Elevated chloride levels in the skin can be used to diagnosis CF.
The sweat test is considered the “gold standard” for the diagnosis of CF. Though many centers may perform sweat test “screenings”, the only acceptable procedure for diagnosis is called quantitative pilocarpine iontophoresis. This test is performed at CF Centers throughout the country. There are two sweat test methods approved by the Cystic Fibrosis Foundation – the Gibson-Cooke procedure and the Wescor Macroduct® Sweat Collection System. Both methods stimulate localized sweating on the forearm or on the thigh using a chemical called pilocarpine. Sweat is then collected on filter paper or gauze (Gibson-Cooke) or in microbore tubing (Macroduct®). The sweat collected is weighed and analyzed for chloride concentration. Results are interpreted as follows:
Chloride Concentration
Result
< 40 mmol/L
Normal
40-60 mmol/L
Borderline or indeterminate
> 60 mmol/L
Abnormal
An abnormal test is most likely due to CF. To be able to accurately interpret sweat test results, an appropriate volume of sweat must be obtained. Acceptable volumes of sweat are 75 mg for the Gibson-Cooke procedure and 15 µL for the Macroduct® procedure. Sweat testing cannot be performed accurately in the first two days after birth, and it may be difficult to obtain an adequate sweat sample during the first two to three weeks after birth, especially for premature babies. If the volume of sweat obtained is inadequate the test should be repeated.
