What is a Sweat Test?

A sweat test is a medical test used to measure the amount of chloride excreted from the body along with sweat. The amount is measured over a specific period of time and is used to determine whether a child has, or may have, cystic fibrosis. This disease causes victims to produce increased concentrations of both chloride and sodium in their sweat.

Cystic fibrosis can cause children not to grow, to have digestive problems and to have a variety of serious respiratory problems. Cystic fibrosis was a death sentence in the first half of the 1900s and affected children usually did not live beyond their first year. Improved treatments mean people diagnosed with cystic fibrosis in the 21st century can expect to live into adulthood, though the disease requires constant management.

The excess sodium produced in sweat — as much as two to five times greater than normal — is often the first indicator that there is a problem. Parents often report to the child’s physician that the child tastes salty. This observation is generally enough to cause the doctor to order a sweat test. Such children will frequently have a positive test, resulting in a diagnosis of cystic fibrosis.

A sweat test helps doctors diagnose this disease in children as young as 2 days old, though infants this age may not produce enough sweat for an accurate reading. The test doesn’t require any special preparation, and children who are going to be tested may go about their normal activities until the time of the test. Once at the office or lab, a sweat test will take about an hour.

The procedure for the sweat test is fairly simple. A chemical that causes the child to produce sweat is applied to his or her arm. A weak, painless electrical current is then used to further trigger sweating in the test area. The sweat is then collected and tested for elevated amounts of chlorine and sodium.

Although a sweat test is often used to confirm a diagnosis of cystic fibrosis, it can be inconclusive. In these instances, the doctor may order additional testing to confirm the presence or absence of the disease. Deoxyribonucleic acid (DNA) is sometimes collected from saliva or blood and examined for genetic indicators of cystic fibrosis. No matter how the diagnosis is made, treatment for this disease begins as early as possible. Patient care is a lifelong process.