Cystic Fibrosis Genetic Carrier Testing

Medical technology now offers information about the future health of individuals. Using genetic information, tests can be offered to find out if a person might have a child who may have certain diseases or health care needs. Genetic testing looks at the genes that a person has to learn if that person carries an altered gene for cystic fibrosis. An altered gene also may be called a CF mutation. For someone to have this disease, two copies of an altered CF gene must be present-one from each parent. A "carrier" has only one copy of an altered CF gene and therefore does not have CF.

Genetic carrier testing can be used to tell if a person carries one of the altered genes that causes CF. The test looks at a person's DNA (genetic material), which is taken from cells in a blood sample or from cells that are gently scraped from inside the mouth. There are more than 1,000 known alterations of the gene that causes CF. Current tests look for the most common CF gene alterations. A negative test means the person does not carry most of the alterations, but the person still may carry a rare CF gene.

The altered genes that are looked for by the test vary according to a person's race or ethnic group, or if CF occurred already in the family. About 95% of Americans with CF are white or Caucasian. One person in every 29 people who are Caucasian are carriers of an altered CF gene. In other race or ethnic groups, one in 46 Hispanic Americans, one in 65 African Americans and one in 90 Asian Americans carry an altered CF gene. If you have a relative with CF, or who is known to carry the altered CF gene, your chance of carrying the gene is greater because of your family's history. If you are pregnant or planning to have a child, you should discuss this test and the results with a health professional who is knowledgeable about genetic testing, such as a genetic counselor.

What the Results Mean

A positive genetic carrier test for CF means that a person has an altered copy of the CF gene; this result is more than 99% accurate. A "negative" carrier test, which says that someone does not carry the CF gene, is not as accurate. With more than 1,000 different alterations or mutations of the CF gene, there are some rare ones that the test does not identify. If your test is negative for an altered CF gene, there is still a small chance you could be a carrier of one of these rare mutations. This chance depends on your race or ethnic group and the type of genetic carrier test you receive.

One Altered CF gene

If you have one copy of the altered CF gene, it is possible your child may have CF. However, your child will only have CF if your partner also is a carrier of an altered CF gene. If you are both carriers, each baby you have together has a 25% chance of having CF. If one partner is a carrier and the other partner tests negative for the altered gene (or if there is no information on whether or not they carry the altered gene), there is still a slight chance the child could have CF. Your doctor or a genetic counselor can tell you about your chances of having a child with CF.

When No One Has Had the Disease

Since one copy of an altered CF gene does not cause symptoms, this copy can be passed down to family members without any impact on their health. Unless they have a child with CF, most people who carry an altered CF gene do not know they carry it. Once parents have a child with CF, all of their children should be tested for CF. Other relatives, such as brothers, sisters or cousins, have a chance of being CF carriers and might want to have carrier testing.

Getting the Test

Your decision to get a genetic test to learn if you carry an altered CF gene is a difficult and personal choice. You may want to talk to your medical or religious advisors to help you decide.  The American College of Obstetricians and Gynecologists (ACOG) suggests that all couples who are thinking about having a baby — or those who are already pregnant — should get genetic carrier testing for CF. Although Caucasians have a higher risk of carrying the altered CF gene, ACOG favors testing for anyone who requests it. 

Inheriting the Altered Gene

People inherit genes from their parents. To inherit CF, a child must get one copy of the altered CF gene from each parent. In other words, the child must have two copies of the altered gene to have CF. A person who has only one altered gene does not have CF, but is considered a "carrier" of the CF gene. Both males and females may inherit the disease.

Types of Altered Genes Affect Severity

The type of altered CF gene can affect the type of symptoms of CF. But genetic testing cannot tell how severe a person's CF will be in advance. The most common altered gene, delta F508, causes the most common CF symptoms. But, this altered gene can vary in how severely it affects someone with CF. Some less common mutations of the CF gene may cause milder symptoms.  

Genetic Testing for Diagnosis

Genetic tests can diagnose CF, but these tests are often used to find out if a person carries an altered gene. The standard test to diagnose CF is called the sweat test. It measures the salt in a person's sweat. People with CF have saltier sweat than most people. If someone is diagnosed with CF through a sweat test, a genetic test can show what the altered CF genes are. Learn more about the sweat test.

Tests Before a Baby’s Birth

A baby can be tested for CF before birth. If the mother and father are both carriers of altered CF genes, or if the mother is a carrier, genetic testing for CF can be done before birth. To learn more, ask your doctor, obstetrician, midwife or genetic counselor.

Outlook for People with CF

When scientists found the CF gene in 1989, they gained a great tool for finding new CF treatments and, someday, a cure. There is exciting research to change the gene that causes CF, and other research to fix the symptoms of CF. Today, there are new treatments that help many people with CF to live full, active lives and more treatments are coming in the future.

Other Resources


Content adapted with permission from the Cystic Fibrosis Foundation.