TREC vs KREC: comparing newborn immunodeficiency screens

The assay does not diagnose every inborn error of immunity. It detects defined failures in lymphocyte production by quantifying DNA markers generated during T-cell and B-cell receptor rearrangement.

The distinction in TREC vs KREC newborn screening is therefore biological and diagnostic. TREC primarily reflects the production of new T lymphocytes in the thymus. KREC reflects the production of new B lymphocytes during immunoglobulin gene rearrangement in the bone marrow. The two markers overlap in laboratory workflow, but they do not cover the same immunodeficiency phenotype.

Molecular mechanisms: how TREC and KREC markers function

TREC and KREC are non-replicating circular DNA fragments. They are generated as byproducts of V(D)J recombination, the process that creates antigen-receptor diversity in developing lymphocytes.

A T-cell receptor excision circle, or TREC, is produced during T-cell receptor gene rearrangement in the thymus. Newly generated, naive T lymphocytes carry these circular DNA fragments after leaving the thymic compartment. Because TRECs do not replicate during cell division, their concentration decreases as T cells proliferate. The measured quantity therefore functions as a surrogate marker for recent thymic output rather than as a direct count of all T lymphocytes.

KREC formation occurs during immunoglobulin kappa light-chain gene rearrangement in developing B cells in the bone marrow. Like TRECs, KRECs are non-replicating. Their quantity provides an indirect measure of newly produced B lymphocytes. Reduced KREC levels indicate impaired B-cell generation or maturation, particularly when the abnormal result is confirmed by cellular and molecular testing.

The markers are measured from dried blood spots, normally collected 48–72 hours after birth. A laboratory punches a small disc from the dried blood spot, commonly a 3 mm punch, and extracts DNA. Multiplex quantitative real-time PCR can then quantify TREC, KREC, and a reference gene from the same sample.

Reference genes may include RNASP, ACTB, or albumin. They serve as internal indicators of amplifiable genomic material and help identify specimens with inadequate DNA recovery or PCR performance. A low TREC or KREC value is not interpretable in isolation if the reference signal is poor.

TREC is a marker of recent thymic T-cell production. KREC is a marker of recent bone-marrow B-cell production. Neither is a universal immunodeficiency assay.

The assay architecture is similar, but the biological signal is not interchangeable:

Diagnostic scope: SCID versus XLA and related disorders

TREC screening is primarily a screen for severe combined immunodeficiency and other conditions associated with profound T-cell lymphopenia. SCID is clinically urgent because affected infants have severely impaired adaptive immunity and may develop life-threatening infections before a diagnosis is established through symptoms alone.

The value of TREC screening is the separation of molecular risk from clinical presentation. A newborn can appear clinically stable while having a severely reduced capacity to generate functional T cells. The dried blood spot assay is not measuring immune competence directly, but it can identify an abnormal production signal before recurrent infection, persistent thrush, chronic diarrhea, or growth failure becomes evident.

A low TREC result is not specific to SCID. It can also occur with other forms of T-cell lymphopenia, syndromic disorders, prematurity-related factors, or technical problems affecting DNA extraction and amplification. The screening output is therefore a triage signal. It identifies infants who require confirmatory immunological assessment.

KREC screening extends the diagnostic field toward B-cell production defects. The principal target is congenital agammaglobulinemia, including X-linked agammaglobulinemia caused by pathogenic variants affecting BTK and non-X-linked forms involving other molecular mechanisms. These disorders can produce a marked reduction or absence of circulating mature B cells and immunoglobulin production.

TREC alone does not reliably identify X-linked agammaglobulinemia or isolated B-cell disorders. The relevant biological failure occurs in the B-cell compartment, so a T-cell production marker is not an adequate proxy. This is the central reason to compare a TREC-only program with a combined TREC/KREC assay rather than treating the two markers as alternative versions of the same screen.

What each marker can and cannot establish

The diagnostic scope can be summarized as follows:

1. Low TREC with normal KREC indicates a possible T-cell production defect or profound T-cell lymphopenia. SCID is one important possibility, but not the only one. Flow cytometry is required to measure T-cell number and phenotype, including naive T-cell populations where clinically appropriate.

