immunoreactive trypsin (IRT)

Last edited 07/2019

Infants with cystic fibrosis 1 to 2 weeks of age show increased levels of immunoreactive trypsin in the plasma in the neonatal heel-prick test.

The test is positive if immunoreactive trypsin is greater than 80 mcg/litre. It is currently the best screening test for cystic fibrosis but cannot be done after the 8 weeks of life since the immunoreactive trypsin falls as pancreatic insufficiency develops.

Immunoreactive trypsin is also raised in haemorrhagic shock and encephalopathy.


  • The National Screening Committee recommends that newborn blood spot screening include: Phenylketonuria (PKU), Congenital Hypothyroidism (CHT), Sickle Cell disorders, Cystic Fibrosis (CF) and Medium Chain Acyl Co-A Dehydrogenase Deficiency (MCADD)
  • Blood spot screening is offered to all babies up to one year of age, with the exception of CF where the test is valid up to 8 weeks of age only CF screening may detect some but not all carriers, SCD screening also detects carriers.