Pediatric Immunology

How the Diagnosis is Made

At a Glance

After a low TREC newborn screen, FOXN1 haploinsufficiency is evaluated with flow cytometry to measure T-cell number and maturity, genetic testing, and checks of B and NK cells to distinguish it from SCID or 22q11.2 deletion.

When a newborn screening test comes back with a “low TREC” result, it triggers an urgent series of evaluations to understand how your baby’s immune system is developing. Understanding the biology behind these tests can help you navigate the next steps with your medical team.

The Role of the Thymus “School”

To understand FOXN1 haploinsufficiency, it helps to think of the immune system as an educational system. T cells are born in the bone marrow (the “nursery”) but they cannot fight infections until they mature in the thymus (the “school”) ^[1].

The FOXN1 gene is the “headmaster” of this school. It provides the instructions for thymic epithelial cells (TECs), which make up the physical structure and the “curriculum” of the thymus ^[2]^[3]. These cells produce essential signals (like DLL4 and CCL25) that tell young immune cells to become T cells and teach them how to identify germs ^[4]. In FOXN1 haploinsufficiency, having only one working copy of this gene means the “school” may be smaller or less efficient, leading to fewer “graduating” T cells at birth ^[1]^[5].

Decoding the TREC Result

The newborn screen measures TRECs (T-cell Receptor Excision Circles). These are small, circular pieces of DNA created when a T cell is being “educated” in the thymus ^[6].

A High TREC Count: Indicates the thymus is producing new T cells.
A Low TREC Count: Acts as a “flag” indicating that the output of new T cells is low ^[7].

It is important to remember that a low TREC result is a surrogate marker—it tells us the thymic output is low, but it doesn’t tell us why ^[8]. Many things can cause a low TREC result, including prematurity, certain medications taken during pregnancy, or other medical conditions ^[9]^[10]. Therefore, the diagnostic sequence is: Low TREC screen → confirmatory immune testing → genetic testing/variant interpretation.

Key Diagnostic Tests

If the TREC result is low, your doctor will order more specific tests to find the cause:

1. Flow Cytometry

This test uses lasers to count different types of white blood cells. Doctors look for:

Total T Cells (CD3): The overall absolute number of T cells.
Helper (CD4) and Killer (CD8) T Cells: The two main branches of the T-cell family. In FOXN1 haploinsufficiency, CD8 counts are often lower than CD4 counts ^[11].
Naïve T Cells (CD45RA+): These are cells with a phenotype suggesting they recently left the thymus ^[12]. A low percentage suggests the thymus is struggling to produce new recruits ^[13].

2. Genetic Sequencing

Genetic sequencing looks at your baby’s DNA to see if there is a variant in the FOXN1 gene itself ^[14]. Finding a variant requires expert interpretation to ensure it is pathogenic and matches the child’s immune phenotype, as a Variant of Uncertain Significance (VUS) does not definitively prove the diagnosis.

Ruling Out Other Conditions

The diagnostic process is often about “ruling out” other possibilities to ensure the correct diagnosis. Doctors will check for:

Hematopoietic SCID: In these cases, the problem is in the bone marrow (the “nursery”). These babies often lack B cells or NK cells as well as T cells ^[15]. In FOXN1 haploinsufficiency, B and NK cells are usually normal ^[16].
22q11.2 Deletion (DiGeorge Syndrome): This is another “thymus problem” where a small piece of chromosome 22 is missing ^[17]. While it can look like FOXN1 issues on an immune test, it often involves other physical features like heart defects or low calcium levels ^[18]. Genetic testing can easily distinguish between the two ^[19].

Because low TRECs can be urgent, doctors often recommend a tiered infection prevention plan and delaying live vaccines until they are sure how well your baby’s immune system is working ^[20]^[21]. You must also inform emergency and transfusion teams that your child has a suspected T-cell disorder and requires irradiated, leukoreduced, and CMV-negative blood products until the evaluation is complete.

Common questions in this guide

What does a low TREC result mean for my baby's newborn screen?

A low TREC result means the screen detected fewer signs of newly produced T cells than expected. It is a warning signal, not a diagnosis: prematurity, medications taken during pregnancy, and other medical conditions can also lower the result. Follow-up immune testing is needed to find the cause.

How do doctors confirm FOXN1 haploinsufficiency?

Doctors usually combine flow cytometry, which counts and characterizes T cells, with genetic sequencing of FOXN1. A detected variant must be interpreted by experts to determine whether it is disease-causing and fits the baby's immune findings. A variant of uncertain significance does not by itself confirm the diagnosis.

What do CD3, CD4, CD8, and naive T-cell results show?

Flow cytometry measures the total T-cell count and the main CD4 helper and CD8 killer T-cell groups. It can also measure naive T cells, which are cells that appear to have recently left the thymus. In FOXN1 haploinsufficiency, CD8 counts may be lower than CD4 counts, while a low naive-cell proportion can suggest reduced thymic output.

How do doctors distinguish FOXN1 haploinsufficiency from other causes of low T cells?

Doctors check B cells and NK cells, because hematopoietic forms of SCID often reduce these cell types along with T cells, whereas they are usually normal in FOXN1 haploinsufficiency. They may also evaluate for 22q11.2 deletion, especially if heart defects, low calcium, or certain physical features are present. Genetic testing helps distinguish these conditions.

Why might my baby need vaccine and blood-product precautions during testing?

A suspected T-cell disorder can increase concern about infections while the immune evaluation is underway. Clinicians may delay live vaccines and recommend a tiered infection-prevention plan until they understand the baby's immune function. If a transfusion is needed, tell emergency and transfusion teams that irradiated, leukoreduced, and CMV-negative blood products are required during the evaluation.

What happens if the initial immune test results are borderline?

The medical team may repeat flow cytometry to see whether absolute T-cell counts and the proportion of naive T cells change over time. The timing depends on the initial results and your baby's clinical situation, so ask the team when testing should be repeated and what result would change the plan.

Get notified when new evidence is published on Combined immunodeficiency due to FOXN1 haploinsufficiency.

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References (21)

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This page explains how clinicians evaluate suspected FOXN1 haploinsufficiency after a low TREC screen for educational purposes only and does not replace medical advice. Your baby's immunology and genetics team should interpret the results and advise on vaccines, infection precautions, and transfusions.