A sweat test is a diagnostic procedure that measures the amount of chloride, a component of salt, in perspiration. It involves stimulating a small area of skin to produce sweat, which is then collected and analyzed in a laboratory. The test is safe and involves no needles.
Purpose of the Sweat Test
The primary reason a sweat test is performed is to diagnose cystic fibrosis (CF), an inherited condition affecting cells that produce mucus, sweat, and digestive juices. In individuals with CF, a genetic change impacts the cystic fibrosis transmembrane conductance regulator (CFTR) protein. This protein normally regulates the movement of chloride and water across cell membranes, including those in sweat glands. When the CFTR protein does not function properly, chloride cannot move efficiently in and out of cells, leading to unusually high levels of chloride in sweat.
The detection of elevated sweat chloride levels through a quantitative pilocarpine iontophoresis test is considered the gold standard for diagnosing cystic fibrosis in suspected cases. While newborn screening programs in many regions identify potential CF cases, a sweat test is typically needed to confirm the diagnosis.
How the Sweat Test is Performed
The sweat test involves a specific method to induce and collect sweat, typically from a small area on the forearm or leg. The procedure begins with cleaning and drying the chosen skin area. Next, an odorless chemical called pilocarpine is applied to the skin, often soaked into a gauze pad or gel. A weak electrical current is then passed through an electrode placed over the pilocarpine for about five minutes. This process, known as pilocarpine iontophoresis, safely stimulates the sweat glands.
During the electrical stimulation, individuals may experience a mild tingling sensation or a feeling of warmth, but it is not painful. After this initial five-minute period, the electrode and pads are removed, and the stimulated skin area is cleaned again. Sweat is then collected for approximately 30 minutes using a specialized device, such as a plastic coil, filter paper, or gauze, placed directly onto the skin. Once sufficient sweat is collected, the sample is sent to a laboratory for quantitative analysis of its chloride content. The entire test usually takes about an hour to complete.
Understanding Your Results
After a sweat sample is collected, it is analyzed to determine the concentration of chloride, typically measured in millimoles per liter (mmol/L). The results are generally categorized into three ranges, each indicating a different likelihood of cystic fibrosis. A chloride level of 29 mmol/L or less is typically considered normal, suggesting that cystic fibrosis is unlikely. However, a normal result does not completely rule out CF, as some individuals with specific CFTR mutations may have normal sweat test results.
A sweat chloride level between 30 and 59 mmol/L falls into an intermediate or “borderline” range. In such cases, additional testing, such as repeating the sweat test or undergoing genetic testing, is often recommended to clarify the diagnosis. A chloride level of 60 mmol/L or greater is highly suggestive of cystic fibrosis. If a positive result is obtained, confirmatory tests, including genetic testing for CFTR gene mutations, are usually performed to establish a definitive diagnosis. While CF is the primary condition identified by this test, it is worth noting that very rare medical conditions other than CF can sometimes lead to elevated sweat chloride levels.