass the newborn screening test will be referred for a sweat test: this test can typically be performed by the age of 2 weeks, though some babies may not produce enough sweat until they are slightly older than 2 weeks of age.

Other infants are referred for sweat testing when early symptoms are present: a newborn with meconium ileus (a bowel obstruction in the newborn period), rectal prolapse, failure to thrive, and /or frequent respiratory infections may be referred for sweat testing.

Older children are typically referred due to symptoms: frequent respiratory infections, sinus infections, growth failure, greasy stools, and pancreatic insufficiency are common indicators for testing.

My own son was referred for a sweat chloride test at the age of 5 years. He has had chronic sinus and ear infections (5 sets of ear ventilation tubes placed by the age of five) and is failure to thrive (severe reflux and chronic diarrhea). His pediatrician felt the symptoms warranted further investigation, and wanted to rule out cystic fibrosis. There are over 1,600 mutations for cystic fibrosis, and the symptoms vary according to the specific mutation.

Cystic Fibrosis Symptoms

While many people think of respiratory symptoms when they hear the words "cystic fibrosis," the disorder affects many systems and may present in different ways. Common symptoms include:

  • Salty tasting skin
  • Failure to thrive
  • Respiratory infections
  • Sinus infections
  • Nasal polyps
  • Rectal prolapse
  • Meconium ileus (bowel obstruction) in the newborn period
  • Chronic diarrhea (malabsorption)
  • Cirrhosis of the liver
  • Diabetes
  • Infertility due to congenital absence of the vas deferens in males

How the Sweat Chloride Test Is Performed

Fortunately, the test is not difficult to perform and the results are typically available within the next 24 hours. While sweat collection devices differ (some testing centers use filter paper/gauze to collect the sweat, while others use a plastic coil), the method is similar:

  1. Pilocarpine is placed onto the child's arm and a mild electrical current is applied. This portion of the test takes 5 minutes.
  2. The site is cleaned and the sweat collection device is placed over the site. The arm is wrapped. This portion of the test takes 30 minutes.
  3. The process is repeated on the other arm.
  4. After 30 minutes, the collection device is removed and the sweat is analyzed.

My son sat still for the test and watched the timer count down as the electrical current was applied. The sensation is a strong tingling feeling, similar to when one's foot falls asleep. The child does not feel a shock, though the sensation may be unpleasant.

Understanding the Results: Positive, Negative, and Indeterminate

Two sets of sweat chloride test result criteria exist, based on the age of the patient. Children under the age of six months will have a negative sweat chloride result if the value is less than 30 mmol/L. For children older than six months, a sweat chloride value less than 40 mmol/L is considered negative for cystic fibrosis.

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A negative result on the sweat chloride test indicates that cystic fibrosis is extremely unlikely.

Babies less than six months of age who obtain sweat chloride values between 30 - 59 mmol/L have an indeterminate result. Patients older than six months of age who have a sweat chloride value of 40-59 mmol/L also have an indeterminate result.

An indeterminate result on the sweat chloride test indicates that cystic fibrosis is possible. In this case, the child's symptoms will be reviewed to obtain a clinical picture. In addition, genetic testing is often ordered to determine if there are disease-causing mutations present on the CFTR gene (the gene that causes cystic fibrosis). The genetic test results and clinical symptoms will determine whether or not the child has cystic fibrosis.

Some children will receive a diagnosis of cystic fibrosis-related metabolic syndrome, or CRMS. CRMS is a newly recognized entity in children with borderline sweat chloride values. Patients with CRMS are generally monitored by a cystic fibrosis clinic on a routine basis.

For all patients, a sweat test with chloride values greater than 59 mmol/L is considered positive for cystic fibrosis. A positive result on the sweat chloride test indicates that cystic fibrosis is nearly certain.

Cystic Fibrosis Sweat Test Results

Chloride Levels (mmol/L)Patient AgeInterpertation


6 months and younger

Negative (CF is very unlikely)


6 months and younger

Indeterminate (CF is possible)


6 months and younger

Positive (CF is probable)


>6 months of age

Negative (CF is very unlikely)


>6 months of age

Indeterminate (CF is possible)


>6 months of age

Positive (CF is probable)

Coping With the Results

The results from a sweat test are typically available within 24 hours. The time waiting for the test results is generally filled with anxiety.

