At a glance

Also known as

Trypsin-like immunoreactivity; serum trypsinogen, IRT

Why get tested?

To screen for cystic fibrosis (CF) in new-born infants

When to get tested?

It forms part of the newborn screening programme.

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 spot of blood drawn from the infant’s heel and put onto filter paper

Test preparation needed?


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, mucous plugs can block the pancreatic ducts that lead into the small intestines, preventing trypsinogen from reaching the small intestine and resulting in a build-up of trypsinogen in the blood. 

As part of the newborn screening programme infants are screened for cystic fibrosis using a trypsinogen test called immunoreactive trypsinogen (IRT). 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 by pricking a newborn's or very young infant's heel and 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.

About Reference Intervals

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

What other tests might my doctor do to check for pancreas function?

Your doctor may request a stool test for faecal fat or faecal elastase or a blood test for amylase or rarely lipase to look at other aspects of pancreas and digestive function.

Last Review Date: February 14, 2020

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