Tests can help find out if a person has cystic fibrosis or has the changed gene that can be passed on to a child. Adults may want to get tested during a pregnancy or when they are planning for a pregnancy. People can be tested at any age to see if they have cystic fibrosis.
A medical history and a physical exam are often the first steps in diagnosing cystic fibrosis, followed by screening or lab tests.
The diagnosis of cystic fibrosis requires one of the following:
- Your child has early symptoms. These may include diarrhea that does not go away; large, greasy or very smelly stools; constipation; not wanting to eat; or losing weight. To learn more, see Symptoms.
- A brother or sister who has cystic fibrosis.
- A positive newborn screening test.
Also, there must be at least one of the following:
- A genetic test showing that a person inherited one or two defective cystic fibrosis transmembrane regulator (CFTR) genes. This testing can be done using blood or a sample from the mother's womb before birth (chorionic villus sampling or amniocentesis).
- Two positive sweat tests on different dates. Sweat tests measure the level of salt in sweat. People with cystic fibrosis have more than the normal amount of salt in their sweat. If a genetic test detects two defective CFTR genes, then just one sweat test result may be all that is needed to confirm a diagnosis.
- An abnormal nasal potential difference test. This test uses electrodes on the lining of the nose to see how well salts flow into and out of cells.
Monitoring cystic fibrosis
Certain tests can help your doctor monitor your child's cystic fibrosis. These tests include:
- Lung function tests to find out how healthy the lungs are by checking how well air moves into and out of the lungs.
- A throat culture or sputum culture to see what kinds of bacteria are causing any infections your child may have.
- A chest X-ray to take a picture of the chest, including the heart and lungs.
- A CT scan to find any serious disease in the lungs, pancreas, or other organs.
- Blood tests, such as glucose (blood sugar) level and liver function. These tests are to see if there are any complications of cystic fibrosis.
- A stool analysis to see how well your child is absorbing and digesting fat and other nutrients.
- An arterial blood gas analysis to measure the levels of oxygen and carbon dioxide in the blood. This test shows how well the lungs are working.
Both newborns and adults can be tested for the changed (mutated) gene that causes cystic fibrosis. These tests include:
- Newborn screening. Levels of a type of digestive enzyme are measured from a blood sample. High levels of this enzyme suggest cystic fibrosis. Some newborns may also have a genetic test.
- Genetic test for adults. These tests identify the most common defects in the CFTR gene. Genetic testing can be done during pregnancy through chorionic villus sampling or amniocentesis. The test can also be done before pregnancy, to help couples find out if either or both of them carry a defective CFTR gene.
- If both parents carry the changed gene, there is a 25% (1-in-4) chance that their child will have no genetic problem. There is a 25% chance that their child will have cystic fibrosis. And there is a 50% (1-in-2) chance that their child will be a carrier.
- If only one parent is a carrier of the changed gene, the child will not have cystic fibrosis. But there is a 50% chance that the child will be a carrier.
If you are interested in a genetic test for cystic fibrosis, talk with your doctor about the test. Genetic counseling can help you understand your test results. Genetic testing may involve certain ethical, legal, and religious issues.