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, infant or child 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, following abnormal results in the newborn screening (blood spot) test for CF.
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. Chloride's level usually mirrors that of sodium.
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 elevated immunoreactive trypsinogen (IRT) and CFTR gene mutations routinely 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 by specially trained staff. Sweat production is stimulated with a chemical the sweat glands called pilocarpine delivered to these glands by an electrical current. The sweat produced for approximately 30 minutes following stimulation is collected and sent to laboratory for analysis. This should be a painless procedure that may create a tingling or warm sensation.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed. However, you may be instructed to avoid applying creams or lotions to your skin 24 hours before the test. It is also important to be well-hydrated by drinking plenty of water before undergoing the procedure.
Can I have a sweat chloride test done at my doctor's office?
It is unlikely because this test should only be performed at specialised centres with staff specifically trained in the this test.
If one of my children has CF and a positive chloride sweat test, should my other children be tested?
You are best to talk to your doctor and a genetic counsellor about testing the other siblings for CF gene mutations, regardless of whether they have symptoms or not, to determine whether they are carriers.
What other tests may my doctor recommend if my child has CF?
Your doctor may do other tests, such as chest X-rays as well as lung function and pancreas function testing, including faecal elastase, to find out how severe your child’s CF is and to guide them in recommending a treatment plan and/or a referral to a special CF centre where your child can get expert help, advice and monitoring.