What Is the IRT Newborn Screen for Cystic Fibrosis?

Immunoreactive trypsinogen, or IRT, is the blood marker used in the first step of newborn screening for cystic fibrosis. A small sample of blood taken from a baby’s heel in the first few days of life is tested for IRT levels, and if those levels are elevated, the screening program flags the baby for further evaluation. IRT itself is a pancreatic enzyme precursor, and it leaks into the bloodstream at abnormally high levels in newborns whose pancreatic ducts are already being damaged by thick mucus, even before any clinical symptoms appear. The test has been the cornerstone of CF newborn screening worldwide for decades, but the path from a high IRT result to an actual diagnosis is more complex than most parents realize.

What IRT Measures and Why It Works

In a baby with cystic fibrosis, the protein that regulates salt and water movement across cell membranes is defective. This causes thick secretions to block the pancreatic ducts before birth, damaging the tissue and forcing trypsinogen, a digestive enzyme precursor, into the blood. IRT is essentially a snapshot of how much prenatal pancreatic injury has occurred. Research has confirmed that newborn IRT serves as a biomarker of prenatal exocrine pancreatic disease in CF patients, and that genetic variants in other genes can modify the degree of that damage, contributing to variability in IRT levels from one CF baby to the next.¹

Most screening labs set a cutoff above which a baby’s IRT is considered elevated. In one large UK dataset, about 2.4% of all babies fell above the laboratory cutoff, meaning the vast majority of flagged newborns do not have CF.² The challenge for screening programs is finding the right balance: set the cutoff too low and you overwhelm families and labs with false alarms, but set it too high and you miss affected babies.

What Happens After a High IRT Result

A single elevated IRT is not a diagnosis. It is the starting gun for a multi-step process, and different states and countries handle that process differently. The two dominant strategies are called IRT/IRT and IRT/DNA.

In an IRT/IRT protocol, a baby with an elevated first IRT is called back for a second heel prick, usually around two weeks of age. If the IRT is still high on the second sample, the baby is referred for a diagnostic sweat test. In an IRT/DNA protocol, the original blood spot is immediately tested for known CF-causing gene variants. If one or two variants are found, the baby is referred for a sweat test. Programs have debated the merits of each approach for years, weighing factors like turnaround time, cost, and the equity implications of gene panels.³

Some programs have explored floating daily percentile cutoffs instead of fixed IRT thresholds. A study of 15 years of California screening data found that switching to a daily percentile-based cutoff at the 4-5% level would catch more cases but would also send roughly 150 to 210% more specimens on to molecular testing, a major increase in lab workload and cost.⁴ That trade-off illustrates why no single cutoff strategy dominates everywhere.

The Sweat Test and Why It Matters

Regardless of which algorithm flags a baby, the gold standard for confirming or ruling out CF is the sweat chloride test. A small electrical current is used to stimulate sweat production on the baby’s forearm, and the chloride concentration in that sweat is measured. A value at or above 60 mmol/L on two occasions confirms CF; below 30 mmol/L is considered normal; and the 30-59 range is intermediate and requires further investigation.

Getting enough sweat from a tiny newborn is not always easy. One study comparing two collection methods found that the more traditional approach had a “quantity not sufficient” rate of over 15%, compared with about 2% for the newer Macroduct system. After adjusting for age, the odds of getting an inadequate sample were more than eight times higher with the older method.⁵ A failed sweat test means the family has to come back and repeat the process, which adds stress during an already anxious time.

When IRT Gets It Wrong

IRT screening misses some babies with CF, and the most well-known reason is meconium ileus, a bowel obstruction caused by thick meconium that occurs in a subset of CF newborns. Counterintuitively, these babies often have IRT levels in the normal range. One study found that 42% of infants with meconium ileus had IRT values below the screening cutoff, and their average IRT was significantly lower than that of CF infants without the bowel complication.⁶ The likely explanation is that the severe early pancreatic obstruction in these babies has already depleted the trypsinogen that would otherwise leak into the blood.

A Brazilian study of children with false-negative newborn screens reinforced this pattern, noting that meconium ileus was present in a notable proportion of missed cases and that false-negative IRT in this group is a well-documented phenomenon.⁷ The clinical takeaway is that a normal IRT does not rule out CF if a newborn presents with meconium ileus. Clinicians should pursue diagnostic testing regardless of the screening result in those cases.

