At a glance
Also known as
Sweat chloride; iontophoretic sweat test
Why get tested?
To diagnose cystic fibrosis (CF)
When to get tested?
When a newborn or infant has symptoms that suggest CF, such as frequent respiratory infections and chronic cough, persistent diarrhoea, foul-smelling bulky greasy stools and malnutrition; and as a follow-up, confirmatory test to help diagnose CF in older patients
A sweat sample collected using a special sweat stimulation procedure
What is being tested?
The chloride sweat test measures the amount of sodium and chloride in sweat. Sodium and chloride are part of the body's electrolyte balance and combine to form the salt found in sweat. They help regulate tissue fluid balance. Normally, chloride travels in and out of the body's cells, helping to maintain electrical neutrality and water balance. This movement occurs through a protein, the cystic fibrosis transmembrane conductance regulator (CFTR) that serves as a channel, letting chloride out of cells and into the surrounding fluid and also reducing sodium absorption. Sodium levels thus usually mirror those of chloride.
Cystic fibrosis (CF) is caused by a in each of the two copies of the CFTR (one copy from each parent). With two mutations, the CFTR protein may be dysfunctional or totally absent. Since CFTR protein levels are usually highest in the epithelial cells lining the internal surfaces of the pancreas, sweat glands, salivary glands, intestine, and reproductive organs, these are the areas most affected by CF. Dysfunctional or absent CFTR protein in the exocrine tissues mentioned above causes deranged transport of chloride and other ions such as sodium and bicarbonate. This abnormality causes mucous secretions to become thick and sticky and the sweat much saltier than normal. This elevation in salt in the sweat can be measured, as a chloride sweat test.
Diagnosis of classic CF depends on the presence of clinical disease and an elevated chloride in sweat (≥ 60 mmol/L). There are also people with what is called non-classic CF. They may have milder diseases and only one organ system e.g. the lungs or gut may be affected. They may not be diagnosed until later in childhood or as adults. They often have normal or equivocal chloride sweat test results. In these cases the diagnosis of CF then rests on the results of other tests and the detection of two mutations in the CFTR genes.
Cystic Fibrosis is the most prevalent genetic disorder among Caucasian populations, with a frequency of 1 in 2000-3000 live births. Newborn screening for CFTR gene mutations and Immunoreactive Trypsinogen (IRT) take place in Australia for early detection of Cystic Fibrosis.
How is the sample collected for testing?
A sweat sample is collected using a special sweat stimulation procedure. A tiny amount of a clear, sweat-stimulating liquid is applied to a small patch of skin on the arm or leg. An electrode is then placed over the site and a weak electrical current stimulates the area. This is a painless procedure that may create a tingling or warm sensation. After several minutes, the area is cleaned and sweat is collected for about thirty minutes, either into a plastic coil of tubing or onto a piece of gauze or filter paper. The sweat obtained is then sent to the laboratory for analysis.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.