Immunoreactive trypsinogen (IRT) is a protein produced by the pancreas that serves as the primary screening marker for cystic fibrosis in newborns. It is the inactive precursor to trypsin, a digestive enzyme that breaks down proteins in the small intestine. Every baby born in the United States has their IRT level measured as part of routine newborn screening, typically through a small blood sample taken from the heel within 24 to 48 hours of birth.
How IRT Relates to Pancreas Function
The pancreas releases trypsinogen into the digestive tract, where it gets activated into trypsin to help digest food. A small amount of trypsinogen also leaks into the bloodstream, and its concentration there reflects how much the pancreas is producing overall. This makes it a useful window into pancreatic health. The “immunoreactive” part of the name simply refers to the lab method used to detect it: an immunoassay that identifies the protein using antibodies.
In a healthy newborn, IRT circulates at relatively low levels. When something blocks or damages the pancreatic ducts, trypsinogen backs up into the blood, causing levels to spike. This is exactly what happens in cystic fibrosis. The thick, sticky mucus characteristic of the disease obstructs the pancreatic ducts, creating a backup that pushes more trypsinogen into the bloodstream than normal. This secretory obstruction is present to some degree in virtually all newborns with cystic fibrosis, which is what makes IRT such an effective screening tool.
How the Newborn Screening Works
A nurse or technician pricks the baby’s heel and collects drops of blood on a special filter paper card. The sample is sent to a state laboratory, where it is tested for dozens of conditions, IRT among them. The blood should ideally be collected after 24 hours of age but before the baby is one week old. Samples drawn too early can produce unreliable results.
Most screening programs use a cutoff around 60 to 70 ng/mL for the first IRT measurement. If the result falls above that threshold, what happens next depends on which state you live in. Some states move directly to DNA analysis, looking for mutations in the gene responsible for cystic fibrosis. These are called IRT-DNA states. Other states repeat the IRT test on a second blood sample before ordering DNA analysis, following an IRT-IRT-DNA protocol. The goal of both approaches is to narrow down which babies truly need further evaluation and which simply had a high reading for other reasons.
What a Positive IRT Result Means
An elevated IRT is not a diagnosis of cystic fibrosis. It is a flag that prompts additional testing. The vast majority of babies with a high first IRT will not have the disease. The test is deliberately set to cast a wide net so it catches as many true cases as possible, which means it also picks up many babies who are perfectly healthy.
If both IRT levels come back elevated, or if DNA testing identifies one or more cystic fibrosis gene mutations, the baby is referred for a sweat chloride test. This remains the gold standard for confirming a cystic fibrosis diagnosis. A small area of the baby’s skin (usually the forearm) is stimulated to produce sweat, and the chloride concentration in that sweat is measured. A result of 60 mEq/L or higher is considered diagnostic. Babies may also be sent for a sweat test if their IRT exceeds 170 ng/mL, even without an identified gene mutation.
Why IRT Can Be Elevated Without Cystic Fibrosis
Several factors can push IRT levels above the screening cutoff in babies who do not have the disease. The most well-documented is perinatal stress. Research comparing babies with false-positive results to the general newborn population found significantly lower Apgar scores (a measure of how well a baby is doing right after delivery) in the false-positive group, suggesting that birth-related stress on the body can temporarily raise IRT. That said, most babies with a false-positive result had perfectly normal Apgar scores, so stress is only one piece of the puzzle.
Prematurity and being a carrier of a single cystic fibrosis gene mutation (without having the disease) can also contribute to borderline elevations. Because so many non-disease factors can nudge the number up, an elevated IRT on its own should not cause alarm. It is a starting point, not an endpoint.
When IRT Can Miss Cystic Fibrosis
False negatives, where a baby with cystic fibrosis gets a normal IRT result, are rare but do occur. One recognized scenario involves meconium ileus, a bowel obstruction present at birth that affects some babies with cystic fibrosis. Paradoxically, these newborns can have normal or low IRT readings despite having the disease. Delays in collecting the blood sample also increase the risk of a missed result, because IRT levels in affected babies tend to decline over the first weeks of life and can fall below the cutoff if testing happens too late.
Laboratory errors, including problems with how the blood sample is processed from the filter paper, account for a small number of missed cases as well. For this reason, babies who show clinical signs suggestive of cystic fibrosis, such as persistent respiratory infections, poor weight gain, or salty-tasting skin, should be evaluated with a sweat test regardless of their newborn screening result. The same applies to babies born with meconium ileus or who have a sibling with the disease.
IRT and Long-Term Pancreatic Health
In babies who are confirmed to have cystic fibrosis, the initial IRT level may offer a rough signal about future pancreatic function. Higher IRT values at screening, combined with higher sweat chloride levels, appear to be associated with a greater likelihood of developing pancreatic insufficiency, a condition where the pancreas can no longer produce enough digestive enzymes and the child needs enzyme supplements with meals. This correlation is not precise enough to serve as a definitive predictor, but it gives clinicians an early indication of how the disease may progress.
Outside of newborn screening, IRT measured in dried blood spots is also being studied as a biomarker for pancreatic function in children with other forms of pancreas disease, since its blood concentration tracks with how much digestive enzyme the pancreas is actually releasing. For now, though, the overwhelming majority of people encounter IRT in one context: the routine heel-prick test their baby receives in the first days of life.