2. Normal TREC with low KREC indicates a possible B-cell production or maturation defect. This pattern is compatible with disorders such as congenital agammaglobulinemia, but it does not establish a genetic diagnosis. B-cell enumeration, immunoglobulin testing, and targeted molecular analysis may be required.

3. Low TREC and low KREC raises concern for a combined defect affecting both lymphocyte compartments or for a broader disturbance in lymphocyte development. The result requires expedited clinical evaluation and confirmatory testing.

4. Normal TREC and KREC reduces the likelihood of the severe T- and B-cell production defects targeted by the assay. It does not exclude all primary immunodeficiencies. Disorders involving antibody function, innate immunity, complement, phagocyte function, immune regulation, or later-onset phenotypes may remain outside the screening window.

The screen is therefore defined by its biomarkers. It is not a general-purpose test for pediatric immune health and should not be described as one.

The multiplex advantage: one specimen, two immune compartments

A combined TREC/KREC assay quantifies both markers in a single dried blood spot specimen. This offers a straightforward laboratory advantage: one collection and one DNA extraction can provide information about both T-cell and B-cell production.

Multiplex qPCR can also include a reference gene in the same analytical workflow. The resulting assay measures the target markers alongside a control for sample adequacy and amplification. This improves analytical interpretation compared with treating an isolated low copy number as definitive evidence of disease.

The practical value of multiplexing is greatest when the screening program is designed to identify more than SCID. TREC screening has a narrower biological target. Adding KREC increases coverage for congenital B-cell deficiencies, particularly agammaglobulinemia phenotypes that may not be visible through TREC analysis.

The expansion is not equivalent to universal immune screening. KREC does not detect every B-cell disorder. A child may have adequate early B-cell production but later develop functional antibody failure, defective class switching, impaired antigen-specific responses, or an immunodeficiency caused by a pathway outside the KREC signal. The assay detects a specific production abnormality, not the entire B-cell phenotype.

Analytical advantages and operational constraints

The main strengths of the combined approach are technical and programmatic:

• Shared specimen type. Both markers can be quantified from a dried blood spot collected during routine newborn screening.
• Low sample volume. A single 3 mm punch can support multiplex analysis when DNA recovery and assay validation are adequate.
• Compartment-specific information. TREC and KREC provide separate signals for T-cell and B-cell production.
• Early risk stratification. Testing is performed before most affected infants develop infection-related clinical findings.
• Scalable throughput. Real-time PCR is compatible with batch processing and standardized laboratory workflows.

The constraints are equally specific:

• Marker biology limits sensitivity. A normal marker does not exclude immunodeficiencies that do not impair the measured rearrangement process.
• Cut-offs are platform-dependent. Copy-number thresholds vary with assay design, calibrators, reference genes, laboratory population data, and regional percentile calculations.
• False-positive results require resolution. Abnormal screening results generate follow-up testing, not a final diagnosis.
• Pre-analytical quality affects performance. Poor blood-spot collection, insufficient saturation, transport problems, or DNA extraction failure can distort results.
• Clinical context remains necessary. Prematurity, transfusion history, maternal treatment, and other neonatal factors can alter interpretation.

Throughput is not the same as clinical accuracy. A high-volume laboratory can process large numbers of specimens, but the screening program still depends on validated cut-offs, repeat testing rules, referral pathways, and access to pediatric immunology services.

Interpreting TREC and KREC screening results

There is no single internationally standardized TREC or KREC cut-off that can be transferred unchanged between laboratories. Assay chemistry, instrument platform, extraction method, reference normalization, and local newborn population data influence the reported copy number.

Examples from pilot programs illustrate the variation. One Brazilian pilot used thresholds below 15 TRECs per microliter and below 14 KRECs per microliter. A South Kazakhstan pilot used thresholds below 3,165 copies per 10^6 cells for TREC and below 2,554 copies per 10^6 cells for KREC. These values are not universal clinical constants. They belong to specific analytical and population contexts.

