Cystic Fibrosis Diagnosis

Your doctor may diagnose cystic fibrosis based on your symptoms and results from certain screening tests, such as genetic and sweat tests.

Screening for cystic fibrosis

Genetic testing may be performed to look for carriers, as well as to screen relatives of people who have cystic fibrosis. Genetic testing may also be used as prenatal screening tool to look for a mutated CFTR gene. All newborns in the United States are now screened for cystic fibrosis. Since universal screening for cystic fibrosis began relatively recently, there are still young people and adults who have not been screened.

Carrier screening to detect CFTR mutations

Genetic testing can tell you if you carry a mutation of the CFTR gene. This is called carrier testing. People who have inherited 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. Genetic testing looks at your DNA from a blood or saliva sample, or cells from the inside of your cheek.

The standard test to check for possible cystic fibrosis carriers looks for 23 of the most common disease-causing gene mutations. If you have a positive test, there is a 99% chance you are a carrier. However, if you have a negative test, there is still a small chance that you could carry a CFTR mutation that did not show up on the test.

Siblings of a person who has cystic fibrosis may want to be tested for cystic fibrosis whether or not they have symptoms. Other relatives, such as first cousins and half-siblings, may be tested if they have symptoms, or if the family is concerned that the individual may have cystic fibrosis.

After a positive screening test, the diagnosis should be confirmed with further testing.

Prenatal screening

Couples who are planning to have children may want to be tested to see if they are cystic fibrosis carriers. Genetic testing, such as the carrier screening described above, may be done before or during pregnancy. Often, the mother is tested first. However, if you are already pregnant, you and your partner may choose to be tested at the same time. If the father has a family history of cystic fibrosis, he may be tested first. Similar to standard genetic testing, prenatal screening uses a sample of blood, saliva, or cells from the inside of your cheek to check your DNA.

If one partner is a carrier for a cystic fibrosis gene mutation, then the next step is to test the partner if this has not been done. If both parents are cystic fibrosis carriers, then prenatal diagnostic testing may be performed to see whether your unborn baby has cystic fibrosis or is a carrier.

Newborn screening

When a child has cystic fibrosis, it is very important to diagnose it early to help prevent complications. Newborn screening for cystic fibrosis is performed during a baby’s first 2 to 3 days of life. A few drops of blood from a heel prick are placed on a special card and analyzed in labs.

The type of newborn screening that is performed varies from state to state. Every state and the District of Columbia begins with a blood test to check for levels of a chemical made by the pancreas called immunoreactive trypsinogen (IRT). 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 may also be high if the baby is premature, had a stressful delivery, or is a carrier of cystic fibrosis.

Some states test only IRT for cystic fibrosis newborn screening. Other states test IRT and also perform DNA testing. In states that test both IRT and DNA, if IRT is high, then the hospital will test the baby’s DNA for some of the gene mutations that cause cystic fibrosis.

After a positive screening test, the diagnosis should be confirmed with further testing.

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 have symptoms that may indicate cystic fibrosis or to confirm a positive diagnosis from a screening of your newborn baby. A normal sweat chloride test 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.

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.

How is a sweat chloride test performed?

The sweat test detects a higher amount of chloride — a component of salt — in the sweat of people who have cystic fibrosis. In order to make sweat for this test, a colorless, odorless chemical and a little electrical stimulation are applied to a small area of a person’s arm or leg. The sweat is collected and sent to a hospital lab for testing.