Immunoreactive trypsin
At a Glance
Why Get Tested?
To screen for cystic fibrosis (CF) in new-born infants
When to Get Tested?
When a newborn infant has signs and symptoms of cystic fibrosis particularly meconium ileus (no stools in the first 24 to 48 hours of life) or where there is a high risk of the baby having the condition
Sample Required?
A blood sample drawn from an infant’s heel, a spot of blood that is put onto filter paper
Test Preparation Needed?
None
The Test Sample
What is being tested?
Trypsinogen is a proenzyme, an inactive precursor to the proteolytic enzyme trypsin. Normally, trypsinogen is produced in the pancreas and transported to the small intestine. In the small intestine, it is activated, turned into trypsin by an enzyme in the intestinal mucosa, forming a powerful chemical responsible for breaking down the protein in food into smaller pieces called peptides.
In babies with cystic fibrosis, muc0us plugs can block the pancreatic ducts that lead into the small intestines, preventing trypsinogen from reaching the intestines resulting in a build-up of the protein in the blood. This can be measured as immunoreactive trypsin (IRT). 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 pancreatic 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 drawn from a newborn's or very young infant's heel, a spot of blood is put onto filter paper.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
Immunoreactive trypsin (IRT) is used to selectively screen for cystic fibrosis (CF) in new-born infants.
When is it requested?
This test is used as part of a newborn screen for cystic fibrosis, in cases of meconium ileus (no stools in the first 24 to 48 hours of life), where there are other symptoms and signs suggestive of CF or where a baby is known to be at high risk of having the condition.
IRT testing is not diagnostic; there are a number of false positives and diseases other than CF and pancreatic dysfunction that can cause a positive IRT. An elevated level must be followed with other testing. When diagnosing CF this may include 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 symptomatic, 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 degree of IRT elevation does not reflect the severity of the disease.
Common Questions
Ask a Laboratory Scientist
* indicates a required field!
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.
