Notice: Users may be experiencing issues with displaying some pages on stanfordhealthcare.org. We are working closely with our technical teams to resolve the issue as quickly as possible. Thank you for your patience.
A medical history and a physical exam are often the first steps in diagnosing cystic fibrosis. This is followed by screening or lab tests.
The diagnosis of cystic fibrosis requires one of the following:
Early symptoms of cystic fibrosis. These may include diarrhea that does not go away; large, greasy or very smelly stools; constipation; not wanting to eat; or losing weight.
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.
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 finds two defective CFTR genes, then just one sweat test result may be all that's 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.