Sweat Chloride Test
At a Glance
Why Get Tested?
To diagnose cystic fibrosis (CF)
When to Get Tested?
When a newborn or infant has symptoms that suggest cystic fibrosis, such as frequent respiratory infections and chronic cough, persistent diarrhoea, foul-smelling bulky greasy stools, and malnutrition; and as a follow-up, confirmatory test to help diagnose CF
Sample Required?
A sweat sample collected using a special sweat stimulation procedure
The Test Sample
What is being tested?
Sodium and chloride are part of your body’s electrolyte balance. They help regulate the fluid balance in your blood and tissues. Normally, chloride travels in and out of the body’s cells, helping to maintain electrical neutrality and water balance. Chloride’s level usually mirrors that of sodium.
When a patient has cystic fibrosis (CF), they have inherited an abnormal (mutated) pair of genes on chromosome number 7. These genes normally produce a protein (cystic fibrosis transmembrane conductance regulator (CFTR)) that lets chloride out of cells and into the surrounding fluid. When a mutation is present, the CFTR protein may not work properly or may be totally absent. Since CFTR levels are usually highest in the cells lining the internal surfaces of the pancreas, sweat glands, salivary glands, intestine, and reproductive organs, these are the areas most affected by CF.
The cells absorb more water than normal, making liquid produced by the cells thick and sticky and the sweat about five times saltier than normal. This increase of salt in the sweat can be measured, and is called the sweat chloride test.
How is the sample collected for testing?
A sweat sample is collected using a special sweat stimulation procedure. A tiny amount of a clear, sweat-stimulating liquid is applied to a small patch of skin on the arm or leg. An electrode is then placed over the site and a weak electrical current stimulates the area. This is a painless procedure that may create a tingling or warm sensation. After several minutes, the area is cleaned and sweat is collected for about thirty minutes, either into a plastic coil of tubing or a piece of gauze or filter paper is used for this purpose.. The sweat obtained is then sent to the laboratory for analysis.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
The sweat chloride test is used to test for CF in symptomatic patients. It is also used to help confirm or rule out a diagnosis of CF in patients who have had positive or equivocal (uncertain) results using other tests. It is not used to monitor CF since elevated levels do not correlate with the severity of the disease or its symptoms.
When is it requested?
The sweat chloride test is performed when a patient (usually an infant) has symptoms of CF, such as noticeably salty sweat, frequent respiratory infections and chronic cough, persistent diarrhoea, foul-smelling bulky greasy stools, or malnutrition, and/or has a close relative who has been diagnosed with CF. The sweat chloride test may also be used to help confirm a diagnosis in patients who have an elevated immunoreactive trypsin test (IRT).
CF gene mutation analysis tests for the most common CF gene mutations (of almost 1000 possible mutations). If the analysis is negative, then the patient may not have CF or they may have a rarer mutation not tested for. In this situation, sweat chloride testing may detect CF even if the genetic mutations causing it cannot be identified.
What does the test result mean?
A positive sweat chloride test indicates that there is a good chance that the patient has CF. Positive sweat chloride tests are usually repeated for verification and confirmed, wherever possible, with a positive CF gene mutation analysis.
A negative sweat chloride test must be considered alongside the patient’s clinical findings. A few people with CF will have normal sweat chloride levels and will have to be evaluated using other tests.
The sweat test may not always be useful in newborns. They may not produce enough sweat for a reliable diagnosis, so the test may have to wait until the baby is several months old. Other situations where the sweat test is not recommended would be where the child is dehydrated or where there is a skin problem such as eczema covering the area where sweat would be collected from. The sweat chloride test may not need to be done if the patient has an elevated IRT and a CF gene mutation is identified.
Is there anything else I should know?
The sweat chloride test will not pick up carriers of cystic fibrosis. It takes two mutated (abnormal) copies of the CFTR gene to cause cystic fibrosis. Those who are only carriers, and therefore have one normal and one abnormal gene, will not have sweat chloride abnormalities.
There are other conditions besides CF that can cause positive sweat tests. These include Addison’s disease, nephrogenic diabetes insipidus, and hypothyroidism. Conditions, such as oedema in the skin, or poor collection technique can result in false negatives.
The sweat chloride test should only be performed at specialised centres with experience with this kind of testing. Otherwise, problems in accuracy can arise.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
