Table 2.

Definition of SCID variants based on Primary Immune Deficiency Treatment Consortium (PIDTC) criteria

Typical SCID	Leaky (Atypical) SCID	Omenn syndrome
Absence or very low number of T cells (CD3 T cells <300/uL) AND No or very low T cell function (< 10% of lower limit of normal) as measured by lymphocyte proliferative response to mitogen phytohemagglutinin (PHA) AND/ORM T cells of maternal origin present	Reduced number of CD3 T cells for age up to 2 years < 1000/uL for > 2 years up to 4 years < 800/uL for > 4 years < 600/uL Absence of maternal engraftment < 30% of lower limit of normal T cell function (as measured by lymphocyte proliferative response to mitogen PHA Hypomorphic mutations in known SCID gene	Generalized skin rash Absence of maternal engraftment Detectable CD3 T cells, ≥ 300/mL Absent or low (up to 30% of normal) T cell proliferation to antigens to which the patient has been exposed If the proliferation to antigen was not performed, but at least 4 of the following supportive criteria, at least one of which must be among those marked with an asterisk (*) below, are present, the patient is eligible: Hepatomegaly Splenomegaly Lymphadenopathy Elevated IgE Elevated absolute eosinophil count *Oligoclonal T cells measured by CDR3 length or flow cytometry *> 80% of CD3+ or CD4+ T cells are CD45RO+ *Proliferation to PHA is reduced < 30% of lower limit of normal *Proliferative response in mixed leukocyte reaction is reduced < 30% of lower limit of normal *Mutation in SCID-causing gene

Adapted from references [14, 15]