Sweat Chloride Test
A sweat sample collected using a special sweat stimulation procedure
None. However, you may be instructed to avoid applying creams or lotions to your skin 24 hours before the test. Also, it is important to be well-hydrated before undergoing the procedure.
Sodium and chloride are part of your body’s electrolyte balance. They help regulate the fluid balance in your blood and tissues. Normally, chloride travels in and out of the body’s cells, helping to maintain electrical neutrality and water balance. Chloride’s level usually mirrors that of sodium.
When a patient has cystic fibrosis (CF), they have inherited an abnormal (mutated) pair of genes on chromosome number 7. These genes normally produce a protein (cystic fibrosis transmembrane conductance regulator (CFTR)) that lets chloride out of cells and into the surrounding fluid. When a mutation is present, the CFTR protein may not work properly or may be totally absent. Since CFTR levels are usually highest in the cells lining the internal surfaces of the pancreas, sweat glands, salivary glands, intestine, and reproductive organs, these are the areas most affected by CF.
The cells absorb more water than normal, making liquid produced by the cells thick and sticky and the sweat about five times saltier than normal. This increase of salt in the sweat can be measured, and is called the sweat chloride test.
How is the sample collected for testing?
The sweat sample will be collected by specially trained nurses or laboratory staff. It is collected using a special sweat stimulation procedure.
Two gel discs containing pilocarpine (a chemical that stimulates the sweat glands) are placed on a clean area of skin on the arm or leg. Alternatively liquid pilocarpine may be applied to the skin surface. In order to get the pilocarpine into the skin, the area is stimulated using a small electrical current from a torch battery. This takes about five minutes.
Then the area is cleaned and a small plastic device resembling a wristwatch is strapped over the area where one of the gel discs was in contact with the skin. This is left in place for about 30 minutes to collect the sweat. Alternatively the sweat may be collected into filter paper.
The ‘wristwatch’ device, or filter paper, containing the sweat is sent to the laboratory where the sweat will be weighed and the concentration of chloride will be measured.
Is any test preparation needed to ensure the quality of the sample?
No special 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 before undergoing the procedure.
How is it used?
When is it requested?
The sweat chloride test is performed when a patient (usually an infant) has symptoms of CF, has a close relative who has been diagnosed with CF or has had a positive or equivocal (uncertain) result from a blood spot in the Newborn Screening programme. CF symptoms include noticeably salty sweat, frequent respiratory infections and chronic cough, persistent diarrhoea, foul-smelling bulky greasy stools, or malnutrition.
CF gene mutation analysis tests for the most common CF gene mutations. If the analysis is negative, then the patient may not have CF or they may have one of the, almost 1000, rarer mutations not tested for. In this situation, sweat chloride testing may detect CF even if the genetic mutations causing it cannot be identified.
What does the test result mean?
A positive sweat chloride test indicates that there is a good chance that the patient has CF. Positive sweat chloride tests are usually repeated for verification and confirmed, wherever possible, with a positive CF gene mutation analysis.
A negative sweat chloride test must be considered alongside the patient’s clinical findings. A few people with CF will have normal sweat chloride levels and will have to be evaluated using other tests.
The sweat test may not always be useful in newborns. They may not produce enough sweat for a reliable diagnosis, so the test may have to wait until the baby is several months old. Other situations where the sweat test is not recommended would be where the child is dehydrated or where there is a skin problem such as eczema covering the area where sweat would be collected from.
Is there anything else I should know?
The sweat chloride test will not pick up carriers of cystic fibrosis. It takes two mutated (abnormal) copies of the CFTR gene to cause cystic fibrosis. Those who are only carriers, and therefore have one normal and one abnormal gene, will not have sweat chloride abnormalities.
The sweat chloride test should only be performed at specialised centres with experience with this kind of testing. Otherwise, problems in accuracy can arise.
If one of my children has CF and a positive sweat chloride test, should my other children be tested?
Yes, if they are symptomatic. If not and/or if the genetic mutations causing your child’s CF are known, your doctor may prefer to test the brothers or sisters (siblings) for these specific mutations. This will let them know if your children have CF or are just carriers. Carriers are not ill and do not need treatment, but they may want to consider genetic counselling before they decide to have children themselves. Some doctors do not recommend testing on siblings unless they are symptomatic; testing and counselling for reproductive choices can be done later.
What other tests may my doctor recommend if my child has CF?