In our son's case, I called our pediatrician the morning after the sweat test was performed. It took several hours for the nurse to return our call with the results: our little boy had a negative sweat chloride test. He does not have cystic fibrosis.

While the test did not give us a reason for his poor growth (he is feeding-tube dependent), chronic diarrhea, and chronic sinus and ear infections, it did rule out cystic fibrosis as a cause. This test was important because our treatment strategy would have changed if he had received a positive sweat chloride result.

Many parents receive indeterminate results, and living in the "gray area" of a diagnosis is extremely difficult. Some children with indeterminate results will later prove to have cystic fibrosis. Others will be diagnosed with CRMS. Still, others will be found to be unaffected by cystic fibrosis.

Some children will receive a positive result from the sweat chloride test. Genetic testing is often performed to find the specific mutations and routine care from an accredited cystic fibrosis center is required. With early detection and multiple, promising new drugs on the frontier, early intervention means a brighter future for those diagnosed with CF.

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.


Leah Lefler (author) from Western New York on August 29, 2014:

Hazel, Nolan was finally diagnosed with Ehlers Danlos Syndrome, a connective tissue disorder. He has laryngomalacia and very severe reflux, so he has chronic ear infections. He has had a Nissen Fundoplication and his ear and sinus infections have reduced dramatically following this surgery. He still gets them occasionally, but we are fortunate to see a reduction. He has had 5 sets of ear tubes in his young life!

hazelbrown from Central PA on August 29, 2014:

I teach two boys with CF, and while they're doing really well, it's still so stressful for their family. I'm glad Nolan tested negative! Have you had any success figuring out what's going on with him? I hope so!

Leah Lefler (author) from Western New York on May 15, 2013:

He is a very brave little boy, teaches12345 - he has been through quite a lot in his 5 years of life! We are grateful for the negative result on the cystic fibrosis test, though we are still at a loss for what is causing his various health problems. We have an appointment with a pediatric specialist who might be able to help us.

Dianna Mendez on May 12, 2013:

What a sweet child! I only wish I could give him a hug for the courage he has in facing such a test. Thank you for the education and background on this important method of testing for Cystic Fibrosis. Prayers and hugs sent your way.

Leah Lefler (author) from Western New York on April 17, 2013:

The test is fairly short and non-invasive - while it was uncomfortable for Nolan, it has been one of our easier tests to go through. It is so important to rule out (or in) cystic fibrosis if a child has symptoms! I am glad your son received a negative result, hyphenbird. Going through any medical test is stressful, particularly when you have to wait for results.

Brenda Barnes from America-Broken But Still Beautiful on April 17, 2013:

Your Hub is fantastic. My little boy had the test when he was about 15 months old. He is 10 now so I do not remember a lot about the procedure. His was also negative. I am so happy your son's test was negative and pray he will be healed and begin to thrive. As always, your writing is impeccable, interesting and informative.

Leah Lefler (author) from Western New York on April 16, 2013:

We were quite concerned about Nolan, Sheri - while we don't have a family history, he has respiratory issues, GI issues, and a mixed hearing loss (partly due to constant sinus and ear infections). It is a good thing to rule out for anyone with constant upper respiratory or GI troubles. The sweat tester at our local CF center said the oldest person she has diagnosed was 54 years old! There are over 1600 mutations and the symptoms depend on the specific mutations.

Sheri Dusseault from Chemainus. BC, Canada on April 16, 2013:

My nephew has CF so after he was diagnosed all the children in our extended family had to have that test. Thank goodness no one else has it. He just received a lung transplant and is doing very well! This is well done and will be so useful to people going through it.

Leah Lefler (author) from Western New York on April 16, 2013:

It is one of the most common genetic disorders for those of European descent, rasta1 - our son was tested because he was showing symptoms similar to cystic fibrosis and we needed to determine whether or not he had the disorder. His test was negative, which rules it out as a potential cause for his problems!

Marvin Parke from Jamaica on April 16, 2013:

Never heard of it until now. Thanks for the contribution.

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