What is the Sweat Test for Cystic Fibrosis?
A Sweat Test, also called a Sweat Chloride Test, measures the amount of chloride (a component of salt) in sweat. It is considered the standard laboratory test for confirming cystic fibrosis (CF) when the condition is suspected or when a newborn screening suggests possible CF.
Cystic fibrosis is an inherited disease caused by mutations in the CFTR gene. Normally the CFTR protein moves chloride in and out of cells. When the protein is not working properly, chloride gets trapped inside cells instead of moving in and out of sweat glands as it should. As a result, people with cystic fibrosis have higher-than-normal chloride levels in their sweat.
Although the test is mainly used to diagnose cystic fibrosis, the result is always interpreted together with symptoms, newborn screening results, family history, and, when appropriate, genetic testing.
Why Is the Sweat Test Considered the Standard Test for Cystic Fibrosis?
Many people assume that a genetic test alone confirms cystic fibrosis. Often, however, diagnosis also includes genetic information combined with evidence that the CFTR protein is not functioning properly.
The sweat test measures how CF affects salt transport in the body, not the gene itself. Thus, it remains the gold standard diagnostic test for cystic fibrosis, even where genetic testing is available.
Some people have rare CFTR gene changes that require both genetic testing and sweat chloride testing for an accurate diagnosis.
What are the Symptoms that Prompt this Test?
Your healthcare provider may recommend a Sweat Test for Cystic Fibrosis if you or your child experiences:
- Chronic cough
- Frequent lung infections
- Wheezing or shortness of breath
- Production of thick mucus
- Good appetite but poor weight gain
- Failure to thrive of infants and children
- Frequent loose or bulky stools
- Chronic diarrhoea or constipation
- Salty tasting skin
- Chronic sinus infections or nasal polyps
Cystic fibrosis is usually diagnosed in infancy or childhood, but some people with milder forms of the disease may not be diagnosed until adolescence or adulthood.
How is the Test Performed?
Preparation
- Do not put any cream or lotions on the skin before the test.
- Drink lots of fluids before the test to ensure you are well hydrated.
- Discuss with your healthcare provider any medications your child or you are taking.
- Follow any other directions given prior to the test.
Collection
The test involves a special technique for collecting sweat from the skin.
- A colourless chemical is placed on a small area of the skin, usually on the arm or leg.
- Mild electrical stimulation is used to encourage sweating. It may feel warm or tingly but it doesn’t hurt.
- Sweat is collected on filter paper, on gauze or in a special collection device.
- Collected sweat sample is then sent to the lab for analysis.
The entire process usually takes 30 to 60 minutes.
What are the Normal Ranges for This Test?
The Sweat Test for Cystic Fibrosis measures the amount of chloride in sweat and reports the result in millimoles per liter (mmol/L).
General reference ranges include:
- Sweat Test for Cystic Fibrosis Normal Range: Less than 30 mmol/L
- Sweat Test for Cystic Fibrosis Intermediate or Borderline Range: 30 to 59 mmol/L
- Sweat Test for Cystic Fibrosis Higher or Positive Range: 60 mmol/L or higher
In some cases, repeat testing or additional investigations may be required if the results fall within the borderline range.
“The reference ranges vary depending on the laboratory and the methodology used. Individuals must consult their healthcare provider for correct interpretation of the results.”
What Does an Intermediate Result Mean?
An intermediate (borderline) sweat chloride result does not automatically confirm or exclude cystic fibrosis.
Some people with results in the intermediate range may:
- Have cystic fibrosis
- Have a CFTR-related disorder
- Be carriers of a CFTR gene variant
- Require repeat sweat testing or additional genetic investigations
For this reason, healthcare providers often recommend repeating the sweat test or performing CFTR genetic testing before reaching a final diagnosis.
Can a Normal Sweat Test Completely Rule Out Cystic Fibrosis?
A normal sweat chloride makes cystic fibrosis less likely, but does not exclude the diagnosis.
Those with rare variants of the CFTR gene or a milder form of cystic fibrosis may have normal or near-normal sweat chloride levels.
If the symptoms are strongly suggestive of cystic fibrosis, your health care provider might recommend:
- Repeat sweat test
- CFTR genetic testing
- Other pancreatic or lung function tests
Diagnosis is based on the total clinical picture, and not on a single laboratory finding.
What Happens if Test Results Show Abnormal Levels?
If sweat chloride levels are high or borderline further investigations may include:
• CFTR gene test
•Sweat Chloride test (repeat)
- Follow-up of newborn screening (for infants)
- Pancreatic-function tests
- Stool elastase test
- Lung function testing (as appropriate to age)
- Chest X-ray
- Respiratory cultures
These tests confirm the diagnosis and also establish whether other organs are involved.
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