A sweat test helps diagnose cystic fibrosis. It measures the amount of salt chemicals (sodium and chloride) in sweat. People with cystic fibrosis have 2 to 5 times the normal amount of sodium and chloride in their sweat, much of which is not reabsorbed properly and so remains on the surface of their skin.

During the sweat test, medication that causes a person to sweat is delivered through the skin. The sweat is then collected on a paper or a gauze pad and the amount of salt chemicals in the paper or gauze is measured in a lab. Generally, chloride is measured.

Sweat Test
Results

Adults generally have higher sodium and chloride concentrations in their sweat than children. Also, sweat test results in adults can vary widely. This is especially true in women, because the amount of salt in their sweat can vary with the phase of their menstrual cycle. Enough sweat must be collected to get accurate test results.

Normal values may vary from lab to lab. Sweat Chloride must be measured to diagnose cystic fibrosis. Some labs also measure sodium. Normal and abnormal sweat sodium values vary slightly from sweat chloride values. 

Greater than normal values may mean

• Sweat chloride levels greater than 60 mmol/L are generally diagnostic of cystic fibrosis. Some people with cystic fibrosis have borderline or even normal sweat chloride levels.
• Some other conditions that can cause a higher-than-normal amount of salt in sweat include:
  • Poorly functioning adrenal glands (adrenal insufficiency).
  • Hypothyroidism.
  • A condition caused by an inability to keep normal levels of water in the body (diabetes insipidus).
  • Kidney failure.
  However, the sweat test is never used to diagnose these conditions.