Sweat Chloride Test
What is the sweat chloride test?
The sweat chloride test is a common and simple test used to
evaluate a patient who is suspected of having cystic fibrosis (CF),
the most common lethal genetic disease affecting Caucasians. If not
diagnosed previously, CF is often clinically suspected when there is
poor growth during infancy or recurrent serious intestinal or
respiratory diseases in a toddler or young child.
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.
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 minute (painless) electrical current that allows the penetration of
a medication which maximizes sweat stimulation.
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?
Usually about a half hour to an 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.
What are normal sweat chloride levels?
The normal sweat chloride values are 10-35 milliequivalents per
liter.
What is the sweat chloride level in CF?
Patients with CF usually have a sweat chloride value greater than
60 milliequivalents per liter.
What if the sweat chloride is intermediate (between normal and
the usual CF levels)?
Intermediate values between 35 and 60 milliequivalents per liter
may be seen in some CF patients (and in some normal children). In
those cases, the sweat chloride test should be repeated in the very
near future.
Can a CF child have a normal sweat chloride level?
In a severely malnourished patient with CF, the sweat chloride
level may be normal. However, once the malnutrition is corrected, the
test becomes positive.
Can the sweat chloride be high and the child not have CF?
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 CF by their clinical
condition and molecular tests for CF can be done to clarify the
diagnosis.
Last Editorial Review: 4/24/2002
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