On the other end, there are biological reasons IRT can spike in the absence of CF. Prematurity, perinatal stress, and certain other conditions can transiently raise IRT. These false positives are far more common than true positives, which creates an emotional burden for families that screening programs increasingly try to manage.

The Equity Problem With Gene Panels

When a screening program uses an IRT/DNA approach, the specific gene variants included in the panel matter enormously. Most panels were originally designed around the variants most common in people of European descent, such as the F508del mutation. This creates a real gap for families of other backgrounds. A multi-state analysis found that variant detection was significantly lower in Hispanic, Black, Asian, and Native American newborns compared with white newborns across the most commonly used screening panels.⁸

The practical consequence is that a baby of Hispanic or Black ancestry with CF is more likely to have a false-negative screen in an IRT/DNA program, because the baby’s second CFTR variant may not be on the panel. Researchers have argued that if an IRT/DNA method is used, the panel needs to include more rather than fewer variants to avoid a disproportionate number of missed cases in ethnic minorities.⁹ This is one of the strongest arguments for moving toward broader sequencing approaches.

Next-Generation Sequencing as an Alternative

Some programs have started replacing limited gene panels with next-generation sequencing (NGS), which reads the entire CFTR gene rather than checking a short list of known variants. A feasibility study showed that pairing IRT with NGS could offer higher sensitivity, better specificity, and improved positive predictive value compared with standard panels.¹⁰

New York State has been refining an IRT/NGS approach over several phases. After initially including variants whose clinical significance is uncertain, the program learned that doing so identified too many babies who turned out not to have CF. Removing those uncertain variants, with one exception, brought the rate of confirmed two-variant detection up to 94% of CF patients, while also reducing the ratio of uncertain diagnoses to confirmed cases to less than half.¹¹ A validated NGS assay developed alongside this effort analyzes 338 clinically relevant CFTR variants in a tiered strategy, with full gene sequencing available when the first tier does not give a clear answer.¹²

NGS addresses the equity problem directly: if you read the whole gene, you do not need to predict in advance which variants a baby might carry. But it also raises new questions, because broader sequencing finds more variants of uncertain clinical significance, which can leave families in a gray zone.

The Gray Zone Between CF and Healthy

One of the most unsettling outcomes for families is a designation called CRMS/CFSPID, which stands for CFTR-related metabolic syndrome or CF screen positive, inconclusive diagnosis. These are babies who screen positive and are found to carry CFTR variants but do not meet the full criteria for a CF diagnosis. They show no clinical evidence of disease, yet their screening results suggest some risk of developing CF or a CFTR-related disorder down the road.¹³

How common is this outcome? In New York’s first three years of IRT/DNA/sequencing screening, nearly 60% of all referred newborns ended up classified as CRMS/CFSPID rather than as having CF. The vast majority carried one disease-causing variant alongside a variant of uncertain or varying significance. Over one to three years of follow-up, none had abnormal fecal elastase results and only two ultimately converted to a CF diagnosis with diagnostic sweat chloride levels.¹⁴ So most of these babies will likely remain healthy, but they still need periodic monitoring, and their families live with lingering uncertainty.

Why Early Detection Changes Outcomes

The whole point of screening is to catch CF before symptoms cause irreversible damage. The evidence that this works is strong. A Welsh study comparing children diagnosed through screening with those born just before screening was introduced found that screened children were on average about 2.7 cm taller and 1.7 kg heavier by age 10. Their lung function, measured by the volume of air they could forcefully exhale in one second, was roughly 9% higher at the same age.¹⁵

Longer follow-up data from another cohort showed that the advantages persisted through age 15. Screened individuals gained lung function over a nine-year period while unscreened individuals lost it, and body mass index was higher in the screened group.¹⁶ Even within the screening era, babies who were diagnosed late, meaning they slipped through the screen or were born to families who did not follow up, fared worse. Compared with babies caught early, late-diagnosed children had higher rates of hospitalization for respiratory illness, worse lung function, and higher rates of chronic infection with Pseudomonas aeruginosa, a stubborn lung pathogen in CF.¹⁷

The arrival of CFTR modulator drugs, which target the underlying protein defect rather than just treating symptoms, has raised the stakes further. If these highly effective treatments can be started in the first months of life, the benefit of catching CF early through IRT screening could be even greater than historical studies suggest.¹⁸

