What is a Sweat test?
The sweat test measures the amount of chloride (a component of salt) in the sweat. There are no needles involved in this test. In the first part of the test, a medicine (pilocarpine) is applied to a small area of the arm or leg to encourage the sweat glands to produce sweat. Then little electrical stimulation is given with small electrodes. A person may feel tingling in the area, or a feeling of warmth but no pain. This part of the test lasts about five minutes.
Then the child is asked to wait for 30 minutes. The sweat is then collected on a piece of filter paper or gauze or in a plastic coil. The collected sweat is then sent to a hospital laboratory to measure how much chloride is in the sweat. The sweat test usually takes about an hour, but it may take longer. There is no special preparation required for the test.
When do children need sweat test?
Sweat test is done on a child if he/she shows symptoms of a disease called Cystic Fibrosis. Some of the symptoms suggestive of cystic fibrosis are recurrent chest infections, wet cough, not gaining weight, loose stools, anemia, skin rash etc.
Infants who test positive on newborn screening also need sweat test between 10 days to 4 weeks of life.
Understanding Sweat test results
Children with cystic fibrosis have high chloride content than children who don’t have cystic fibrosis. To understand what the sweat test results mean, a chloride level of:
- Less than or equal to 29 mmol/L = CF is unlikely regardless of age
- Between 30 – 59 mmol/L = CF is possible and additional testing is needed
- Greater than or equal to 60 mmol/L = CF is likely to be diagnosed
When sweat chloride test results fall between the range of 30-59 mmol/L, the sweat test is usually repeated. If it remains intermediate the second time, genetic testing is advised.
Genetic Testing for CF
If sweat test results are intermediate i.e. Chloride level is between 30-59mmol/L, genetic testing for mutations responsible for causing CF is done. For this test only a blood sample is required.
