Immunoreactive trypsin (IRT) is used as part of a newborn screening program to screen for an increased risk of cystic fibrosis and is used to help diagnose pancreatic insufficiency in both children and adults.

This test is ordered 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), and as an initial test for cystic fibrosis in symptomatic young infants who are not creating enough sweat to do a sweat chloride test. A trypsinogen test is also ordered when children or adults present with symptoms suggesting cystic fibrosis and pancreatic dysfunction such as persistent diarrhea, foul-smelling bulky greasy stools, malnutrition, and vitamin deficiency.

Trypsinogen testing is not diagnostic; there are a fair number of false positives and problems 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 another IRT in a month, CF gene mutation testing, and/or sweat chloride testing.

If an IRT level is elevated, an infant may have cystic fibrosis; an infant or adult may have abnormal pancreatic enzyme production, pancreatitis, or pancreatic cancer; 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 infant is symptomatic, other testing for CF, such as sweat chloride and/or CF gene mutation testing, should be considered.

IRT testing will not identify heterozygous carriers of a CF mutation. 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.