Severe Combined Immunodeficiency (SCID) Genetic Test: Immune Deficiency Genes and Results

A severe combined immunodeficiency genetic test looks for inherited changes that prevent T cells—and often B cells or natural killer cells—from working normally. SCID is a medical emergency because an affected baby may appear well at birth but can quickly develop severe, persistent, or unusual infections. Genetic testing helps confirm the diagnosis, identify the exact SCID subtype, guide treatment, and clarify risks for parents, siblings, and future pregnancies. It is usually ordered after an abnormal newborn screen, profound T-cell lymphopenia, concerning infections, or a known family history. A positive result can influence the urgency and type of stem cell transplant, enzyme replacement, or gene therapy. A negative panel does not always exclude SCID, because some disease-causing variants may lie outside the regions tested or in genes not yet recognized. Results therefore need to be interpreted with immune-cell counts, T-cell function tests, clinical findings, and specialist review.

• The test identifies disease-causing variants in SCID-related genes, commonly including IL2RG, ADA, RAG1, RAG2, JAK3, IL7R, DCLRE1C, and several DNA-repair genes.
• A positive result can confirm a molecular diagnosis, but the gene and variant must match the child’s immune-cell pattern and symptoms.
• An abnormal newborn TREC screen is not itself a genetic diagnosis; it signals low newly formed T cells and requires urgent confirmatory testing.
• Testing usually uses blood, buccal cells, or another DNA sample, and no fasting or medication changes are generally needed.
• Infants with suspected SCID need immediate specialist precautions, including avoidance of live vaccines and careful selection of blood products while evaluation continues.
• A negative or uncertain result may require broader sequencing, deletion/duplication analysis, or reanalysis if laboratory findings still strongly suggest SCID.

Table of Contents

• What the SCID Genetic Test Shows
• When SCID Genetic Testing Is Ordered
• Major SCID Genes and Immune Patterns
• How the Test Is Performed
• Understanding SCID Genetic Test Results
• How Results Guide Treatment
• Inheritance, Family Testing, and Pregnancy
• Limitations and Next Steps

What the SCID Genetic Test Shows

SCID genetic testing examines DNA for variants that disrupt the development or function of T lymphocytes. T cells coordinate much of the adaptive immune response. When they are absent or profoundly impaired, the body cannot control many viral, fungal, bacterial, and opportunistic infections. B cells may be present but unable to make effective antibodies without T-cell help. Natural killer, or NK, cells may also be reduced depending on the genetic subtype.

SCID is not one disease caused by one gene. It is a group of inborn errors of immunity with overlapping clinical features. More than 20 genes can produce a SCID or SCID-like phenotype, and the number continues to grow. A laboratory may use a focused SCID panel, a broader primary immunodeficiency genetic panel, exome sequencing, or genome sequencing. The test chosen depends on the urgency, immune phenotype, family history, and local laboratory workflow.

The genetic result answers several different questions:

• Is there a molecular explanation for the low T-cell count or impaired T-cell function?
• Which pathway is affected—cytokine signaling, antigen-receptor recombination, purine metabolism, DNA repair, thymic development, or another process?
• Is the disorder X-linked, autosomal recessive, or rarely autosomal dominant?
• Does the subtype create special risks, such as sensitivity to radiation or chemotherapy?
• Could a targeted treatment, such as adenosine deaminase enzyme replacement or gene therapy, be appropriate?
• Which relatives may carry the same variant?

Genetic testing does not replace immune testing. A diagnosis usually combines the DNA result with an absolute lymphocyte count, flow cytometry for T-, B-, and NK-cell numbers, naïve T-cell markers, T-cell receptor excision circles, or TRECs, and lymphocyte proliferation studies. Maternal cells may cross the placenta and persist in an infant with SCID, so laboratories may also test for maternal T-cell engraftment. These cells can make the T-cell count appear less low than it truly is and can contribute to rash or graft-versus-host-like illness.

Typical SCID is characterized by a profound lack of functional autologous T cells. Leaky or atypical SCID allows some T-cell development, often because the variant retains partial activity. Omenn syndrome is a form of leaky SCID with abnormal, restricted T-cell expansion and features such as a widespread red rash, enlarged lymph nodes or liver and spleen, eosinophilia, and high IgE. Genetic testing helps distinguish these related presentations.

When SCID Genetic Testing Is Ordered

SCID genetic testing is ordered urgently when newborn screening or clinical findings suggest severe T-cell deficiency. In places that screen newborns for SCID, the first signal is usually a low TREC value from a dried blood spot. TRECs are small DNA circles formed as T cells mature in the thymus. A low value means few newly produced T cells are entering the blood, but it does not identify the cause.

