Cystic fibrosis is an inherited condition. It is caused by a faulty gene that allows too much salt and not enough water into cells. This results in a build-up of thick, sticky mucus in the body’s tubes and passageways. These blockages damage the lungs, digestive system and other organs, resulting in inflammation (swelling) and, in the lungs, repeated infections.

A child born with cystic fibrosis will have inherited the faulty gene from both their mother and father. In the UK, it is thought that 1 person in every 25 carries the faulty gene for cystic fibrosis, which is why it is so common. A carrier can be completely healthy and have no symptoms of cystic fibrosis.

If two carriers of the faulty gene have a baby, there is:

• a one-in-four chance that the child will not inherit either of the faulty genes (the child will not have cystic fibrosis and will not be a carrier of the condition)
• a one-in-two chance that the child will inherit one copy of the faulty gene from either their father or mother (the child will not have cystic fibrosis but will be a carrier of the condition)
• a one-in-four chance that the child will inherit both copies of the faulty gene (the child will have cystic fibrosis)

Diagnosing cystic fibrosis

Most cases of cystic fibrosis in the UK are now diagnosed through screening tests, which are carried out very early in life. However, some babies, children and even young adults are diagnosed later following unexplained illness.

There are four main ways of diagnosing cystic fibrosis:

• newborn testing
• antenatal testing
• carrier testing
• sweat testing

Newborn screening

Babies are screened for cystic fibrosis at birth as part of the NHS newborn 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 other inherited conditions, such as sickle cell anaemia and phenylketonuria.

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.

Carrier testing

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

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 pilocarpine 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 has symptoms of cystic fibrosis
• 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

A genetic test checks for the faulty cystic fibrosis gene by either analysing a saliva sample taken from inside the cheek using a swab or a blood sample. 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.

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