Testing for cystic fibrosis carriers in families

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

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.

A person with only one copy of a CF gene mutation is called a carrier. A carrier does not have CF, but if their partner is also a carrier there’s a chance that their children will inherit CF. 

Some research has suggested that carriers may have a very small chance of having mild symptoms of CF, but most carriers will not have any symptoms. Carriers do not need any CF treatment. If you have any concerns about your health, you can speak to your GP.

For a baby to be born with CF they need to inherit two CF gene mutations – one from each parent. The parents of a child with CF must have at least one copy of a CF gene mutation, being either a carrier or a person with CF themselves.

If both parents are carriers, with only one copy of a CF gene mutation, their children have:

• a 1 in 4 (25%) chance of inheriting two copies of a CF gene mutation 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.

Around 1 in 25 people in the UK are carriers of a faulty gene that causes CF – about 4% of the population. Relatives of someone with CF have a higher chance of being carriers.

If you are related to someone with CF and you have not been carrier tested, you can find the chances that you will have a child with CF in our carrier testing factsheet (page 6).

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%).

Carrier testing is available as a free NHS service if:

• you are related to someone who has CF, or someone who is a carrier of a CF gene mutation
• your partner has CF or is related to a person with CF, or is a carrier of a CF gene mutation

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 support access
• 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.

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 support access
• 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 mutations your relative has. If your relative’s CF gene mutations are not known, they will have tests first to find out what these are.

Some CF gene mutations are too rare to be picked up with these tests. If this is the case, carrier testing can be a little more complicated as more detailed genetic testing will need to be done. The parents of the person with CF will have their entire CF gene tested for the mutations they are carrying. Once these are found, other relatives can be tested for these mutations too.

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 mutations. Standard carrier tests pick up about 90–95% of CF gene mutations, so even if your test is negative there is still a small chance you may be a carrier of a rarer CF gene mutation. If you later find out you are a carrier, this first negative test result is called a ‘false negative’.

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

If your test result is negative, you should be told the chances that you could still be a carrier of a rare CF gene mutation. Standard carrier tests pick up about 90–95% of CF gene mutations. Getting a negative result from one of these tests means that the chance of you being a carrier is reduced by 10 times. It means you are not a carrier of the common CF mutations that were tested for, but it does not mean you are definitely not a carrier of rarer CF mutations.

Getting a positive result from a test for any CF gene mutations means you are definitely a carrier, even if you don’t have a family history of CF. If your test is positive and you are a carrier, you will usually be invited to talk to a genetic counsellor. This can be by telephone or in person. The genetic counsellor can tell you what your result means, and about any tests and support that is available to you.

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

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.

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.