A positive newborn screen can reflect classic SCID, leaky SCID, another combined immunodeficiency, a syndromic condition, prematurity, major congenital heart disease, lymphocyte loss, or temporary effects of illness or treatment. Confirmatory immune testing should begin promptly. Genetic testing may be ordered at the same time or soon afterward because the molecular diagnosis can affect treatment planning.

Doctors may also suspect SCID in an infant who was not screened or whose screen was missed. Concerning findings include:

• persistent thrush that does not respond normally to treatment
• chronic diarrhea, poor weight gain, or failure to thrive
• severe pneumonia, bloodstream infection, meningitis, or recurrent infections
• infection with organisms that rarely cause serious disease in healthy infants
• prolonged viral illness, including cytomegalovirus, respiratory viruses, or rotavirus
• complications after a live vaccine, such as rotavirus or BCG vaccine
• absent or very small tonsils and lymph nodes
• marked lymphopenia on a complete blood count
• a family history of infant deaths, severe infections, known SCID, or male relatives affected through the maternal line

A low absolute lymphocyte count in an infant deserves prompt assessment because normal infant lymphocyte counts are higher than adult values. A count that might appear acceptable by an adult reference range can be abnormally low for age.

Until SCID is excluded, clinicians commonly advise measures that reduce avoidable exposure and treatment risks. Live vaccines are withheld. If transfusion is needed, blood products are generally irradiated, leukocyte-reduced, and cytomegalovirus-safe according to the treating center’s protocol. Breastfeeding decisions may depend on the mother’s cytomegalovirus status and the infant’s diagnosis. Families should follow the immunology and transplant team’s instructions rather than making changes on their own.

Genetic testing may also be performed before symptoms develop when a familial variant is known. This is a targeted presymptomatic genetic test, often providing a faster and clearer answer than a broad panel.

Major SCID Genes and Immune Patterns

The pattern of T, B, and NK cells helps narrow the likely gene, although patterns can overlap. Laboratories often describe phenotypes using plus and minus signs—for example, T−B+NK− means T cells are very low, B cells are present, and NK cells are very low.

Gene or pathway	Common pattern	Important features	Typical inheritance
IL2RG	T−B+NK−	Most common cause of X-linked SCID; disrupts the common gamma-chain used by several cytokine receptors	X-linked
JAK3	T−B+NK−	Produces a phenotype similar to IL2RG deficiency	Autosomal recessive
IL7R	T−B+NK+	Blocks signaling needed for T-cell development; B and NK cells are usually present	Autosomal recessive
RAG1, RAG2	T−B−NK+	Prevent normal rearrangement of T- and B-cell antigen receptor genes; partial-function variants may cause Omenn syndrome or later-onset immune dysregulation	Autosomal recessive
DCLRE1C	T−B−NK+	Artemis deficiency; impaired DNA repair during antigen-receptor formation and increased sensitivity to radiation and alkylating chemotherapy	Autosomal recessive
ADA	Often T−B−NK−	Toxic purine metabolites damage lymphocytes; may have skeletal, neurologic, hearing, liver, or lung involvement	Autosomal recessive
PNP	Mainly profound T-cell deficiency	Purine metabolism disorder; neurologic problems and autoimmunity may occur	Autosomal recessive
AK2	T−B−NK− with low neutrophils	Reticular dysgenesis; severe congenital neutropenia and possible sensorineural hearing loss	Autosomal recessive
LIG4, NHEJ1, PRKDC	Often T−B−NK+	DNA-repair defects; may cause microcephaly, growth restriction, developmental issues, and treatment-related toxicity	Usually autosomal recessive

Other genes can affect thymic development, calcium signaling, CD3 components, or additional immune pathways. For example, FOXN1 variants can impair thymic epithelial development, while ORAI1 and STIM1 variants can cause combined immunodeficiency with muscle, dental, or skin features. Some chromosomal disorders and syndromes create severe T-cell lymphopenia without fitting classic hematopoietic-cell SCID. That distinction matters because a stem cell transplant may not correct a primary thymic stromal defect.

A gene name alone does not predict severity. Different variants in the same gene may leave no protein function, partial function, or an altered function. RAG1 and RAG2 are good examples: complete loss can cause classic T−B− SCID, while hypomorphic variants may produce Omenn syndrome, granulomas, autoimmune disease, or a combined immunodeficiency diagnosed later in childhood or adulthood.

How the Test Is Performed

Most SCID genetic tests use a blood sample. A buccal swab, saliva sample, cultured skin cells, or another tissue may be used in specific circumstances. No fasting is usually required. The laboratory extracts DNA and analyzes a defined set of genes or a broader portion of the genome.

