What Do TREC and KREC Tests Screen For in Newborns?

When you receive a call that your baby’s newborn screening had an abnormal TREC or KREC result, it is completely normal to feel scared and overwhelmed. The most important thing to know right now is that this screening is an early warning system, not a final diagnosis ^[1]. These tests are designed to catch serious immune system defects before a baby gets their very first infection ^[2]. Catching these conditions early allows doctors to protect your baby and plan treatments that can save their life ^[3].

What To Do Today

• Confirm contact information: Make sure you have the direct callback number for the screening team or pediatric immunologist.
• Ask about vaccines: Do not allow your baby to receive live vaccines (like the oral rotavirus vaccine) until cleared by an immunologist. You should ask your doctor which routine inactivated vaccines are safe to continue.
• Seek urgent care for illness: If your baby develops a fever (a rectal temperature of 100.4°F / 38.0°C or higher), feeds poorly, is unusually sleepy, has breathing trouble, persistent diarrhea, or a rash, seek emergency medical care immediately. Tell the medical staff right away that your baby had an abnormal newborn immune screen.
• Ask about feeding and medical care: Ask the immunologist if you need to temporarily pause breastfeeding while waiting for maternal CMV (cytomegalovirus) testing. Also ensure any medical team knows your baby might require specialized (irradiated and CMV-safe) blood transfusions if blood products are ever needed.

What do TREC and KREC mean?

A baby’s immune system has different types of white blood cells that fight off infections. The newborn screening uses a few drops of blood from a heel prick to check for small DNA circles that are produced when the body builds these crucial cells ^[4][5]. They are indirect markers of recent immune cell production, not direct tests of immune function.

• TREC (T-cell receptor excision circles) are markers of newly formed T-cells ^[4]. T-cells are cells that coordinate the immune response and directly attack infected cells. If the TREC level is low or undetectable, it suggests the baby may have reduced T-cell production, which is the hallmark of a severe condition called Severe Combined Immunodeficiency (SCID) ^[6][4].
• KREC (Kappa-deleting recombination excision circles) are markers of newly formed B-cells ^[7]. B-cells are cells that mature to produce antibodies against bacteria. A low KREC result suggests reduced B-cell production, which can be an early warning for conditions like agammaglobulinemia (a disorder where the body struggles to make antibodies) ^[8]. Not all regions include KREC in their newborn screening programs.

Does an abnormal result mean my baby definitely has an immune deficiency?

No. An abnormal result simply means your baby needs more testing to find out why the numbers were out of range ^[1].

While a low TREC or KREC can indicate a serious primary immunodeficiency like SCID or severe B-cell defects, there are many other reasons a baby might have an abnormal result:

• Prematurity or low birth weight: Babies born early or very small often have lower TREC and KREC levels that naturally improve as they grow ^[9][1].
• Medical conditions: Being born with a congenital heart defect, an infection, or other conditions requiring neonatal intensive care can temporarily lower these numbers ^[1][10].
• Mothers’ medications: If the mother was taking certain immunosuppressive medicines (drugs that lower immune responses, such as azathioprine) during pregnancy, it can cause the baby to have temporarily low KREC levels ^[5][1].

Sometimes, babies with abnormal screening results have no immune system problems at all ^[1].

What happens next?

Because these tests are designed to find babies before they get sick, your baby will likely look perfectly healthy right now ^[3]. However, an apparently well baby can become ill quickly if they lack a functioning immune system, so their doctor will treat the situation with urgency ^[11]. The exact steps depend on your baby’s specific results and local screening program rules.

1. Follow-up Testing: For some premature babies or borderline results, the program may just ask for a repeat heel-prick blood test. If the result is severely low or absent, your doctor will urgently order a specialized blood draw (called flow cytometry or lymphocyte subsets) to count the exact number and types of T-cells and B-cells in your baby’s blood ^[12].
2. Seeing a Specialist: You will likely be referred to a pediatric immunologist. They will review the blood tests, examine your baby, and determine if further testing is needed to check how well the cells function or to look for genetic causes ^[13]. A normal cell count does not rule out every single immune problem, so the specialist’s review is vital.
3. Interim Precautions: Your immunologist will give you tailored instructions on how to protect your baby while waiting for results. Rather than a strict protective bubble for everyone, precautions depend on the baby’s specific lab values. You may be asked to avoid crowds, practice strict handwashing, and ensure close caregivers are up-to-date on safe vaccines, while delaying live vaccines for the baby ^[3].

Finding out your newborn might have a health issue is terrifying, but the newborn screening did exactly what it was supposed to do. If your baby does have a primary immunodeficiency, knowing early gives them the best chance. For example, in one early California newborn screening program, infants diagnosed with SCID and treated promptly had a 94% survival rate ^[2]. While every baby’s individual prognosis depends on their specific diagnosis and treatment, early identification is a powerful tool to protect their health.