Immunoreactive Trypsin
At a Glance
Why Get Tested?
To screen for cystic fibrosis (CF) in newborn infants
When to Get Tested?
When a newborn infant has signs and symptoms of cystic fibrosis particularly when it passes no stools in the first 24 to 48 hours of life (meconium ileus) or where there is a high risk of the baby having the cystic fibrosis due to family history
Sample Required?
A blood sample taken from an infant’s heel, a spot of blood that is put onto filter paper
The Test Sample
What is being tested?
Trypsin is produced from trypsinogen. Trypsinogen is normally produced in the pancreas and is carried to the small intestine where it changes from an inactive ‘proenzyme’ to the active ‘enzyme’ trypsin which is used to help the body digest proteins.
In babies with cystic fibrosis, mucous can block the passage ways (ducts) from the pancreas into the small intestine. The mucos prevents trypsinogen from reaching the intestines resulting in a build-up of the protein in the blood. This can process can be detected and measured because ‘immunoreactive’ trypsin (IRT) levels increase in the blood of the infant. Infants with cystic fibrosis 1 to 2 weeks of age show increased levels of IRT in the plasma in the neonatal heal-prick test. It is currently the best screening test for cystic fibrosis but cannot be done after the first few weeks of life since it falls as the disease if the pancreas (pancreatictic insufficiency) develops. Several factors other than cystic fibrosis can affect the test result and therefore not all babies with high IRT will have cystic fibrosis. Where an elevated level is obtained, further investigation either by sweat testing or genetic analyses may be required.
How is the sample collected for testing?
A blood sample is taken from the heel of a newborn or very young infant, the spot of blood is then put onto filter paper.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
Immunoreactive trypsin (IRT) is used to selectively screen for cystic fibrosis (CF) in new-born infants. In the UK, it is now part of the routine neonatal screening programme.
When is it requested?
This test is used as part of a newborn screen for cystic fibrosis, in cases where no stool is produced in the first 24 to 48 hours of life, where there are other symptoms and signs suggestive of cystic fibrosis or where a baby is known to be at high risk of having the condition.
IRT testing can not detect all cases of cystic fibrosis (CF) and a positive test can be due to diseases other than CF and other diseases of the pancreas can cause a positive IRT. An elevated level must be followed with other testing. When diagnosing CF this may include: another IRT in a month, CF gene mutation testing, and/or sweat chloride testing.
What does the test result mean?
If an IRT level is elevated, a baby may have cystic fibrosis or the elevated IRT may be a false positive. Elevated levels need to be followed with further testing. If the IRT level is negative but the baby is has symptoms, other testing for CF such as sweat chloride and/or CF gene mutation testing may be considered.
Is there anything else I should know?
IRT testing will not identify CF carriers. Their trypsinogen production and function will not be affected. In patients who do have CF, the level of the IRT elevation does not indicate how bad the disease is.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