A common testing sequence is:

1. Rapid clinical and immune assessment. The team checks blood counts, lymphocyte subsets, naïve T cells, TRECs, immunoglobulins, and T-cell function while treating active infections.
2. Focused or broad sequencing. A panel reads the coding regions and nearby splice boundaries of genes known to cause SCID and related immune disorders.
3. Copy-number analysis. The laboratory looks for deletions or duplications that sequencing alone may miss. Some panels include this automatically; others require a separate method.
4. Variant confirmation and family testing. A clinically important result may be confirmed by another method, followed by testing of parents or relatives.
5. Broader testing when needed. Exome or genome sequencing may follow if the first panel is negative and the phenotype remains convincing.

Turnaround time varies from a few days for urgent rapid testing to several weeks for comprehensive analysis. The clinical team should not delay protective measures or definitive planning while waiting for DNA results when the immune findings already support SCID.

Recent transfusion, maternal cell engraftment, stem cell transplant, or gene therapy can complicate which DNA is present in blood. The laboratory must know about these events. After a transplant, blood DNA may mainly represent the donor rather than the patient. A pre-transplant sample, buccal cells, skin fibroblasts, or another non-blood source may be needed to establish the patient’s germline genotype.

The test is different from a TREC screen. TREC testing measures a marker of new T-cell production; genetic testing identifies a causal DNA change. It is also different from HLA typing, which is used to compare donor and recipient tissue markers. If transplant is planned, a separate HLA matching test is performed for the child and potential donors.

Understanding SCID Genetic Test Results

A laboratory report usually classifies variants as pathogenic, likely pathogenic, uncertain significance, likely benign, or benign. The interpretation depends on the gene’s inheritance pattern, the number of variants found, their phase, and whether the immune phenotype fits.

Positive or diagnostic result

A diagnostic result identifies the disease-causing variant pattern expected for the gene. Examples include one pathogenic IL2RG variant in an affected boy, two pathogenic ADA variants on opposite copies of the gene, or two disease-causing RAG1 variants inherited one from each parent.

A positive result can:

• confirm the SCID subtype
• identify treatment-specific risks
• support targeted donor and conditioning choices
• open access to gene-specific therapy or clinical trials
• allow accurate testing of parents and siblings
• clarify recurrence risk in future pregnancies

The report may use the term “likely pathogenic.” In clinical genetics, that classification generally means there is strong evidence that the variant causes disease, even if the evidence is not yet sufficient for the highest category. A likely pathogenic result can be clinically actionable when it fits the inheritance and phenotype.

Carrier result

A carrier result means a person has one pathogenic variant for an autosomal recessive SCID condition but usually does not have that condition. In an affected child, finding only one variant in a recessive gene is not a complete explanation. The second variant might be a deletion, a deep intronic change, a structural rearrangement, or a variant that current methods cannot detect.

Females with one IL2RG pathogenic variant are usually carriers of X-linked SCID. Most are healthy because of X-chromosome inactivation, although unusual immune findings are possible. Carrier testing should use the known familial variant whenever available rather than relying on a generic screen.

Variant of uncertain significance

A variant of uncertain significance, or VUS, means current evidence cannot determine whether the change causes disease. A VUS should not be treated as a confirmed diagnosis by itself. It may become more or less suspicious when combined with functional studies, RNA analysis, parental testing, population frequency, and the child’s cellular phenotype. The general principles are described in a genetic variant result interpretation.

For a recessive condition, two uncertain variants are not automatically diagnostic. The laboratory may need to determine whether they are in trans—on opposite chromosome copies—or in cis on the same copy. Testing both parents often resolves this question.

Negative result

A negative result means the test did not find a reportable disease-causing variant. It does not prove that SCID is absent. The result may be negative because:

• the causal gene was not included
• the variant lies in a region the method does not assess well
• a structural change or mosaic variant was missed
• the disorder is caused by a non-genetic or secondary process
• the gene-disease relationship has not yet been discovered

When immune testing strongly supports SCID, clinicians continue management and expand the molecular investigation rather than relying on a negative panel alone.

How Results Guide Treatment

SCID treatment must begin before infections and organ damage become advanced. The genetic subtype helps the team choose the safest curative approach, but age, infection status, donor availability, organ function, and center expertise also shape the plan.

Hematopoietic stem cell transplantation can provide healthy donor stem cells capable of producing functional immune cells. Outcomes are generally better when treatment occurs early, especially before serious infection. Donor selection may include an HLA-matched sibling, matched unrelated donor, haploidentical relative, or other donor source. The need for conditioning chemotherapy varies by genotype, donor, graft strategy, and treatment goal.

Genetic findings can change conditioning decisions. Patients with DCLRE1C, LIG4, NHEJ1, or related DNA-repair defects may be unusually sensitive to ionizing radiation and certain chemotherapy agents. Recognizing that subtype before treatment can reduce avoidable toxicity. ADA-SCID has additional options, including polyethylene glycol-conjugated ADA enzyme replacement as a bridge or treatment and gene therapy in appropriate settings. Gene therapy is also available or under study for selected other SCID forms, including X-linked SCID.

