A sweat test measures the amount of chloride (a component of salt) in your sweat to diagnose cystic fibrosis (CF). It is the gold standard for CF diagnosis, and it’s most commonly performed on newborns and young children after an abnormal newborn screening result. The test is painless, takes about an hour, and requires almost no preparation.
Why Chloride Levels Matter
Cystic fibrosis is caused by problems with a protein that controls how salt and water move across cell surfaces throughout the body, including the lungs, digestive tract, and sweat glands. In healthy sweat glands, this protein acts as a channel that moves chloride ions out of cells, helping maintain the right balance of salt and water.
In people with CF, genetic mutations cause this protein to malfunction, get produced in insufficient quantities, or not get produced at all. When the protein isn’t working, chloride becomes trapped inside cells instead of moving freely. In the sweat glands specifically, this means chloride can’t be reabsorbed on its way to the skin surface, so it ends up in the sweat at much higher concentrations than normal. That’s what the test detects.
How the Test Works
The sweat test uses a technique called pilocarpine iontophoresis, which is a technical name for a simple process. A technician applies a sweat-stimulating chemical to a small area of skin, usually on the forearm. A mild, painless electrical current helps the chemical absorb into the skin and activate the sweat glands in that spot.
After about five minutes of stimulation, the electrode is removed and replaced with a collection device, either a piece of gauze or a small plastic coil, that sits on the skin and absorbs sweat. The collection period lasts about 30 minutes. The sample is then sent to a lab, where the chloride concentration is measured. The entire visit typically takes about an hour from start to finish.
The electrical current feels like a light tingling or tickling. It does not hurt, and it leaves no marks. Babies and young children tolerate it well.
What the Results Mean
Results are reported in millimoles per liter (mmol/L) of chloride. Three ranges guide the diagnosis:
- Below 30 mmol/L: Normal. CF is unlikely.
- 30 to 59 mmol/L: Borderline or intermediate. This range doesn’t confirm or rule out CF. Additional testing, usually genetic analysis, is needed.
- 60 mmol/L or higher: Consistent with a CF diagnosis. A second sweat test is typically performed to confirm the result.
A borderline result is not uncommon, and it doesn’t necessarily mean your child has CF. Some people carry one copy of a CF gene mutation without having the disease, and certain other medical conditions can occasionally affect sweat chloride levels. Genetic testing can usually clarify the picture when sweat test results fall in this middle range.
When the Test Is Done on Newborns
Most sweat tests are ordered after a newborn screening comes back positive for CF. Newborn screening is a blood test done in the first days of life, and a positive result is a flag for further evaluation, not a diagnosis. The sweat test is the next step.
Newborns need to meet a few criteria before the test can be performed reliably. The baby must be at least 48 hours old, because sweat electrolyte levels can be temporarily elevated in the first day of life and produce misleading results. The baby should also weigh more than 2 kilograms (about 4.4 pounds) and be at least 36 weeks of corrected gestational age. Premature or very small infants may not produce enough sweat for a valid sample.
The Cystic Fibrosis Foundation recommends that newborns with a positive screen have the test done as soon as possible after 10 days of age, ideally before the end of the first month of life. Early diagnosis matters because starting treatment sooner leads to better long-term outcomes for children with CF.
What You Need to Do Before the Test
Preparation is minimal. You should avoid applying any creams, lotions, or moisturizers to the skin for 24 hours before the test, since these can interfere with sweat collection. Beyond that, there are no dietary restrictions and no activity limits. All regular medications can be continued as usual, as they do not affect the results. Babies should be fed at their normal times and in their normal amounts.
Adults and Older Children
While the sweat test is most commonly associated with newborn screening, it can be performed at any age. Some people with milder forms of CF aren’t diagnosed until adolescence or adulthood, particularly if their symptoms were attributed to other conditions like chronic sinusitis, recurring lung infections, or unexplained infertility. The test procedure is identical for adults, and the same chloride thresholds apply.
If you’re an adult being referred for a sweat test, it usually means a doctor noticed a pattern of symptoms that could point to CF or a related condition. The test remains the most reliable first step in confirming or ruling out that possibility, and genetic testing can follow if the results are borderline or positive.
Why a Repeat Test May Be Needed
A single positive result is generally confirmed with a second sweat test, often at a CF Foundation-accredited care center. Occasionally, the first test produces an insufficient quantity of sweat, which means the result is labeled “QNS” (quantity not sufficient) rather than positive or negative. This happens more often in very young or small infants. When this occurs, the test simply needs to be repeated, usually after waiting a few weeks for the baby to grow. An insufficient sample is not a result at all and carries no diagnostic meaning.