The sweat test is used for the diagnosis of a disease called cystic fibrosis (CF) or mucoviscidosis.
The test has no direct relation to allergy or asthma. It is performed sometimes as part of the differential diagnosis of asthma, allergic rhinitis, sinusitis or food allergy. Symptoms of CF may resemble those of these conditions, and the test is performed to rule out CF. Furthermore, a person may have CF in addition to those conditions.
The sweat in persons with CF has a higher concentration of salt (sodium chloride). Two electrodes are placed in the forearm, and a mild electric current is applied, which stimulates the secretion of sweat. This is collected and the concentration of chloride is measured. The result is immediately available.
Values below 30-40 mEq/L, depending on age, are considered normal. Values over 60 mEq/L are associated to diagnosis of CF. Values between 30-40 and 60 are inconclusive, and may need repetition of the test. The interpretation of the test is done considering other clinical data and other tests. There are other medical conditions that can yield false positive or false negative results in the test.
The test is painless and takes around 30 minutes. No specific preparation is needed if you are scheduled for a sweat test.
Patients with CF can develop chronic and very severe respiratory symptoms. For more information, visit websites specifically dedicated to CF.
Angel Mazon MD, PhD