Sweat test

A simple test that is used to evaluate a patient who is suspected of having cystic fibrosis (CF).

The goal of the test is to stimulate the patient's skin to produce a certain amount of sweat, which may then be absorbed by a special filter paper and analyzed for its chloride content.

In a technique called iontophoresis, a small painless electric current is applied to the forearm or back, allowing penetration of a medication that maximizes sweat stimulation.

Normal sweat chloride values are 10 to 35 milliEquivalents/liter; patients with CF usually have a value greater than 60 milliEquivalents/liter per liter.

Intermediate values (between 35 and 60 milliEquivalents/liter) may be seen in some CF patients and in some normal children.

In those cases, the sweat test should be repeated.

In a severely malnourished patient with CF, the sweat chloride may be normal; when the malnutrition is corrected, however, the test becomes positive.

A few rare conditions that may produce a false positive test include diseases of adrenal, thyroid, or pituitary glands; rare lipid storage diseases, and inflammation of the pancreas.

Children with these conditions are generally easily differentiated from patients with CF by their clinical condition.

Molecular tests for CF can be done to clarify the situation.

Also known as the sweat chloride test.

See also: Cystic fibrosis