Immunoreactive trypsin (IRT) is used to selectively screen for cystic fibrosis (CF) in new-born infants. Currently, in the UK, it is not part of routine neonatal screening programmes.

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 fair 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: another IRT in a month, CF gene mutation testing, and/or sweat chloride testing.

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.

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.