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PubMed This is a summary of 83 peer-reviewed journal articles Updated
Pediatrics

What is SCID and Understanding the Newborn Screen

At a Glance

An abnormal SCID newborn screen means your baby has low or missing T-cells. You must immediately isolate your baby at home, require strict handwashing, and avoid all live vaccines like the rotavirus vaccine while awaiting confirmatory testing.

Receiving an abnormal newborn screening result is terrifying. It is natural to feel a sense of panic or overwhelming worry for your child’s future. However, it is important to know that this screening is exactly what it was designed to do: find children who need help before they ever get sick [1][2]. Because this was caught early, you have already given your baby the single greatest advantage possible in managing this condition [3][4].

Immediate Action Checklist

If you just received the call about an abnormal screen, here is what you need to do right now:

  • Isolate your baby: Keep your baby at home. Do not take them to public places like grocery stores or places of worship [5].
  • Stop all live vaccines: Inform your pediatrician that your baby cannot receive live vaccines. The rotavirus vaccine (often given at 2 months) is a live virus and can be deadly for a baby with SCID [6][7].
  • Strict handwashing: Anyone touching the baby must wash their hands with soap and water for 20 seconds [5].
  • Manage siblings: Keep older siblings who attend daycare or school away from the baby as much as possible to prevent them from passing on common childhood germs [5].
  • For a detailed list of rules, read the Protecting Your Infant: Immediate Precautions page.

What is SCID?

Severe Combined Immunodeficiency (SCID) is a rare genetic condition where a baby is born without a functional immune system [8]. In a healthy body, the immune system acts like a specialized security force. In babies with SCID, the “T-cells” (the leaders of that security force) are missing or do not work [8][9].

Without these cells, the body cannot defend itself against common germs that most of us encounter every day without issue. This is why SCID is often called a “pediatric emergency”—not because the baby is sick now, but because they have no natural shield against future infections [8][10].

Understanding the Newborn Screen and “TREC”

The test that identified this risk is called the TREC assay [1]. It was performed using the small drop of blood taken from your baby’s heel shortly after birth.

  • What is a TREC?: A T-cell receptor excision circle (TREC) is simply a chemical “receipt” left over when the body successfully builds a new T-cell [11][12].
  • The Logic of the Test: If the laboratory sees plenty of these receipts in the blood, it means the baby’s body is successfully “printing” new T-cells [11][13]. If the TREC count is very low or zero, it is a signal that the immune system is not producing the cells it needs [14].

An abnormal screen does not always mean your baby has SCID; it can sometimes be caused by other health factors or even the mother’s medications during pregnancy [14][15]. However, an abnormal result must be followed by immediate diagnostic testing.

What Happens Next?

Your journey over the next several weeks will generally follow these steps. You can explore each topic using the links below:

01

Confirming the Diagnosis: Flow Cytometry and Genetics

Learn how doctors confirm a SCID diagnosis using flow cytometry and genetic testing. Understand your baby's T, B, and NK cell counts and SCID genetic subtype.

02

Curative Treatments: Transplant and Gene Therapy

Learn about curative treatments for Severe Combined Immunodeficiency (SCID). Understand bone marrow transplants, gene therapy, conditioning, and PEG-ADA.

03

Building Your Care Team and Preparing for the Hospital

Learn how to build an expert SCID care team, evaluate transplant centers, and prepare for your baby's hospital stay in a HEPA-filtered isolation room.

04

Life After Treatment: Monitoring and Recovery

Learn what to expect after SCID treatment. Understand chimerism, immune reconstitution, immunoglobulin therapy, and monitoring for late effects like GVHD.

While you wait for more tests, remember: Your baby is not currently sick. The newborn screen is a “warning light” to help us keep them that way. Please proceed to the Protecting Your Infant page to understand how to keep the environment safe right now.

Common questions in this guide

What does an abnormal SCID newborn screen mean?
An abnormal newborn screen means your baby's blood showed low or zero T-cells. While it can be a sign of Severe Combined Immunodeficiency (SCID), it can also be caused by other temporary factors. Your doctor will need to order confirmatory blood tests immediately.
What is a TREC assay test?
A TREC assay is a newborn screening test that measures T-cell receptor excision circles, which are chemical markers left behind when the body makes new T-cells. A low TREC count alerts doctors that the baby's immune system may not be developing properly.
What immediate precautions should I take after an abnormal SCID screen?
You must immediately isolate your baby at home, enforce strict 20-second handwashing for anyone touching the baby, and keep older siblings away if possible. It is also critical that your baby does not receive any live vaccines, such as the rotavirus vaccine.
Is my baby sick right now?
No, an abnormal newborn screen does not mean your baby is currently sick. The test is designed to catch immune deficiencies before the baby is exposed to everyday germs, allowing doctors to protect and treat them early.
What are the next steps to confirm a SCID diagnosis?
Doctors will perform specialized blood tests to accurately count your baby's immune cells. They will also use genetic testing to look for specific mutations that cause SCID in order to guide the best treatment plan.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was the specific TREC value from the newborn screen, and what does it tell us about my baby's T-cell levels?
  2. 2.What are the next diagnostic steps to confirm if this is SCID or another type of T-cell lymphopenia?
  3. 3.Should I stop breastfeeding or take specific precautions if I (the mother) am CMV positive?
  4. 4.What immediate changes should we make to our home environment and who is allowed to visit?
  5. 5.Can you refer us to a specialized center that performs hematopoietic stem cell transplants (HSCT) or gene therapy for SCID?

