How Do Doctors Diagnose Cystic Fibrosis?

Medically Reviewed by Renee A. Alli, MD on June 05, 2021

Doctors use many different tests to confirm that you or a loved one has cystic fibrosis (CF). These include tests that check your blood and sweat, and sometimes your stool. Your symptoms will also need to be reviewed by a doctor who specializes in this genetic disease.

There are a number of ways to diagnose CF. This article examines the most common.

Newborn Screening

All 50 states and the District of Columbia screen babies for severe health conditions such as CF. This is often done with a blood test a few days after birth.

A doctor or nurse will draw a few drops of blood from your baby’s heel and send the sample to a lab. There, it’s checked for a chemical made by the pancreas called immunoreactive trypsinogen (IRT).

The IRT-based test is just a screening test -- it’s not meant to make a diagnosis. Most babies who “fail” the IRT screen don’t have cystic fibrosis -- they just have a higher risk and need another test to confirm. If results from the second test are normal, there’s no further risk or need for more testing.

Chloride Sweat Test

A “sweat test” is thought to be the most reliable way to tell if someone has CF. It checks the amount of salt in your sweat. People with CF have higher levels of chloride, a compound in salt.

This test can be done on people of any age. But some babies might not sweat enough and may need to be tested more than once.

To do it, a doctor rubs a small area of the skin on your arm with a special gel that activates the sweat glands. They’ll use a weak electrical current to push this medicine into the skin. It doesn’t hurt, but may cause a tingling, warm feeling.

Sweat will collect for 30 minutes in a special sweat collection device that is attached to your arm, and then the sample will be sent to a lab. High amounts of salt will confirm CF.

Carrier Testing

Cystic fibrosis is caused by a faulty gene that’s passed down from both parents. You can carry this gene without having symptoms. But if you and your partner are both carriers, there’s a risk that your baby will be born with CF.

If you’re thinking about starting a family, your DNA can be checked to see if you have this gene. To do this, your doctor will take a sample of your blood or swab the inside of your cheek to get a sample of cells.

You may want this test if:

  • A close family member (like a sibling or parent) has CF
  • A close family member is known to be a carrier of the CF gene
  • Your partner is a carrier

A genetic counselor -- someone who is trained in diseases that run in families -- can explain your results to you. They can also help explain your options if your results show that you’re a carrier.

Prenatal Testing

“Carrier testing” is also offered to newly pregnant women and their partners. If the results show that you both have the CF gene, then you may decide to have further testing to check the health of your fetus.

This can be done by:

Amniocentesis: Your doctor checks the amniotic fluid (which surrounds your baby in the womb) for any signs of birth defects. This is often done between the 15th and 20th week of pregnancy.

Chorionic villus sampling (CVS): A sample of tissue from the placenta can also be checked for the faulty CF gene. You can have this done between your 10th and 12th week of pregnancy.

Adult Testing

If your doctor thinks you might have CF and you weren’t tested at birth, they may suggest that you have both a genetic test and a sweat test. Repeated bouts of pancreatitis (inflamed pancreas), nasal polyps, or frequent sinus and lung infections, are some things that may trigger your doctor’s concern.

Show Sources


National Heart, Lung, and Blood Institute: “How is Cystic Fibrosis Diagnosed?”

Mayo Clinic: “Cystic Fibrosis.”

Cystic Fibrosis Foundation: “Newborn Screening for CF,” “Testing for CF,” Sweat Test.”

KidsHealth: “Cystic Fibrosis (CF) Chloride Sweat Test.”

March of Dimes: “Cystic fibrosis and pregnancy.”

NHS: “Causes of Cystic Fibrosis,” “Diagnosing Cystic Fibrosis.”

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