Testing for cystic fibrosis carriers in families

Dhyuti, 8 months, playing with family

Carrier testing

Carrier testing is the process of testing relatives and partners of someone with CF to see if they are a carrier of a faulty CF gene.

Genetic testing is available for family members of someone who has cystic fibrosis (CF), to find out if they are carriers of CF gene variants (mutations).

Genetic testing is a complicated topic. If you have any questions please discuss it with your GP, or CF team if you have access to one. They can refer you to a genetic counsellor (expert on inherited conditions). Our factsheet on carrier testing also goes into more detail.

Download the factsheet

  • What is carrier testing?

    Carrier testing is a type of genetic test to find out if you have a variant (mutation) in one copy of the CF gene and are a carrier of CF. This testing is offered to relatives of someone who has CF or partners of known CF carriers.

    This is sometimes called cascade screening. The name ‘cascade’ is used because whenever a carrier is found, their relatives will then be offered testing, and so on.

  • What is a carrier? Are carriers healthy?

    Genes are our bodies’ instructions for how to grow and function. Differences in the make-up of our genes are what make us unique. These differences are called ‘variants’ or ‘mutations’. Most of the time, gene variants are harmless, but sometimes they can stop a gene from working properly and affect our health. 

    We all inherit two copies of every gene, one from each of our biological parents. A person with CF has inherited a variant in both copies of the CF gene. A carrier is a person who has inherited a variant in only one copy of the CF gene. Carriers do not have CF because they still have one copy of the CF gene working as it should. However, if their partner is also a carrier, there is a chance that their children will have CF.

    Most carriers will not have any symptoms. Some research has found that carriers have a very small chance of having very mild symptoms, such as a higher risk of sinusitis or pancreatitis. Carriers do not need any CF treatment. If you have any concerns about your health, you can speak to your GP. 

  • How does a child inherit CF?

    A baby will be born with CF if they inherit two CF gene variants – one from each biological parent. This means that each biological parent of a child with CF is either a carrier or a person with CF themselves. 

    If both parents are carriers (so each has only one CF gene variant), their children have: 

    • a 1 in 4 (25%) chance of inheriting two CF gene variants and having CF 
    • a 1 in 2 (50%) chance of being a carrier, like their parents, but not having CF 
    • a 1 in 4 (25%) chance of not being a carrier or having CF.

    The chances are the same for each child the couple has.

    Chances of CF diagram
  • I have not had a carrier test. How likely is it that I am a carrier of CF?

    Around 1 in 25 people in the UK are carriers of a faulty gene that causes CF – about 4% of the population. If you’re related to someone with CF you have a higher chance of being a carrier.

    If you are related to someone with CF and you have not had carrier testing, you can find the chances that you are a carrier in our carrier testing factsheet (page 7).

    The likelihood that your child will have CF depends on whether you are a carrier of CF or if you have CF. Find out more in our carrier testing factsheet (page 8). If you have CF and you do not know whether your partner is a carrier, the chance that you will have a child with CF is 1 in 50 (2%).

  • Who can have a carrier test?

    Carrier testing is available as a free NHS service if: 

    • you are related to someone who has CF
    • your partner has CF, is related to a person with CF, or is a carrier of a CF gene variant 
    • you and your partner are blood relatives (for example, first cousins) and are from an ethnic group where CF is more common (such as white Northern European)

    If you have difficulty getting a carrier test through your GP, you can:

    • show them the information on this page
    • send them our GP letter, which contains information for GPs to explain who is eligible for carrier testing in England
    • contact our Helpline.

    If you are not in the list above and want to have a carrier test, it may be difficult to get one on the NHS but private testing is available. Speak to your GP to see if this is a good option for you. 

  • How can I find out if my partner and I are carriers?

    Speak to your GP if you would like to have carrier testing.

    If you have difficulty getting a carrier test through your GP, you can:

    • show them the information on this page
    • send them our GP letter, which contains information for GPs to explain who is eligible for carrier testing in England
    • contact our Helpline.

    If you are a relative of someone with CF, your test will look at whether you’re a carrier of the CF gene variants your relative has. This test may also look at other common CF gene variants. If your relative’s CF gene variants are not known, they will have tests first to find out what these are. 

    If you are not related to someone with CF, but your partner is, or has CF themselves, your test will only look for the more common variants. Standard carrier tests pick up about 85–95% of CF gene variants, so even if your test is negative there is still a small chance you may be a carrier of a rarer CF gene variant. If you later find out you are a carrier, this first negative test result is called a ‘false-negative’. 

  • What do my test results mean?

    The results will tell you whether you have a positive test and are a carrier, or a negative test and are not a carrier.

    Standard carrier tests pick up about 85–95% of CF gene variants. Getting a negative result from one of these tests means that the chance of you being a carrier is reduced by almost 10 times. It means you are not a carrier of the common CF variants that were tested for, but it does not mean you are definitely not a carrier of rarer CF variants.

    Getting a positive result from a test for any CF gene variants means you are definitely a carrier, even if you don’t have a family history of CF.

    After you and your partner have a test for common CF gene variants, the chances of you having a child with CF can be worked out more accurately. If one, or neither, of you are found to be a carrier, the chances of you having a child with CF is low. Tests for CF will likely not be offered during pregnancy, but you can talk to your doctor if you are worried. If both you and your partner are found to be carriers, you have a 1 in 4 (25%) chance of having a child with CF together. A genetic counsellor will be able to tell you about any tests and support available to you.  

    For more detail on carrier test results, read our factsheet.

  • What if my partner and I are both carriers of a CF gene mutation?

    If you and your partner are carriers, you will be offered genetic counselling. A genetic counsellor will give you information that will help you make decisions that are right for both of you.

    You can find out more about the options available in our factsheet.

  • Do you recommend home testing kits for cystic fibrosis?

    There are home testing kits for inherited conditions including CF, but these are not recommended by Cystic Fibrosis Trust. Testing should be done with the support of a genetic counsellor. Home genetic testing kits cannot give you this support and the results are also less reliable.

Swab testing

Carrier testing factsheet

Check out our carrier testing factsheet for more information on being a carrier of a CF mutation.

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