Sweat chloride
At a Glance
Why Get Tested?
To diagnose cystic fibrosis (CF)
When to Get Tested?
When a newborn or infant has symptoms that suggest CF, 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?
There is a high level of chloride salt dissolved in the body fluids. Chloride normally travels in and out of the body’s cells as needed
However, in patients with cystic fibrosis (CF), this movement of chloride is abnormal. People with CF have a mutated (abnormal) pair of genes on chromosome number 7. These genes normally produce a protein called cystic fibrosis transmembrane conductance regulator (CFTR) that serves as a channel for chloride to move in or out of cells.
The abnormal movement of chloride in CF affects the production of certain body secretions. 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 abnormality causes mucous secretions to become thick and sticky and the sweat much saltier than normal. This elevation in salt in the sweat can be measured, as a 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 onto a piece of gauze or filter paper. The sweat obtained is then sent to the laboratory for analysis.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
The sweat chloride test is used to test for cystic fibrosis (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?
If the chloride level in the sweat is above a certain level, this is regarded as a positive sweat chloride test, which means 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 evaluated along with 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 (cystic fibrosis transmembrane regulator) 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. However, these conditions are not likely to be confused with CF.
On the other hand, the result can be negative even if the patient does have CF, for example, if there is swelling oedema in the skin, or if the sweat has not been collected properly.
The sweat chloride test should only be performed at specialised centres with experience with this kind of testing. Otherwise, problems in accuracy can arise.
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