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Cystic Fibrosis

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Laboratory tests may be used to screen for and help diagnose cystic fibrosis (CF), to determine whether someone is a genetic carrier of CF, and/or to evaluate a person with CF to help manage the condition.

Tests for screening and diagnosis:

Even when there is no family history of the disorder, the American College of Obstetricians and Gynecologists (ACOG) recommends that all couples planning a pregnancy or having their first visit for prenatal care be offered a carrier screening test for cystic fibrosis.

All newborn screening programs test babies for cystic fibrosis. The following can be used in testing for CF. Some states use only the IRT blood test, while others use IRT and CF gene mutation testing for screening. Positive screening results may be followed by sweat chloride testing for confirmation.

  • CF Gene Mutations Testing—this testing may be used to screen for CF or to determine whether someone is a CF carrier. The American College of Medical Genetics (ACMG) and ACOG recommend a panel of 23 of the most common CFTR mutations. This panel detects most carriers and cases of CF. Some laboratories offer expanded panels of up to 100 or more mutations. Less common mutations may not be detected. People who undergo CF gene mutation testing should receive genetic counseling to understand the implications of the results.
  • Immunoreactive Trypsinogen (IRT)—this is a blood test that measures the level of immunoreactive trypsinogen. Trypsinogen is produced in the pancreas and transported to the intestine, where it is activated to form the enzyme trypsin. In CF, thick mucus can obstruct pancreatic ducts and prevent trypsinogen from reaching the intestine. Blood IRT levels are elevated in people with CF, but positive results must be followed by confirmatory sweat chloride testing.
  • Sweat Chloride Test—this test involves measuring the chloride level in a sweat sample collected through a special procedure. The sweat of a person with CF may be up to five times saltier than normal. This test is considered the "gold standard" and is used to confirm positive newborn screening tests as well as to diagnose older children and adults.

Nasal (transepithelial) Potential Difference (NPD)—this test is used less frequently than those listed above. The procedure is technically very difficult to perform and is available at only a few CF centers around the country. For the test, an electrode is placed on the lining of the nose, which is then bathed with a series of fluids that contain different salts. The fluids change the flow of sodium and chloride (ions) across the nose lining, generating a potential difference (PD) that can be measured. The abnormal sodium and chloride transport in the nose lining of people with CF is associated with a different pattern of nasal PD compared to the nose lining of people without CF. Results of the NPD must be considered along with signs and symptoms as well as sweat chloride test and CF gene mutation results when diagnosing CF.

Several other tests may be used to help evaluate a person with CF. Examples include:

Non-laboratory tests that may be done include bone and chest X-rays, upper GI and small bowel series, and lung function tests. See to learn more about these.

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