The units themselves matter. A result expressed as copies per microliter cannot be compared directly with a result expressed as copies per 10^6 cells without understanding the normalization method. Comparing raw numerical values across assay systems can create a false impression of diagnostic discrepancy.

Most screening programs use an algorithm rather than a single decision point. A specimen may be classified according to the initial result, reference-gene performance, repeat testing, and the degree of marker reduction. A borderline result may trigger repeat analysis, while a markedly abnormal result may lead directly to confirmatory testing. The exact pathway is program-specific.

Why false-positive results occur

False-positive results are not an incidental feature of TREC/KREC screening. They are an expected consequence of applying a highly sensitive screening strategy to a large newborn population.

For TREC, the principal issue is that a low value can reflect conditions other than SCID. The result may indicate non-SCID T-cell lymphopenia or a specimen and amplification problem. Confirmatory flow cytometry distinguishes a molecular screening signal from an actual reduction in circulating lymphocyte populations.

For KREC, maternal immunosuppressive therapy during pregnancy can produce secondary or transiently reduced KREC levels in the newborn. This can create a false-positive B-cell screening result without establishing congenital agammaglobulinemia. Clinical history is therefore part of laboratory interpretation, not an optional addition after the result is reported.

Other sources of uncertainty include:

• inadequate or uneven blood-spot impregnation;
• insufficient DNA recovery from the punch;
• degraded material or extraction inhibition;
• low target copies near the assay threshold;
• biological variation in newborn lymphocyte production;
• prematurity and other neonatal variables;
• transient effects that resolve on repeat testing.

A screening result should be classified as abnormal, borderline, or uninterpretable according to the validated laboratory algorithm. It should not be translated directly into a disease label.

A low TREC or KREC result is a referral signal. The diagnosis requires cellular characterization and, where indicated, identification of pathogenic variants.

Confirmatory testing after an abnormal screen

The next diagnostic layer is generally flow cytometry. For a low TREC result, flow cytometry can quantify total T cells, helper and cytotoxic T-cell subsets, B cells, and natural killer cells. The precise panel depends on the suspected phenotype and local clinical protocol. Naive and memory T-cell markers may add information about thymic output and maturation.

For a low KREC result, flow cytometry assesses B-cell numbers and phenotype. A markedly reduced or absent B-cell population supports a B-cell production or maturation defect, but it does not by itself distinguish X-linked from non-X-linked causes.

Genetic testing then defines the molecular mechanism when the cellular phenotype is consistent. Depending on the clinical pattern, the diagnostic strategy may include targeted analysis of a known gene, a primary immunodeficiency panel, exome sequencing, or genome-level testing. The choice depends on the phenotype, family history, laboratory findings, and the local diagnostic pathway.

For suspected X-linked agammaglobulinemia, analysis of BTK is relevant. For SCID and other combined immunodeficiencies, the gene set is broader and includes multiple pathways involved in lymphocyte development, receptor signaling, DNA repair, and immune-cell survival. A panel can provide efficient throughput when the phenotype is sufficiently defined. Exome sequencing may be more appropriate when the phenotype is complex or the candidate gene range is wide.

Variants require clinical classification. A detected sequence change is not automatically pathogenic. The laboratory must evaluate the variant against established evidence, inheritance, population frequency, functional data, and phenotype concordance. The final interpretation should distinguish pathogenic variants, likely pathogenic variants, variants of uncertain significance, and benign findings.

Prenatal genetic screening has a different role. It may identify known familial pathogenic variants or assess a pregnancy at risk for a defined disorder. It does not replace postnatal TREC/KREC screening, because newborn screening is designed to identify affected infants without requiring a known family history, while prenatal testing is usually driven by a specific risk assessment.

Clinical impact and screening program design

The clinical utility of TREC/KREC screening depends on more than analytical sensitivity and specificity. A program is effective only when abnormal results lead to timely confirmatory testing and appropriate clinical management.

