Family genetic testing: the family cascade screening programme for cystic fibrosis

Family genetic testing or cascade screening is the process of offering screening for the cystic fibrosis (CF) gene to the family members of someone who has been diagnosed with cystic fibrosis.


Cascade screening offers the family members of someone who has been diagnosed with CF the opportunity to be screened for the CF gene. On this page, we explain:  

  • how cascade screening works;
  • who can have carrier testing;
  • the odds of being a carrier of the CF gene;
  • what it means to be a carrier; and
  • where you can go for testing.

This is a complicated topic; if you would like to know more about cascade testing, you can obtain further information from your CF team, a specialist CF geneticist or your regional genetics centre.

Cystic fibrosis is one of the UK’s most common life-shortening inherited conditions, affecting around one in 2,500 live births – over 10,500 people in the UK.

What is cascade screening?

Cascade screening is used to see whether someone is a carrier of the CF gene, and this testing process is offered to relatives (and their partners) of those diagnosed with cystic fibrosis. Whenever a carrier is detected, carrier testing is made available to the carrier’s relatives and their partners in a cascade fashion.

Why is a baby born with cystic fibrosis?

CF carrier 640x360 diagram

For a baby to be born with CF, both parents must be carriers of the faulty CF gene. The child will have inherited a copy of the faulty gene from each parent.

The carrier parents do not have CF themselves, having one unaffected gene and one faulty CF gene. If both parents are carriers, a child has:

  • a one-in-four chance of being born with cystic fibrosis;
  • a two-in-four chance of being a carrier, like their parents, but not having the disease; and
  • a one-in-four chance of being completely free of the condition – neither having cystic fibrosis nor being a carrier of the faulty CF gene.

It is important to note that these odds are the same for each successive pregnancy.

What is the risk of carrying the faulty gene for relatives?

In the general UK population, 1 in 25 people without a family history of CF carries a faulty CF gene. Relatives of someone with CF have a much higher chance of being carriers. The list below gives the statistical chances of relatives (who have not been tested) being carriers and their risk of having a child with cystic fibrosis.

Before testing

The list below outlines relationship of family member to person with CF, chance of being a carrier, and risk of having a child with CF with an untested partner:

  • Parents: 100%    |    1 in 4 (with each other)
  • Person with CF: 100%    |    1 in 50
  • Parent with new partner: 100%    |    1 in 100
  • Child of woman with CF: 100%    |    1 in 100
  • Brother or sister: 2 in 3     |     1 in 150
  • Aunt or uncle: 1 in 2     |     1 in 200
  • Grandparent: 1 in 2     |     1 in 200
  • First cousin: 1 in 4     |     1 in 400
  • Second cousin: 1 in 8     |    1 in 800

Are carriers healthy?

People with only one faulty copy of the gene that causes CF will not have the condition and are not at risk of developing it, but any children they have with another carrier could be born with cystic fibrosis. There is some limited new evidence which suggests that a very small number of people who carry the gene may experience some mild symptoms which are also common in people who have cystic fibrosis. If you have any concerns about your health, you can speak to your GP.

Can I find out if my partner and I are carriers?

Speak with your GP if you would like to have carrier testing. It is now possible to test for the gene using a simple mouthwash or blood test. For a mouthwash test, cells lining the cheek are collected by spitting saliva into a special tube; DNA can be extracted from these cells and then analysed.

A person with CF will have been tested so that their specific CF genes are identified. When a relative is offered carrier testing, the test will be looking for the specific genotype found in the person with CF’s genes. A test for the more common gene mutations is performed on the partner who does not have a family history of the condition.

Because the test for common mutations only detects about 90-95% of mutations, a ‘negative’ result does not entirely rule out the possibility of the person being a CF carrier, but the statistical likelihood is reduced to less than 1 in 250. Depending on the number of CF mutations tested for in your health region, this figure may differ slightly. A ‘positive’ result shows that the person is definitely a carrier, even if there is no family history.

The person who has CF is tested before any relatives have carrier testing. Very occasionally, the CF mutation cannot be found, which means carrier testing cannot be specific to the genotype of the person with cystic fibrosis. If this is the case for you, more information on this can be found in our factsheet on genetic screening.

After tests that detect 90% of cystic fibrosis genes

After the genetic tests are done (a test which detects 90% of CF genes) you will be given your results at an appointment with a genetic counsellor, by letter or by your GP if they made the enquiries on your behalf.

If it is found that both partners are carriers of the CF gene they will be offered genetic counselling. Each pregnancy has a one in four chance of having a baby with CF (as explained above). A genetic counsellor can explain options available which could include pre-implantation genetic diagnosis.

If the relative of the person with CF is found to be a carrier but their partner has a negative result, the chance of having a baby with CF is less than 1 in 1,000. This risk is low and there is no need to see a genetic counsellor.

If the relative of the person with CF has a negative result but their partner is found to be a carrier the chances of having a baby with CF is less than 1 in 2,000. This risk is low and there is no need to see a genetic counsellor.

(There is an explanation for the difference in the two scenarios above in the factsheet on family genetic testing.)

If both partners do not have the CF gene they are given strong reassurance that the chances of having a baby with CF are less than 1 in 250,000.

What are the options if my partner and I are both carriers of the CF gene?

There is more information in our genetic screening factsheet on the options available if you and your partner both carry the CF gene.

Do you recommend home testing kits for cystic fibrosis?

We recommend that genetic testing is undertaken with professional clinical support and discussed with a genetic counsellor, who can explain in full the results you may get and the options available to you as an individual.

Using a home genetic testing kit may mean that professional support is not available to you if and when you need it, as well as the test being less reliable. For these reasons, we advise against the use of home genetic testing kits.

Who can I contact about carrier testing?               

Genetics centres in the UK offer CF carrier tests to people with a family history of cystic fibrosis. Your GP will be able to refer you to your local genetics centre to be seen by a genetics counsellor.

Download our cascade screening factsheet, or take a look at our other factsheets and information packs.

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