Diagnosing cystic fibrosis 

Most cases of cystic fibrosis in the UK are now identified through screening tests carried out early in life.

However, some babies, children and even young adults are identified later following unexplained illness.

Diagnosing cystic fibrosis

There are three main ways of diagnosing cystic fibrosis:

  • newborn testing
  • antenatal testing
  • sweat testing

Newborn screening

Babies are offered screening for cystic fibrosis at birth as part of the NHS newborn blood spot screening programme.

A small amount of the baby's blood is taken by a heel prick and transferred onto a card. The blood sample on the card is then analysed in the laboratory for cystic fibrosis and eight other conditions, including sickle cell anaemia.

If your baby is thought to have cystic fibrosis, you will be contacted before they are four weeks old.

The sooner cystic fibrosis is diagnosed, the sooner treatment can begin and the better the outlook.

Antenatal testing

A test can be done on a woman when she is pregnant to see if her unborn baby has cystic fibrosis. This can be carried out from 10 weeks of pregnancy.

The test uses chorionic villus sampling (CVS), where a fine needle is passed through the abdomen into the womb. Sometimes, a fine tube is passed through the vagina into the cervix (neck of the womb) instead. A tiny piece of the developing placenta, known as the chorionic tissue, is taken and the chromosomes in the cells of the tissue are examined for the faulty gene that causes cystic fibrosis.

Antenatal testing for cystic fibrosis is usually only offered to mothers who are thought to be at high risk of having a child with the disease, such as women with a family history of the condition. The tests need to be discussed on an individual basis with a genetic counsellor.

Sweat test

If someone has cystic fibrosis, their sweat will have higher levels of salt than normal. A parent may first notice their child's symptoms of cystic fibrosis when they kiss them, as their skin can taste salty.

A sweat test measures the amount of salt in sweat. It is usually done by applying a very weak and painless electric current to a small area of skin to which a harmless chemical has been applied. This causes that area of skin to sweat. A sample of the sweat is then collected and analysed. If the salt content in the sweat is abnormally high, this confirms cystic fibrosis.

A sweat test may be carried out if:

  • newborn screening tests are abnormal
  • a child is born with a serious bowel obstruction known as meconium ileus
  • a child or adult has symptoms suggestive of cystic fibrosis

Genetic testing

genetic test checks for the faulty cystic fibrosis gene by analysing either a blood sample or a saliva sample taken from inside the cheek using a swab. It can be useful to confirm cystic fibrosis if a sweat test gives a borderline result.

It can also be useful to find out which members of a family are carriers of the cystic fibrosis gene.

Carrier testing

There is a simple test that uses a mouthwash to identify whether a person is a carrier of the cystic fibrosis gene.

Swishing the mouthwash collects a sample of cells from the mouth. This sample is then sent to a laboratory and the cells it contains are checked for the faulty cystic fibrosis gene. The results will be discussed on an individual bases with a genetic counsellor, as the results may be misleading in some cases.

It is important to have this test if the person’s partner is a known carrier, or if someone in the family has cystic fibrosis or knows that they carry it.

Testing in later life

Older children and adults with cystic fibrosis require regular testing to check how well (or not) their lungs and digestive system are working.

Tests that can be used for this purpose are described below.


You will be asked to breathe into a machine called a spirometer.

The spirometer takes two measurements: the volume of air you can breathe out in one second (called the forced expiratory volume in one second or FEV1) and the total amount of air you breathe out (called the forced vital capacity or FVC).

You may be asked to do this a few times to get a consistent reading.

The readings are compared with normal measurements for your age, which can show if your airways are obstructed.

Chest X-rays

A chest X-ray can be a useful method for assessing the state of your lungs

Computerised tomography (CT) scan

In a CT scan, a series of X-rays are taken. These are then assembled by a computer into a more detailed ‘3D’ image of your lungs and digestive system. 

Help after diagnosis

If you or your child is diagnosed with cystic fibrosis, it can be extremely difficult to deal with.

It can help to find out as much as you can about the condition. Call the Cystic Fibrosis Trust on 0300 373 1000 for information on its new diagnosis pack.

Page last reviewed: 17/05/2014

Next review due: 17/05/2016