The sweat test is the standard diagnostic test for cystic fibrosis (CF). It measures the concentration of chloride in your sweat, because people with CF have significantly saltier sweat than people without the condition. A chloride level of 60 mmol/L or higher indicates a CF diagnosis, while 29 mmol/L or lower is considered normal.
Why Sweat Reveals Cystic Fibrosis
Cystic fibrosis is caused by a defective protein that normally moves chloride (a component of salt) in and out of cells. This protein sits in the lining of sweat glands, lungs, and other organs. In healthy sweat glands, the protein reabsorbs salt from sweat as it travels through the sweat duct before reaching the skin surface. The result is relatively dilute, low-salt sweat.
When this protein doesn’t work properly, chloride gets trapped in the sweat duct and can’t be reabsorbed. Sodium follows suit, because its absorption depends on chloride moving first. The end result: sweat that reaches the skin contains much higher concentrations of both sodium and chloride. This is why parents of children with CF sometimes notice their baby’s skin tastes unusually salty, and it’s the biological basis the sweat test relies on.
How the Test Works
The test is painless and takes about an hour from start to finish. It has two phases: stimulating sweat production, then collecting and analyzing the sample.
A small area of skin on the arm or leg is used. The technician places a pad soaked in a chemical called pilocarpine on the skin, along with a second pad, and runs a mild electrical current between them for about five minutes. This process, called iontophoresis, drives the pilocarpine into the skin to activate the local sweat glands. Most people feel a slight tingling or warmth, but no pain.
After stimulation, the pads are removed and a collection device is placed over the area. Most centers in the U.S. now use a small coiled tube (called a Macroduct coil) that needs only 15 microliters of sweat for analysis. The older method using gauze or filter paper requires a larger sample of at least 75 milligrams. Sweat is collected for up to 30 minutes. The sample is then sent to a lab to measure the chloride concentration.
What the Results Mean
The Cystic Fibrosis Foundation defines three result ranges:
- 29 mmol/L or lower: Normal. CF is unlikely.
- 30 to 59 mmol/L: Intermediate. This range doesn’t confirm or rule out CF and typically requires repeat testing or genetic analysis to clarify.
- 60 mmol/L or higher: Positive for cystic fibrosis.
An intermediate result can be stressful, but it doesn’t mean your child has CF. Some people with borderline results carry one copy of a CF gene mutation without having the disease, while others may have an atypical form of CF that needs further evaluation. Genetic testing is often the next step in these cases.
When the Test Is Done
Most sweat tests happen after a newborn screening flags a potential concern. Every state in the U.S. screens newborns for CF using a blood test shortly after birth. If that screening comes back positive, the Cystic Fibrosis Foundation recommends scheduling a sweat test as soon as possible, ideally between 10 days and 4 weeks of age. Babies must be at least 48 hours old before testing.
The test can also be ordered for older children or adults who develop symptoms suggesting CF, such as chronic lung infections, poor growth, very salty skin, or persistent digestive problems.
Why It’s Preferred Over Genetic Testing Alone
You might wonder why a sweat test is needed when genetic testing exists. The answer comes down to coverage. Over 2,000 mutations in the CF gene have been identified, and standard genetic panels only screen for the most common ones. Screening tests sometimes miss CF in babies with Asian or African ancestry, because these populations tend to carry rarer mutations that aren’t included on standard panels. The sweat test catches CF regardless of which specific mutation is involved, making it the more reliable first-line diagnostic tool.
Preparing for the Test
Preparation is minimal. Your child (or you, if you’re the one being tested) should be well hydrated and can eat normally beforehand. A few specific guidelines apply:
- No lotions, creams, or oils on both arms from wrist to shoulder on the day of the test.
- No fever within 24 hours of testing.
- Wear a short-sleeved or loose-fitting shirt so sleeves can be easily rolled up.
- One arm must be free of any IV lines.
Parents need to stay for the full duration of the test. For infants who are hospitalized or on ventilators, the test can sometimes be performed at the bedside. Anyone with a pacemaker or defibrillator should contact the testing center in advance.
Insufficient Sweat Samples
One common frustration, especially with very young infants, is getting a “quantity not sufficient” (QNS) result. This means the baby didn’t produce enough sweat for an accurate reading. A study of 144 CF centers found that in infants 14 days old or younger, the average QNS rate was about 10.5%, with the highest rates in premature babies born before 37 weeks. Still, more than 80% of infants under two weeks of age produce adequate sweat volumes. If your baby’s sample is insufficient, the test simply needs to be repeated, usually after waiting a few days for the baby to grow slightly and produce more sweat.
Importantly, sweat should only be collected from one site during a single test. Chloride concentration varies with sweating rate, so combining samples from different areas of the body could produce inaccurate results.