Cystic Fibrosis: Tests and Diagnosis

What tests are used to diagnose cystic fibrosis?

CF is most commonly diagnosed as part of newborn screening. This screening test occurs shortly after birth when a small amount of blood is taken to check for many conditions. A positive screen indicates the baby might have a health condition and that further testing should be done. Testing for CF is now included in the newborn screening protocol for all infants born in the U.S.

A positive (abnormal) newborn screen does not necessarily mean the baby has CF. A different test, called a diagnostic test, is needed to find out if the child has CF.

The two most definitive tests for diagnosing CF are a sweat test and genetic testing for CF-causing gene mutations. In the sweat test, a small, non-painful, electric current is used to carry the chemical pilocarpine into the skin of the forearm. This stimulates sweat glands in the area to produce sweat which is collected in a small tube and the salt (chloride) content is then tested. A sweat chloride concentration greater than 60 mmol/L is diagnostic of CF; chloride concentrations between 30-60 mmol/L are intermediate, and may indicate a milder form of CF. A diagnosis can also be made by identifying two CF mutations on genetic testing.

Several other tests are standard parts of routine care used to care for a child with CF:

  • Chest X-rays or chest CT scans
  • Blood tests to evaluate nutritional status
  • Bacterial studies to test for Pseudomonas aeruginosa, Staphylococcus aureus or Haemophilus influenza bacteria in the lungs (these bacteria are common in CF but may not affect otherwise healthy people)
  • Pulmonary function tests (PFTs) to measure the effects of CF on breathing. PFTs are done as soon as the child is old enough to be able to cooperate in the testing procedure; infant PFTs are currently being studied.

Because genetic screening for CF is available during pregnancy, parents can now learn whether their unborn children may have the condition. But even when genetic tests confirm CF, there's still no way to predict whether a child's CF symptoms will be severe or mild. Genetic testing also can be done on a child after birth, and can be performed on parents, siblings and other relatives who are considering having a family.

How do providers at Children's Hospital Colorado make a diagnosis?

At Children's Colorado, a diagnosis of CF is generally confirmed by finding elevated sweat chloride (greater than 60 mmol/L) and/or two CF-causing mutations through genetic testing. Sometimes children with sweat chloride results between 30 and 60 mmol/L have a more mild form of CF or a related, milder condition called Cystic Fibrosis Related Metabolic Syndrome or CRMS. Your doctor can help determine whether your child has CF or a related condition.

Doctors are diagnosing CF earlier and earlier, usually in infancy. However, about 15% of those with CF are diagnosed later in life, even into adulthood.