The sweat chloride test is used to test for cystic fibrosis (CF) in symptomatic people. It is also used to help confirm or rule out a diagnosis of CF in those who have tested positive or indeterminate with other tests, including a screening sweat test, the immunoreactive trypsinogen test (IRT), the trypsin test, and/or the nasal (transepithelial) potential difference (NPD) test.
It is not used to monitor CF since elevated levels do not correlate with the severity of the disease or its symptoms.
It may also be ordered in the case of people who have tested negative with a standard CF gene mutation panel but who are clinically still suspected of having CF.
The CF gene mutation panel generally tests for the 23 most common CF gene mutations. If the panel is negative, then the person may not have CF or may have rare mutations not included in the screening panel.
Sweat chloride testing may be diagnostic of CF even if the genetic mutations causing it cannot be identified. This testing may also be used to determine if additional mutation analysis is warranted.
When is it ordered?
The sweat chloride test is ordered when a person has symptoms of CF and/or has a close relative who has been diagnosed with CF.
Signs and symptoms of CF include:
- Noticeably salty sweat
- Frequent respiratory infections and chronic cough
- Gastrointestinal symptoms – distal intestinal obstruction (meconium ileus in infants)
- Bulky offensive greasy stools
- Male infertility (obstructive azoospermia)
The sweat chloride test may also be ordered to help confirm a diagnosis when someone has tested positive for a CF screening test, such as a positive immunoreactive trypsinogen test (IRT).
It may also be ordered when someone has tested negative with a standard CF gene mutation panel but is clinically still suspected of having CF.
What does the test result mean?
A positive sweat chloride test indicates that there is a good chance that the person has CF. Positive sweat chloride tests should be repeated for verification and confirmed, wherever possible, by CF gene mutation panel testing.
A few people with CF will have a normal or inconclusive sweat chloride test and will have to be evaluated using other tests, or the diagnosis may be made on clinical grounds alone.
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 weeks old.
The sweat test may not need to be done if the baby has an elevated IRT and a positive CF gene mutation test.
Is there anything else I should know?
The sweat test will not pick up carriers of cystic fibrosis. It takes two mutated copies of the CFTR gene to cause cystic fibrosis. Those who are only carriers do not have CF and will not have sweat chloride abnormalities.
There are other conditions besides CF that can cause positive sweat tests. These include anorexia nervosa, Addison’s disease, nephrogenic diabetes insipidus, and hypothyroidism.
Edema can result in a false-negative result.
The sweat chloride test should only be performed at specialized centers with experience with this kind of testing. Otherwise, problems in accuracy, including false negatives due to poor collection technique, can arise.
What is the aim of the sweat chloride test?
The goal of this test is to painlessly stimulate the patient’s skin to produce a large enough amount of sweat which may then be absorbed by a special filter paper and analyzed for the content of chloride in the sweat.
Patients with cystic fibrosis produce larger quantities of sweat chloride than normal individuals.
How is the sweat chloride test done?
To produce the necessary volume of sweat, a technique called iontophoresis is employed. The technique requires the application of a tiny (painless) electrical current that medically stimulates maximum sweat production.
The patient’s forearm is commonly used. However, in small infants, the back may also serve as an appropriate area to perform this procedure.
How long does the test take?
The sweat chloride test usually takes about one-half to one hour.
What is done with the sweat?
The sweat is collected on a specialized filter paper. After determining that enough sweat has been collected to ensure test reliability, the amount of chloride in the sweat is measured.
This is considered a specialized test, and not all institutions have experience with the techniques, and poor collection techniques can result in false negative results.
What are normal sweat chloride levels?
The normal sweat chloride values are 10-35 milliequivalents per liter.
What is the sweat chloride level in cystic fibrosis?
Patients with cystic fibrosis usually have a sweat chloride value greater than 60 milliequivalents per liter.
What if the sweat chloride is intermediate (between normal and the usual cystic fibrosis levels)?
Intermediate values between 35 and 60 milliequivalents per liter may be seen in some cystic fibrosis patients (and in some normal children). In those cases, the sweat chloride test should be repeated in the very near future.
Can a cystic fibrosis child have a normal sweat chloride level?
In a severely malnourished patient with cystic fibrosis, the sweat chloride level may be normal. However, once the malnutrition is corrected, the test becomes positive.
Can the sweat chloride be high without having cystic fibrosis?
There are a few rare conditions which produce a false positive sweat chloride test. Such situations include diseases of the adrenal, thyroid, or pituitary glands; rare lipid storage diseases; and infection of the pancreas.
Generally, however, these children are easily differentiated from patients with cystic fibrosis by their clinical condition, and molecular tests for cystic fibrosis can be done to clarify the diagnosis.