Since September 2025, neonatal screening for severe combined immunodeficiency (SCID) has been generalized in France. These genetic disorders, which are asymptomatic at birth, cause severe immunodeficiency, exposing infants to serious infections (viral, bacterial, or fungal) as early as the first year of life. Without early treatment and management, infectious complications can be life-threatening.

Studies show that this screening improves survival and quality of life and reduces treatment costs by enabling intervention before complications arise.

In France, the Ministry of Health referred this matter to the Haute Autorité de Santé (HAS), which issued a favorable opinion in January 2022 via a ministerial decree (published on April 16, 2025) regarding the combined screening for DICS and spinal muscular atrophy. These authorizations follow the DEPISTREC study (2015-2017), which demonstrated the effectiveness of this screening: 190,517 children were screened, resulting in a reduction in DICS-related deaths.

The primary objective of the study will be to describe the underlying causes of T-cell lymphopenia identified in newborns through neonatal screening by quantifying TRECs on Guthrie cards. (SCID; variant SCID; syndromic T-cell deficiency; secondary T-cell deficiency; attenuated SCID; Omenn syndrome; immunosuppressive treatment in the mother; not found; isolated prematurity).

Inclusion Criteria:

• Children with a positive Guthrie test result, confirmed by lymphocyte immunophenotyping performed during their first visit with a pediatric specialist.

Exclusion Criteria:

• Children whose parents objected to the collection of data after receiving the informational letter

The target population consists of all children screened for SCID in France, that is, all children born in metropolitan France and the overseas territories whose parents consent to neonatal screening of their child using the Guthrie test.

Within this population, our cohort will include all children with a positive Guthrie card screening result confirmed by lymphocyte immunophenotyping performed during the first visit with a pediatric specialist. This will include the first baby screened on or after September 1, 2025 (the start date of neonatal screening for DICS in France) and will cover a 5-year period.