Supportive treatment may include antimicrobial prophylaxis, immunoglobulin replacement, aggressive treatment of infection, nutritional support, and isolation precautions tailored to the child. Some vaccines given before diagnosis can cause persistent infection and require targeted management. Clinicians also monitor viral testing, liver and lung function, and signs of maternal-cell engraftment.

A molecular diagnosis can indicate whether non-immune organs need surveillance. ADA deficiency, PNP deficiency, reticular dysgenesis, and DNA-repair disorders can affect neurologic, skeletal, hearing, pulmonary, or other systems. Curative immune treatment may not reverse every non-immune feature, so early identification improves anticipatory care.

Inheritance, Family Testing, and Pregnancy

Most SCID conditions are X-linked or autosomal recessive. Family testing should follow the exact gene and variant found in the affected person.

In X-linked SCID caused by IL2RG, a carrier mother has a 50% chance of passing the variant in each pregnancy. A son who inherits it is usually affected; a daughter who inherits it is usually a carrier. A new variant can arise in the affected child, so testing the mother remains important even without family history.

In an autosomal recessive condition, both parents are usually carriers. For each pregnancy together, there is a 25% chance the child will be affected, a 50% chance the child will be an unaffected carrier, and a 25% chance the child will inherit neither familial variant. These probabilities reset with every pregnancy. An autosomal recessive genetic test for relatives should target the known familial variants.

Siblings of a newly diagnosed infant may need urgent immune and genetic evaluation, even if they appear healthy. A sibling could be affected, a carrier, or a potential stem cell donor. Donor suitability is assessed separately; being genetically unaffected does not automatically mean the sibling is the best HLA match.

Once familial variants are known, reproductive options may include prenatal diagnosis through chorionic villus sampling or amniocentesis, preimplantation genetic testing with in vitro fertilization, use of donor eggs or sperm, or natural conception with testing after birth. Prenatal testing should be planned with a genetics professional because the method, timing, and interpretation depend on the family’s exact variants. A prenatal genetic diagnostic test is different from routine prenatal screening and directly tests fetal DNA for the familial change.

Families may also consider banking a pre-treatment DNA sample from the affected child. This can be valuable for future reanalysis, especially if transplant or gene therapy later changes the DNA profile in blood.

Limitations and Next Steps

SCID genetic testing is powerful, but its accuracy depends on the assay and the clinical context. Panels vary in gene content, coverage, copy-number detection, mitochondrial analysis, structural-variant detection, and ability to detect low-level mosaicism. A “comprehensive” panel from one laboratory may not be equivalent to another.

Before testing, ask whether the assay includes:

• sequencing plus deletion/duplication analysis
• genes associated with classic, leaky, and syndromic SCID
• rapid processing suitable for an infant with severe lymphopenia
• reanalysis as gene knowledge changes
• parental or family testing when needed for interpretation

After results return, the immunologist and geneticist should reconcile four layers of evidence: clinical findings, immune-cell phenotype, functional testing, and genotype. A strong mismatch between them deserves investigation. For example, a variant reported as pathogenic may not explain a T−B+NK− pattern if it acts in a pathway expected to remove both T and B cells. Conversely, a convincing cellular pattern may reveal that a negative panel is incomplete.

Further testing can include exome sequencing, genome sequencing, RNA studies, chromosomal microarray, enzyme assays, protein expression, or specialized functional tests. Reanalysis after one or two years may identify a newly established disease gene or a reclassified variant. Broader whole-genome sequencing can detect some noncoding and structural variants that targeted panels miss, although it also creates more uncertain findings.

Urgent medical management should proceed according to the immune phenotype, not wait for a perfect molecular answer. Fever, breathing difficulty, dehydration, worsening diarrhea, reduced feeding, unusual rash, or any sign of infection in a baby with suspected SCID warrants immediate contact with the specialist team or emergency assessment under the child’s established plan.

References

• The diagnosis of severe combined immunodeficiency (SCID): The Primary Immune Deficiency Treatment Consortium (PIDTC) 2022 Definitions 2023 (Position Statement)
• The 2022 Update of IUIS Phenotypical Classification for Human Inborn Errors of Immunity 2022 (Position Statement)
• SCID Newborn Screening: What we’ve learned 2021 (Review)
• Positive Newborn Screening for Severe Combined Immunodeficiency: What Should the Pediatrician Do? 2023 (Review)
• Advances in the treatment of severe combined immunodeficiency 2022 (Review)

Disclaimer

This information is educational and cannot diagnose SCID or replace urgent care from a pediatric immunology or transplant team. An abnormal newborn screen, profound lymphopenia, or suspected SCID requires rapid specialist evaluation, infection precautions, and individualized treatment. Do not delay emergency care while waiting for a genetic result.