Genetic testing plays an important role in cystic fibrosis screening and diagnosis. It can help determine if a person has cystic fibrosis or carries gene mutations that cause it. Other tests can also help diagnose cystic fibrosis.

Cystic fibrosis (CF) is an inherited condition where the effects of a genetic change lead to the accumulation of thick, sticky mucus in the body.

This mucus builds up and can cause blockages and damage that affect many bodily systems, such as the lungs, digestive system, and liver.

According to one 2020 study, about 40,000 people in the United States have CF.

While cystic fibrosis used to cause death in childhood, people’s outlooks are consistently improving. For example, the life expectancy of people with CF born between 2018 and 2022 is now expected to be 56 years, according to the Cystic Fibrosis Foundation.

The improved outlook for people with CF is due to advances in screening and treatment. Genetic testing plays a big role both in identifying people with genetic changes that cause CF and in diagnosing cystic fibrosis.

Genetic changes, often called mutations, in the CFTR gene cause CF. The CFTR gene gives your cells instructions on how to make CFTR protein.

CFTR protein regulates the movement of chloride into and out of your cells and is involved in mucus production. In people with CF, the CFTR protein doesn’t work properly. The result is thicker, stickier mucus.

Genetic testing works by identifying mutations in the CFTR gene that cause CF. There are currently more than 2,000 known CFTR gene mutations that can lead to CF.

Genetic testing uses a sample of blood, saliva, or cells rubbed from the inside of your cheek. Prenatal genetic tests may use a sample of placenta or amniotic fluid. Once collected, genetic testing samples are sent to a lab for analysis.

When would someone request genetic testing?

You may request genetic testing if CF runs in you or your partner’s family. This can let you know if one or both of you carry a mutation that causes CF. Testing can help you assess the chance of future children having CF or being a carrier.

Additionally, if you or your partner has CF or carries a mutation that causes it, you may also request a prenatal genetic test. This can help determine whether your baby has CF or is a carrier.

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CF is inherited in an autosomal recessive manner. This means you need two copies of a mutated CFTR gene, one from each parent, to have CF.

You can also have only one copy of a mutated CFTR gene while the other copy is normal. This is called being a carrier. While carriers don’t develop CF, their children can still potentially inherit a copy of the mutated CFTR gene.

Carrier screening is genetic testing that can see whether you are carrying a mutated CFTR gene. You may request carrier testing if CF runs in you or your partner’s family.

How accurate is carrier screening?

Research estimates that just a few CFTR mutations contribute to almost 85% of all cases of CF. Because of this, CF carrier screening typically only tests for 23 of the most common CFTR mutations.

While this means testing accurately detects CFTR mutations in many cases, people with rare CFTR mutations can have a false-negative result. These people won’t know that they’re a carrier unless more expansive genetic testing is done.

CF is most common in white, non-Hispanic people and much rarer in other racial and ethnic groups. The American College of Obstetricians and Gynecologists (ACOG) notes that the sensitivity of carrier screening also varies accordingly.

For example, ACOG points out that the sensitivity of carrier screening in white, non-Hispanic people is 88%. Meanwhile, sensitivity in Asian Americans, a population in which CF is much less common, is 49%.

If I’m a carrier, what does that mean for potential children?

The table below gives you an idea of what being a carrier may mean for potential children.

Carrier statusPotential of having children with CF
only one parent is a carrierChildren have a 50% chance of being a carrier.
one parent is not a carrier and one parent has CFAll children will be carriers.
both parents are carriersChildren have a 50% chance of being a carrier and a 25% chance of having CF.
one parent is a carrier and one parent has CFChildren have a 50% chance of being a carrier and a 50% chance of having CF.

If the results of carrier screening reveal that you or your partner is a carrier, you can work with a genetic counselor to get a better idea of the chance of your child being a carrier or having CF.

Genetic testing can also see whether a developing fetus has CF or is a carrier. This is often done in situations where there’s a chance of a child being born with CF, such as if:

  • both parents are carriers
  • one parent is a carrier and the other has CF

According to ACOG, prenatal testing can be done as early as 10 weeks of pregnancy. Two methods may be used:

  • Chorionic villus sampling (CVS) can be done between weeks 10 and 13 of pregnancy. It uses a small sample collected from the placenta to detect mutations in the CFTR gene.
  • Amniocentesis is typically done between weeks 15 and 20 of pregnancy. It uses a thin needle to collect a sample of amniotic fluid. The cells present in this sample are then tested for CFTR mutations.

Prenatal testing for CF has a high degree of accuracy. However, the test may not detect rare CFTR mutations and cannot predict the severity of CF.

While the risk is very low, there are complications associated with CVS and amniocentesis. These include:

As such, experts are developing noninvasive prenatal testing methods for CF.

In addition to genetic testing, other tests can also help diagnose CF.

Newborn screening

In the United States, all newborns are screened for CF within the first few days of life. This screening can identify more than half of newborns with CF.

During newborn screening, a few drops of blood are collected from a prick on the baby’s heel. The sample is then placed on a special card that’s tested in a lab.

The test looks at levels of an enzyme called immunoreactive trypsinogen (IRT), which tend to be higher in people with CF.

In some U.S. states, genetic testing may also be done if a baby’s IRT levels are high.

Sweat chloride test

Another test that may be used as a part of CF diagnosis is the sweat chloride test. It’s done by adding an odorless, colorless chemical to a small area of the arm or leg and applying a little electrical stimulation.

The sweat that forms is sent to a lab for testing. People with CF typically have higher levels of chloride in their sweat.

Genetic testing can determine if a person has cystic fibrosis (CF) or carries gene mutations that cause it. Genetic testing works by looking for the presence of mutations in the CFTR gene known to cause CF.

Carriers have one copy of a mutated CFTR gene. While carriers do not have CF, their children can inherit the CFTR mutation. People with a family history of CF may choose to do carrier screening before conceiving.

Prenatal genetic testing can also see whether a developing fetus has CF or is a carrier. This may be done if there’s a chance a child may have CF, such as when both parents are carriers.