A sweat test measures the amount of chloride (salt) in your sweat to determine whether you have cystic fibrosis (CF). It is the gold standard for diagnosing CF and is most commonly performed on infants and young children after a positive newborn screening result, though adults can be tested too. The procedure is painless, takes about 45 minutes total, and results are typically available within 24 hours.
Why Chloride Levels Matter
In healthy sweat glands, your body reabsorbs salt from sweat before it reaches the skin’s surface. A specific protein in your cells acts as a channel that moves chloride ions out of the sweat duct and back into the body. In people with cystic fibrosis, this protein doesn’t work correctly. Chloride gets trapped in the sweat duct, and sodium stays with it, so the sweat that reaches the skin contains significantly more salt than normal, often more than 60 milliequivalents per liter.
This is why parents of children with CF sometimes notice their skin tastes unusually salty. The sweat test takes advantage of this biological difference by collecting a small sample of sweat and measuring its chloride concentration precisely.
What Happens During the Test
The test is done on your forearm or leg and involves three phases: stimulation, collection, and analysis.
During stimulation, a technician places two small electrodes on the skin. One is covered with gauze soaked in a chemical that activates sweat glands, and the other with purified water. A small electrical current runs between them for about five minutes. This pushes the chemical into the skin to trigger localized sweating. The current is mild, and most people feel only a slight tingling or warmth.
Once the electrodes are removed, the collection phase begins. A piece of special filter paper, gauze, or a small capillary tube is placed over the stimulated area and secured. Sweat collects for 30 minutes. During this time you (or your child) can move around, play, eat, and drink normally. After collection, the sample goes to a lab where the chloride concentration is measured. Results are generally reported within 24 hours.
Understanding Your Results
Sweat chloride results fall into three ranges:
- Normal (below 30 mmol/L): CF is unlikely.
- Intermediate (30 to 59 mmol/L): The result is borderline and doesn’t confirm or rule out CF. Genetic testing is typically the next step, and the sweat test may be repeated.
- Positive (60 mmol/L or higher): This is consistent with a cystic fibrosis diagnosis. A second sweat test is usually performed to confirm the result.
An intermediate result can be stressful, but it doesn’t mean your child has CF. Some people carry one copy of a CF gene mutation without having the disease, and certain other conditions can influence chloride levels. Genetic testing can clarify the picture by looking directly at the gene responsible for CF.
When the Test Is Done
Most sweat tests are performed on newborns who had an abnormal result on the standard newborn blood screening done in the hospital. In the United States, every state screens for CF at birth, so the sweat test serves as the confirmatory follow-up. It can also be ordered for older children or adults who have symptoms like chronic lung infections, poor weight gain, or unusually salty skin.
For infants, timing matters. Reliable sweat collection requires the baby to be at least 36 weeks gestational age, weigh more than 2,000 grams (about 4.4 pounds), and be at least 3 days old. Babies younger or smaller than this may not produce enough sweat for an accurate reading. Most centers schedule the test when the infant is about 2 to 4 weeks old.
How to Prepare
No fasting or medication changes are required. The only preparation is to avoid applying any creams, lotions, or moisturizers to the test area for 24 hours before the appointment. These products can interfere with sweat collection or dilute the sample. That’s it. You can eat, drink, and take medications as usual beforehand.
Accuracy and Limitations
The sweat test is highly reliable when performed at an accredited lab following standardized procedures. The main reason for an inconclusive result is an insufficient sweat sample, which happens more often in very young or premature infants. If the lab doesn’t collect enough sweat, the test will need to be repeated rather than giving a misleading number.
A small number of conditions other than CF can elevate sweat chloride, including certain metabolic and endocrine disorders. This is one reason a positive sweat test is almost always confirmed with a second test and supported by genetic analysis. Together, these tools give a clear answer in the vast majority of cases.