Cystic Fibrosis Sweat Chloride Testing

Exam and Test

A Sweat test measures chloride level in sweat and is the standard method for diagnosing cystic fibrosis.

Alternative Names

Sweat test; sweat electrolytes; Iontophoretic sweat test

Dr. Natalie Hayes in the CHKD Cystic Fibrosis Clinic
Dr. Natalie Hayes in the Cystic Fibrosis Clinic.

Why the Test is Performed

People with cystic fibrosis have higher amounts of sodium and chloride in their sweat, which the test can detect.

Some people are referred for testing because of symptoms such as poor growth, frequent respiratory infections, or foul-smelling stools. In some states, newborn screening programs test for cystic fibrosis and the sweat test is used to confirm these results.

How the Test is Performed

In the first part of the test, a colorless, odorless chemical that causes sweating is applied to a small area on an arm or leg. An electrode is then attached to the arm or leg, which allows the technician to apply a weak electrical current to the area to stimulate sweating.

People may feel a tingling sensation in the area or a feeling of warmth. This part of the procedure lasts approximately five minutes. The next part of the test involves cleaning the stimulated area and collecting the sweat on a piece of filter paper or gauze or in a plastic coil. The collected sweat is sent to a hospital laboratory for analysis. The entire collection procedure takes about one hour.

Normal results

Characteristically, sweat chloride concentrations less than 40 mmol/L are normal (does not have CF); values between 40 to 60 mmol/L are indeterminate (borderline), and sweat chloride concentrations greater than 60 mmol/L are consistent with the diagnosis of CF. However, for those infants with newborn screening, sweat chloride concentrations less than 30 mmol/liter are normal. Often tests that are categorized as borderline are repeated for accuracy. Additionally, a CF diagnosis may be confirmed when sweat test are unclear via DNA analysis (a genetic test). A blood sample is collected from the individual with CF or from family members and is sent to a laboratory for DNA/ chromosomal analysis. This test verifies which particular CF mutation may be present in the individual and or immediate family members. However, some patients with cystic fibrosis can have normal sweat chloride test. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

An abnormal test may indicate the presence of cystic fibrosis.