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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 Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) recommends that all couples planning a pregnancy or in their first trimester be offered information on carrier screening test for the more common genetic conditions that affect children, including cystic fibrosis.
All Australian newborn screen programs screen babies for cystic fibrosis. The following can be used in the testing for CF. Positive screening results may be followed by sweat chloride testing for confirmation.
Immunoreactive trypsin (IRT)
This newborn screening test for CF is performed on the blood spots collected on dried blood spot screening cards. 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 are elevated in people with CF, but positive results must be followed by confirmatory sweat chloride testing.
Elevated IRT in the first few days of life can also be caused by perinatal stress, renal failure, congenital infection, bowel atresia and some aneuploidies. An elevated IRT is usually followed with testing for the most common CFTR mutations including the F508del. Early diagnosis is essential so that treatment can be started before damage to the lungs with chronic infection occurs and malnutrition becomes established.
CF gene mutation
The most common CFTR mutation is F508del, which accounts for most of the mutations in those of Northern European descent. Testing of F508del in isolation or a panel of around 50 common mutations of the CF gene has been developed to screen general or targeted populations for CF and CF carrier status (the number of mutations screened within the panel varies between laboratories). 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 counselling to understand the implications of the results.
Sweat Chloride Test
This test involves measuring sodium and chloride from a sweat sample collected in a procedure in which local sweating is stimulated by using the drug, pilocarpine. Since the CFTR protein is altered or missing and chloride travel is restricted, the sweat in somone 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.
Other laboratory tests used to check lung infection, organ function and fertility include:
Non-laboratory tests that may be performed include respiratory function tests, chest X-rays, lung scans including low-dose CT scan to look for air trapping and early bronchiectasis, bronchoscopies, bone scans, upper GI and small bowel X-rays and other gastrointestinal pancreatic and liver investigations such as ultrasound scans.
Last Review Date: March 27, 2019