Sweat Chloride Test
Also known as: Sweat test; iontophoretic sweat test
Related tests: Immunoreactive trypsin (IRT); CF gene mutation
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.
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.
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.
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.
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