Sweat Test for Cystic Fibrosis

Updated on June 18, 2017
leahlefler profile image

My son was referred for a sweat chloride test at the age of 5. He'd had chronic infections, and the doctor wanted to investigate further.

Sweat Chloride Test

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. | Source

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. | Source

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

Sweat Test Pictures

Click thumbnail to view full-size
Preparing the arm: pilocarpine will be placed onto the collection site.The source of the electrical current may look intimidating, but the applied current is mild.Electricity is applied to the pilocarpine, causing the sweat glands to dilate and begin sweating. This portion of the test takes 5 minutes.The same process is applied to the other arm and sweat collection takes place for 30 minutes.Once the test is complete, the patient's arm may be red. The redness will dissipate over the course of the day.
Preparing the arm: pilocarpine will be placed onto the collection site.
Preparing the arm: pilocarpine will be placed onto the collection site. | Source
The source of the electrical current may look intimidating, but the applied current is mild.
The source of the electrical current may look intimidating, but the applied current is mild. | Source
Electricity is applied to the pilocarpine, causing the sweat glands to dilate and begin sweating. This portion of the test takes 5 minutes.
Electricity is applied to the pilocarpine, causing the sweat glands to dilate and begin sweating. This portion of the test takes 5 minutes. | Source
The same process is applied to the other arm and sweat collection takes place for 30 minutes.
The same process is applied to the other arm and sweat collection takes place for 30 minutes. | Source
Once the test is complete, the patient's arm may be red. The redness will dissipate over the course of the day.
Once the test is complete, the patient's arm may be red. The redness will dissipate over the course of the day. | Source

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.

Sweat Testing in Children

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.

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 Age
Interpertation
<30
6 months and younger
Negative (CF is very unlikely)
30-59
6 months and younger
Indeterminate (CF is possible)
>59
6 months and younger
Positive (CF is probable)
<40
>6 months of age
Negative (CF is very unlikely)
40-59
>6 months of age
Indeterminate (CF is possible)
>59
>6 months of age
Positive (CF is probable)

Cystic Fibrosis Foundation Video

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.

Questions & Answers

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      • leahlefler profile imageAUTHOR

        Leah Lefler 

        4 years ago from Western New York

        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 profile image

        hazelbrown 

        4 years ago from Central PA

        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!

      • leahlefler profile imageAUTHOR

        Leah Lefler 

        5 years ago from Western New York

        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.

      • teaches12345 profile image

        Dianna Mendez 

        5 years ago

        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.

      • leahlefler profile imageAUTHOR

        Leah Lefler 

        5 years ago from Western New York

        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.

      • Hyphenbird profile image

        Brenda Barnes 

        5 years ago from America-Broken But Still Beautiful

        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.

      • leahlefler profile imageAUTHOR

        Leah Lefler 

        5 years ago from Western New York

        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 Faye profile image

        Sheri Dusseault 

        5 years ago from Chemainus. BC, Canada

        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.

      • leahlefler profile imageAUTHOR

        Leah Lefler 

        5 years ago from Western New York

        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!

      • rasta1 profile image

        Marvin Parke 

        5 years ago from Jamaica

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

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