Questions For You

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References

References (15)
  1. 1

    Newborn screening for severe combined immunodeficiency and T-cell lymphopenia.

    Puck JM

    Immunological reviews 2019; (287(1)):241-252 doi:10.1111/imr.12729.

    PMID: 30565242
  2. 2

    Severe Combined Immunodeficiency (SCID)-the Irish Experience.

    Burns H, Collins A, Marsden P, et al.

    Journal of clinical immunology 2021; (41(8)):1950-1953 doi:10.1007/s10875-021-01106-3.

    PMID: 34374883
  3. 3

    Outcome of hematopoietic stem cell transplantation for severe combined immunodeficiency and impact of newborn screening on overall survival: A single referral center study.

    Anchoo C, Lev A, Simon AJ, et al.

    The Journal of allergy and clinical immunology 2025; (155(6)):1800-1812 doi:10.1016/j.jaci.2025.01.029.

    PMID: 39900265
  4. 4

    Survival After Hematopoietic Stem Cell Transplantation in Severe Combined Immunodeficiency (SCID): A Worldwide Review of the Prognostic Variables.

    Goebel GA, de Assis CS, Cunha LAO, et al.

    Clinical reviews in allergy & immunology 2024; (66(2)):192-209 doi:10.1007/s12016-024-08993-5.

    PMID: 38689103
  5. 5

    Positive Newborn Screening for Severe Combined Immunodeficiency: What Should the Pediatrician Do?

    Mongkonsritragoon W, Huang J, Fredrickson M, et al.

    Clinical medicine insights. Pediatrics 2023; (17()):11795565231162839 doi:10.1177/11795565231162839.

    PMID: 37025258
  6. 6

    Current status of BCG and OPV vaccination in children with primary immunodeficiency in Chongqing, China.

    Liu Y, Zhang Y, Ning Y, et al.

    Medicine 2025; (104(40)):e44935 doi:10.1097/MD.0000000000044935.

    PMID: 41054950
  7. 7

    Persistent Infection with Rotavirus Vaccine Strain in Severe Combined Immunodeficiency (SCID) Child: Is Rotavirus Vaccination in SCID Children a Janus Face?

    De Francesco MA, Ianiro G, Monini M, et al.

    Vaccines 2019; (7(4)) doi:10.3390/vaccines7040185.

    PMID: 31744133
  8. 8

    Severe combined immunodeficiency diagnosis and genetic defects.

    Aranda CS, Gouveia-Pereira MP, da Silva CJM, et al.

    Immunological reviews 2024; (322(1)):138-147 doi:10.1111/imr.13310.

    PMID: 38287514
  9. 9

    Tackling Infectious Diseases in the Caribbean and South America: Epidemiological Insights, Antibiotic Resistance, Associated Infectious Diseases in Immunological Disorders, Global Infection Response, and Experimental Anti-Idiotypic Vaccine Candidates Against Microorganisms of Public Health Importance.

    Justiz-Vaillant A, Soodeen S, Gopaul D, et al.

    Microorganisms 2025; (13(2)) doi:10.3390/microorganisms13020282.

    PMID: 40005649
  10. 10

    Severe Combined Immunodeficiency-Classification, Microbiology Association and Treatment.

    Justiz-Vaillant AA, Gopaul D, Akpaka PE, et al.

    Microorganisms 2023; (11(6)) doi:10.3390/microorganisms11061589.

    PMID: 37375091
  11. 11

    Identification of 22q11.2 deletion syndrome via newborn screening for severe combined immunodeficiency. Two years' experience in Catalonia (Spain).

    Martin-Nalda A, Cueto-González AM, Argudo-Ramírez A, et al.

    Molecular genetics & genomic medicine 2019; (7(12)):e1016 doi:10.1002/mgg3.1016.

    PMID: 31663686
  12. 12

    Establishing Newborn Screening for SCID in the USA; Experience in California.

    Puck JM, Gennery AR

    International journal of neonatal screening 2021; (7(4)) doi:10.3390/ijns7040072.

    PMID: 34842619
  13. 13

    Investigation of the causal etiology in a patient with T-B+NK+ immunodeficiency.

    Sertori R, Lin JX, Martinez E, et al.

    Frontiers in immunology 2022; (13()):928252 doi:10.3389/fimmu.2022.928252.

    PMID: 35967429
  14. 14

    Newborn Screening for Severe Combined Immunodeficiency.

    Taki M, Miah T, Secord E

    Pediatric clinics of North America 2019; (66(5)):913-923 doi:10.1016/j.pcl.2019.06.007.

    PMID: 31466681
  15. 15

    When Screening for Severe Combined Immunodeficiency (SCID) with T Cell Receptor Excision Circles Is Not SCID: a Case-Based Review.

    Buchbinder D, Walter JE, Butte MJ, et al.

    Journal of clinical immunology 2021; (41(2)):294-302 doi:10.1007/s10875-020-00931-2.

    PMID: 33411155

This information about SCID newborn screening is for educational purposes only and does not replace emergency medical advice. Always follow your pediatrician's and immunologist's instructions regarding infant isolation and immediate care.

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