IRT Test

Cystic fibrosis (CF) is caused by mutations in the CFTR gene, which results in thick mucus building up in the body’s organs and tissues. Early diagnosis and treatment are key to minimizing the impact of the disease and maintaining patients’ quality of life.

The IRT test is used to help diagnose CF. It is done routinely as part of newborn screening, when babies are checked for CF and other diseases shortly after birth.

What is IRT?

IRT, short for immunoreactive trypsinogen, screens for a protein made by the pancreas. Normally, trypsinogen is transported from the pancreas to the small intestine, where it is converted into an enzyme (trypsin) that helps to break down food in digestion.

In CF, the thick mucus that characterizes the disease can build up in the ducts of the pancreas, which blocks the ducts and stops trypsinogen and other pancreatic proteins needed for digestion from reaching the small intestine. This can cause digestive problems, and trypsinogen ends up being secreted into the bloodstream at abnormally high levels.

How the IRT test is performed

The basic idea of the IRT test is to check for elevated IRT levels in a person’s blood, which is indicative of CF.

When the test is done in the context of newborn screening, it typically involves a heel prick — a quick jab with a needle into the baby’s heel. The baby may cry from the brief discomfort of the needle, but the procedure does not cause any lasting harm.

Interpreting test results

While high IRT levels are indicative of CF, they also can be high following a premature or difficult birth. As such, a positive test result must be confirmed by additional testing to ensure an accurate CF diagnosis.

If the initial IRT test is positive, a second IRT test at two weeks after birth, or a DNA test, is carried out to confirm the results. The choice differs by state in the U.S., but both options allow a diagnosis with a success rate of at least 90%.

In cases where two IRT tests are performed and they are both positive for CF, parents in the U.S. will be referred to a Cystic Fibrosis Foundation-approved care center to confirm the diagnosis with a sweat test, and to discuss treatment options.

Two negative IRT test results usually indicate the baby does not have CF.  In cases in which the tests were negative, but the baby shows symptoms of CF, further testing may be carried out. A small number of CF cases may not be detected by IRT or other newborn screening tests.


Last updated: Sept. 21, 2021


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