The Emotional Toll of a Positive Screen

For every baby who is ultimately confirmed to have CF, many more families go through the screening process and come out the other side with a healthy child. That does not mean the experience is benign. A scoping review of research on false-positive newborn screens found that abnormal results, even when repeat testing was normal, were associated with parental anxiety and depression. Among UK mothers who received a false-positive CF result, nearly two-thirds recalled that waiting for the results of repeat tests was difficult and led to feelings of upset, guilt, and heightened anxiety.¹⁹

When genetic testing is involved, the ripple effects extend beyond the baby. Interviews with parents of 44 infants who had false-positive CF screens revealed repercussions including questioning whether the test was accurate, worrying about the child’s future health, wondering whether other relatives might carry CF mutations, and in some cases even questioning paternity. Parents also reported gaining new perspectives, strengthening relationships, and broadly supporting newborn screening despite the scare.²⁰

When Screening Reveals Carrier Status

IRT/DNA screening can uncover something the family did not ask for: the fact that their baby is a carrier of a single CF variant. A carrier will not develop CF but could pass the variant on to future children. When South Carolina switched to a protocol that included CFTR genotyping for babies with elevated IRT, the program detected more carriers than before, and parents of those babies showed increased emotional distress.²¹

Carrier detection sometimes triggers a cascade of family-wide revelations. At one medical center after statewide CF screening was implemented, 101 families were counseled and the majority of parents chose to have their own carrier status tested. In five couples, both parents turned out to be carriers. One of those couples, whose newborn was only a carrier, had an older child who was then unexpectedly found to have CF.²² Genetic counseling at the time of the diagnostic sweat test has been recommended to help parents understand the meaning of carrier results for the whole family, including talking to their children about it later on.²³

Sample Quality and Practical Pitfalls

Before any of the science matters, the blood spot card itself has to be handled properly. IRT, along with dozens of other newborn screening markers, is measured from a dried blood spot on filter paper. These spots are sensitive to how they are stored. A stability study of newborn screening markers found that exposure to high humidity caused dramatic degradation: seven markers lost more than 90% of their initial levels within 30 days under humid conditions, and four of those lost more than half within the first week.²⁴ While that study examined a panel of markers broadly, the principle applies to IRT as well: a blood spot left in a humid mailbox or delayed in transit can give misleadingly low results. Hospitals and labs follow strict handling protocols for exactly this reason, but in practice, shipping delays and environmental exposure still happen.

How Much It Costs and Whether It Is Worth It

Cost-effectiveness analyses consistently find that CF newborn screening is worth doing, regardless of which protocol is used. A simulation study comparing several approaches found that all screening strategies were more favorable than no screening at all. The IRT/PAP method (which uses a second biomarker called pancreatitis-associated protein instead of DNA) had the most favorable ratio at about $28,400 Canadian per CF case detected, while IRT/IRT was the next best option.²⁵ A European study using real-world data found similar results, with cost-effectiveness ratios ranging from about €23,600 to €29,200 per life-year gained depending on the protocol. That study also noted that if early diagnosis reduces treatment costs by even 5%, screening begins to produce net savings.²⁶

An American analysis added an interesting wrinkle: the IRT/IRT strategy saved the healthcare system about $2.30 per newborn overall, but when the costs were broken down by who actually bears them, families ended up with higher out-of-pocket expenses because of the second clinic visit for the repeat heel prick.²⁷ Programs that use IRT/DNA avoid that second visit, shifting the cost to the laboratory side instead.

Where IRT Screening Is Headed

The trajectory is clear: screening programs are moving toward broader genetic analysis after an initial elevated IRT. The appeal of NGS is that it sidesteps the equity problem of limited panels, catches more cases, and can be refined as clinical understanding of CFTR variants improves. But broader sequencing also means identifying more babies in the gray zone of CRMS/CFSPID, more carrier identifications that families did not expect, and more follow-up visits for children who may never develop disease. Programs will need to invest in genetic counseling infrastructure, clear communication protocols for uncertain results, and long-term monitoring systems for the growing cohort of babies who fall between “healthy” and “has CF.” The IRT heel-prick itself has barely changed in decades, but what happens after it flags a baby is being transformed.