The sweat test measures the concentration of chloride that is excreted in sweat. It is used to screen for cystic fibrosis (CF).
Cystic fibrosis may be diagnosed by many different methods including newborn screening, sweat testing, and genetic testing. As of 2006 in the United States, 10 percent of cases are diagnosed shortly after birth as part of newborn screening programs. The newborn screen initially measures for raised blood concentration of immunoreactive trypsinogen. Infants with an abnormal newborn screen need a sweat test to confirm the CF diagnosis. In many cases, a parent makes the diagnosis because the infant tastes salty. Trypsinogen levels can be increased in individuals who have a single mutated copy of the CFTR gene (carriers) or, in rare instances, in individuals with two normal copies of the CFTR gene. Due to these false positives, CF screening in newborns can be controversial. Most states and countries do not screen for CF routinely at birth. Therefore, most individuals are diagnosed after symptoms (e.g. sinopulmonary disease and GI manifestations) prompt an evaluation for cystic fibrosis. The most commonly used form of testing is the sweat test. Sweat-testing involves application of a medication that stimulates sweating (pilocarpine). To deliver the medication through the skin, iontophoresis is used to, whereby one electrode is placed onto the applied medication and an electric current is passed to a separate electrode on the skin. The resultant sweat is then collected on filter paper or in a capillary tube and analyzed for abnormal amounts of sodium and chloride. People with CF have increased amounts of sodium and chloride in their sweat. In contrast, people with CF have less thiocyanate and hypothiocyanite in their saliva (Minarowski et al.) and mucus (Banfi et al.). CF can also be diagnosed by identification of mutations in the CFTR gene