TREC screening has been adopted in multiple newborn screening settings because SCID is severe, clinically actionable, and difficult to identify reliably from early symptoms. The principal benefit is a reduction in diagnostic delay. Early recognition can alter infection-prevention measures, vaccination decisions, specialist referral, and eligibility assessment for definitive treatment.

Adding KREC broadens the program toward B-cell deficiencies. In a combined TREC/KREC screening program covering 2.3 million newborns, the birth prevalence of severe T- and/or B-cell immunodeficiencies was reported as 8.13 per 100,000 live births. A Czech pilot reported a screenable inborn error of immunity incidence of approximately 10.5 per 100,000 newborns. These figures describe program-specific populations and definitions. They should not be converted into a universal prevalence estimate for all primary immunodeficiencies.

The operational differences between TREC-only and dual-marker programs are material:

The screening window also matters. Dried blood spots are routinely collected at 48–72 hours after birth, but neonatal physiology changes rapidly. The timing of collection, gestational age, transfusion status, and neonatal care can affect the result. Programs therefore require defined rules for repeat sampling and referral.

The performance of a newborn screen should be assessed using the complete pathway:

• analytical sensitivity and specificity of the qPCR assay;
• invalid and repeat-test rates;
• positive predictive value in the screened population;
• time from abnormal screen to flow cytometry;
• time from confirmatory testing to genetic diagnosis;
• proportion of affected infants identified before infection;
• loss to follow-up;
• laboratory and clinical capacity for confirmatory testing.

A technically sensitive assay has limited clinical utility if follow-up services cannot process referrals. Conversely, a highly specific cut-off may reduce unnecessary referrals but increase the risk of missed cases. The optimal balance is determined through local validation and outcome monitoring, not by importing a threshold from another country.

TREC versus KREC: selecting the appropriate screening scope

The choice between TREC-only and combined TREC/KREC screening is primarily a policy and coverage decision. It is not a choice between two competing diagnostic technologies.

TREC is the required marker when the clinical objective is newborn screening for SCID and severe T-cell lymphopenia. KREC adds value when the program also aims to detect congenital B-cell maturation disorders, including X-linked agammaglobulinemia and non-X-linked agammaglobulinemia.

The combined assay is technically efficient because both markers can be measured from the same dried blood spot. It is diagnostically broader because it samples two distinct lymphocyte-production pathways. However, the expansion does not eliminate the need for conventional immunology. Flow cytometry remains necessary for cellular confirmation. Genetic sequencing remains necessary for molecular classification when indicated.

The most important limitation is coverage bias. TREC/KREC assays are strongest for severe quantitative defects in T- or B-cell production. They are weaker for disorders in which lymphocyte numbers are preserved but function is impaired. A normal combined screen cannot be interpreted as a general clearance for immune disease.

Final assessment of clinical utility

TREC and KREC are complementary biomarkers, not interchangeable tests. TREC quantifies a surrogate of thymic T-cell output and supports screening for SCID and profound T-cell lymphopenia. KREC quantifies a surrogate of newly produced B lymphocytes and extends screening toward congenital agammaglobulinemia and related B-cell deficiencies.

A combined TREC/KREC assay provides the widest coverage of the two screening models while retaining a high-throughput dried-blood-spot workflow. Its clinical utility depends on validated local cut-offs, reference-gene controls, repeat-testing algorithms, and rapid access to flow cytometry and molecular diagnostics.

The rigid interpretation is straightforward:

• low TREC is not synonymous with SCID;
• low KREC is not synonymous with XLA;
• normal TREC and KREC do not exclude all primary immunodeficiencies;
• numerical cut-offs cannot be transferred across laboratories without validation;
• definitive diagnosis requires cellular and, when appropriate, genetic confirmation.

For newborn programs focused exclusively on SCID, TREC remains the central assay. For programs designed to detect severe defects in both T- and B-cell production, multiplex TREC/KREC screening offers greater diagnostic scope from the same specimen. Its value is measurable, but only within the limits of the biomarkers and the follow-up system supporting them.