Cystic Fibrosis Diagnosis

Diagnosing cystic fibrosis takes several steps. Your healthcare provider may diagnose cystic fibrosis based on your symptoms and the results of genetic and sweat chloride tests.

It is important to diagnose cystic fibrosis as early as possible. Newborn screening makes it possible to detect the disease early in life. Early treatment helps prevent complications and can improve survival.

Universal newborn screening

All newborns in the United States are tested for cystic fibrosis. Newborn screening identifies more than half of cystic fibrosis cases. However, many people born before 2010 have not been screened.

Newborn screening for cystic fibrosis is performed during a baby’s first two to three days of life. A few drops of blood from a heel prick are placed on a special card and tested by a lab. The test will cause your baby brief discomfort.

The blood test checks for levels of immunoreactive trypsinogen (IRT), a chemical made by the pancreas. In people who have cystic fibrosis, IRT tends to be high. However, most babies with high levels of IRT do not have cystic fibrosis. IRT can also be high if the baby was born preterm (premature), had a stressful birth, or is a carrier of cystic fibrosis.

If IRT is high, then the hospital will likely test the baby’s DNA for some of the CFTR mutations that cause cystic fibrosis. The set of mutations in these tests varies by state. After a positive screening test, a diagnosis is confirmed with more tests.

Screening tests for CFTR mutations sometimes miss cases of cystic fibrosis in babies who have Asian or African ancestry. That is because these babies have rarer CFTR mutations that are not tested for.

Carrier screening to detect CFTR mutations

Genetic testing can tell you whether you are a carrier of a mutated CFTR gene. This is called carrier screening. Genetic testing looks at your DNA from a blood or saliva sample or cells from the inside of your cheek. It is your choice whether to have this test.

People who inherit a mutation of the CFTR gene from one parent are cystic fibrosis carriers. People who have inherited a mutation of the CFTR gene from both parents will have cystic fibrosis. Learn more about how cystic fibrosis is inherited.

In previous years, the standard test to check for cystic fibrosis carriers looked for 23 or more of the most common disease-causing gene mutations. This test is more likely to miss carriers who have Asian or African ancestry. Now, many tests look for more CFTR mutations, but there is still a small chance that you could carry a type of mutation that did not show up on the test.

Siblings (brothers and sisters) of a person who has cystic fibrosis may want to be tested for cystic fibrosis even if they do not have symptoms. Other family members, such as first cousins and half siblings, may be tested if they have symptoms or if the family is concerned that those individuals might have cystic fibrosis or be carriers.

After a positive or unclear screening test result, the diagnosis should be confirmed with further testing. A genetic counselor can help you understand the meaning of your carrier screening result.

Preconception and prenatal screening

Couples who are planning to have children may want to be tested to see whether they are cystic fibrosis carriers. Getting tested before pregnancy gives you more options and time to make decisions if results show that both parents are carriers.

The tests use a sample of blood, saliva, or cells from inside your cheek to check your DNA. It is your choice whether to have this screening. Talk with your healthcare provider about screening if you’re thinking about getting pregnant or if you’re already pregnant.

Often the mother is tested first. If the father has a family history of cystic fibrosis, he may be tested first. If one parent is a carrier for a CFTR gene mutation, then the next step is to test the other parent.

If you are already pregnant, both parents may be tested at the same time. If you are both cystic fibrosis carriers, then prenatal testing can check if the unborn baby has two mutated CFTR genes.

Sweat test

A sweat test checks for high levels of chloride in your sweat. The sweat test is the standard test for diagnosing cystic fibrosis. It may be used if you or your child has symptoms that could indicate cystic fibrosis, or to confirm a positive diagnosis from a screening of your newborn. A normal sweat chloride test result alone does not mean you do not have cystic fibrosis. Lower levels of chloride may indicate the need for further testing to diagnose or rule out cystic fibrosis.

How is a sweat chloride test performed?

The sweat test detects a higher amount of chloride — a natural component of salt — in the sweat of people who have cystic fibrosis. To get enough sweat for the test, a chemical and a little electrical stimulation are applied to a small patch of the arm. The sweat is collected and sent to a hospital lab for testing.

Sweat chloride test results for diagnosing cystic fibrosis. The table shows how much chloride in a person’s sweat sample must be present in order to determine whether the diagnosis for cystic fibrosis is positive, unclear, or unlikely. A chloride level of 60 millimoles per liter (mmol/L) or greater indicates cystic fibrosis. A chloride level of 30 to 59 mmol/L indicates that a diagnosis of cystic fibrosis is unclear and that further testing is needed. A chloride level of less than 30 mmol/L indicates that a diagnosis of cystic fibrosis is unlikely. — **Sweat chloride test results for diagnosing cystic fibrosis.** The table shows how much chloride in a person’s sweat sample must be present in order to determine whether the diagnosis for cystic fibrosis is positive, unclear, or unlikely. A chloride level of 60 millimoles per liter (mmol/L) or greater indicates cystic fibrosis. A chloride level of 30 to 59 mmol/L indicates that a diagnosis of cystic fibrosis is unclear, and that further testing is needed. A chloride level of less than 30 mmol/L indicates that a diagnosis of cystic fibrosis is unlikely.