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Cystic Fibrosis Trust

How is cystic fibrosis diagnosed?

Find out more about cystic fibrosis (CF) diagnosis, including the processes of newborn screening, carrier testing and diagnosis in adulthood.

There are three main types of screening for cystic fibrosis: carrier testing, newborn screening and antenatal testing.

Carrier testing

If someone has a history of CF in their family, a partner with CF, or a child with the condition, they may choose to get carrier testing. A simple mouthwash or blood test can determine if someone is a carrier of the faulty gene that causes cystic fibrosis. Carrier testing is often done for people who are thinking of starting a family and have a relative with cystic fibrosis. If you would like more information about carrier testing and genetic screening see our factsheet.

If someone carries the faulty gene that causes CF, will their child have the condition?

If both parents carry the faulty gene, for each pregnancy there is a chance the baby will be born with cystic fibrosis.

If both parents are carriers of the faulty gene, but don’t have cystic fibrosis, 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 condition, and
  • a one-in-four chance of being completely free of the condition – neither having CF nor being a carrier of the faulty gene.

If you are considering carrier screening and have questions about the process or the implications if you are a carrier of the faulty gene, please read our factsheet on genetic screening.

Newborn screening

Since 2003 in Scotland and 2007 in the rest of the UK, all babies born have been screened for CF using the heel prick test. As a result most children are diagnosed with CF shortly after birth (scroll down to find out more about diagnosis in later life).

How does the heel prick test work?

The heel prick test is offered when babies are five days old and involves taking a few drops of blood from the baby’s heel. The blood sample is then tested for several serious but rare conditions including cystic fibrosis.

To have cystic fibrosis, an individual must carry two copies of the faulty gene (one copy inherited from their mother, and one from their father). There are lots of different types of faults (mutations) that can affect the gene causing cystic fibrosis. The heel prick test screens for the most common mutations, but not all of them, so it’s possible for someone with a rare genetic mutation not to be picked up by the test and CF may not be diagnosed until later in life.

A positive result from the heel prick test suggests that a baby may have cystic fibrosis, however further tests will be needed to confirm a diagnosis (see next section on the sweat test). This leaflet from Public Health England gives some further detail about receiving a positive heel prick test.

The heel prick test can also identify some babies who have one faulty gene, meaning that they are carriers of the cystic fibrosis gene but are not thought to have the condition. Take a look at Public Health England’s helpful leaflet on this subject.

What is a sweat test?

If a baby is suspected of having cystic fibrosis, a sweat test may be performed as part of the follow up to the screening process. In people with CF there is a problem in the transport of chloride across cell membranes, which results in higher concentrations of chloride (as salt) in the sweat. The sweat test measures this concentration, and is done by collecting a small amount of sweat from the arm or leg.

To find out more about the sweat test, download our factsheet.

Antenatal testing

If you know that you and your partner are both carriers of the cystic fibrosis gene and you are considering a pregnancy then consider seeing a genetic counsellor before you get pregnant. They can help you to understand more about your genetics and options you may have pre and post-pregnancy. You can be referred to a genetics counsellor by your GP.

Parents who are thought to have increased likelihood of having a child with cystic fibrosis may be offered a chorionic biopsy or amniocentesis during pregnancy, to identify if the baby has the condition. You don’t have to accept the offer of either test, it’s a personal decision. Earlier diagnosis enables treatment to begin at birth, but both these procedures do carry a risk of miscarriage. There’s more detailed information about each test, and the associated risks, on the NHS website:

Antenatal testing is a fast-moving field with new non-invasive technologies now available for some conditions. Non-invasive prenatal testing (NIPD) and non-invasive prenatal diagnosis (NIPD) are possible for cystic fibrosis but currently only available privately.

Diagnosis in adulthood

Since newborn screening was introduced nationally in 2007, most cases of CF are now diagnosed shortly after birth but sometimes the condition may not be diagnosed until later in life.

What are the reasons for late diagnosis?

There are several reasons that could explain why a person with CF was not diagnosed earlier.

  • The CF gene was not identified until l989, and until that time tests used to confirm diagnosis were not so reliable. If a person was born before 1989 it may have been harder for their condition to be diagnosed.
  • The faulty gene that causes CF can be faulty in many different ways. In fact, there are now known to be more than 2,000 mutations that cause cystic fibrosis. If someone has a very rare mutation it may be harder to diagnose.
  • Cystic fibrosis can vary widely in its severity and symptoms, and can mimic other lung diseases such as asthma or bronchitis, making diagnosis challenging.

To find out more, download our diagnosis in adulthood factsheet.

For further information about the support we can offer, please contact our helpline on 0300 373 1000 or email helpline@cysticfibrosis.org.uk.



How cystic fibrosis affects the body

Explore our interactive body to find out all about the different ways CF affects the body, from the lungs and digestive system to the bones and the reproductive system.

Take a look

How is cystic fibrosis treated?

Find out more about medication, exercise, nutrition, transplant and transition on our treatment page.

Find out more

What causes cystic fibrosis?

Find out about the faulty gene that causes cystic fibrosis, how it is passed from parents to children, and how to find out more about your genotype.

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