How is cystic fibrosis diagnosed?

In the UK, all newborn babies are tested for cystic fibrosis (CF). On this page, find out more about how CF is diagnosed.

Cystic fibrosis (CF) is usually diagnosed in newborns with a routine blood test. Often, babies don’t have any symptoms before their diagnosis. Some may have serious constipation (being unable to poo) in the first few days of life – you might have heard this called meconium ileus.

Blood spot test

You’ll be offered a blood spot test for your baby when they’re around five days old. It is sometimes called the heel prick test as a small sample of blood is taken from your baby’s heel. The blood is sent to a lab where it’s tested for a number of conditions, including CF.

If the blood spot test shows your baby could have CF, you’ll be offered more tests to confirm their diagnosis.

Sweat test

A sweat test involves taking a sample of sweat to test how much salt it contains. People with CF have more salt in their sweat than others. This is the best way to diagnose CF in children.

Gene test

A gene test checks for problems with the CFTR gene known to cause CF. This is done using the blood sample from your baby’s heel prick test.

The sweat and gene tests can also be used to diagnose CF in older children and adults who were not tested at birth.

If your baby is diagnosed with CF, your doctor will offer to test their brothers or sisters. This is because siblings might also have inherited the condition.

If one of your family members has CF (such as your brother, sister or a parent), you might have inherited the faulty CFTR gene too. If you only have one copy of the faulty gene, you won’t have CF but will be a ‘carrier’. You can have a carrier test to find out. Your doctor or nurse will take a gene sample using a special mouthwash or blood sample. Read more about how CF is inherited.

Finding out whether or not you’re a carrier helps if you’re planning a family. If two people who are both CF carriers have a baby, there is a one in four (25%) chance the baby will have CF.

Finding out your child has CF can bring a range of emotions. Your child will have a specialist CF care team who can provide support or tell you where you can get support.

You can contact your child’s doctors if you have any questions, or if you’re unsure about something. It’s important to be able to recognise if your child’s health is getting worse, and when you should seek emergency care. Agree a plan with the doctor or nurse about what you should do.

Our helpline is there for you – call 0300 222 5800, Monday to Friday 9am to 5pm. You can also get in touch with the Cystic Fibrosis Trust.

If you or your child is diagnosed with CF later in life, you’ll likely experience a range of emotions. Getting a diagnosis might provide a sense of relief as it explains what’s been happening and getting treatment can reduce symptoms. But being diagnosed with any long-term condition can be difficult. If you’re struggling or want to talk more about your new diagnosis for wellbeing support, talk to your doctor or nurse.

You can also call our helpline on 0300 222 5800 for support and advice – Monday to Friday, 9am to 5pm. You can also get in touch with the Cystic Fibrosis Trust.