What is the specific test to detect T cell immunodeficiency in pediatric patients?

Specific Test to Detect T Cell Immunodeficiency in Children

T-cell proliferation assays measuring mitogen and antigen responses are the hallmark functional test for detecting T-cell immunodeficiency in children, while flow cytometry enumeration of lymphocyte subsets (particularly naive T cells expressing CD45RA) provides essential quantitative assessment. 1

Primary Screening Tests

Complete Blood Count with Differential

• Absolute lymphocyte count is the best initial laboratory test when suspecting primary immunodeficiency 2
• Lymphopenia (defined as <4,500/mm³ in children under 8 months or <1,500/mm³ in older children) suggests T-cell disorder 3, 4
• A complete blood count usually reveals leukopenia, lymphopenia, or both in patients with severe combined immunodeficiency (SCID) 1

Flow Cytometry for Lymphocyte Subset Enumeration

• Lymphocyte subset analysis by flow cytometry (CD3+, CD4+, CD8+, CD19+, CD16/56+) is essential for quantifying specific immune cell populations 3
• CD4 T-cells are identified as CD3+ and CD4+ positive 1
• CD8 T-cells are identified as CD3+ and CD8+ positive 1
• Naive T-cell counts (expressing CD45RA and lacking CCR7) are usually very low or absent in SCID 1
• Flow cytometry characterizing naive T lymphocytes (CD45RA+CD27+) or recent thymic emigrants (CD45RA+CD31+) is recommended when primary immunodeficiency is suspected 3

Definitive Functional Tests

T-Cell Proliferation Assays

• Defects in T-cell proliferative responses to mitogens and antigens in vitro are the hallmark immunologic abnormalities in combined immunodeficiency 1
• These assays exhibit profoundly reduced proliferation to mitogens and antigens in SCID patients 1
• This functional assessment is more specific than cell counts alone for detecting T-cell dysfunction 1

T-Cell Receptor Excision Circle (TREC) Assay

• TREC quantification by real-time quantitative PCR (values <20 copies/mL indicate absent thymic output) is recommended when primary immunodeficiency is suspected 3
• The TREC assay can diagnose T-cell deficiencies at birth using dried blood spots from newborn screening cards 1, 5
• Low TREC numbers are indicative of T-cell primary immunodeficiencies 1
• Infants with SCID (classical or leaky) will have very low or absent TRECs 1
• This test has made universal newborn screening for SCID possible as a public health measure 5

Algorithmic Approach to Testing

Initial Evaluation

1. Complete blood count with differential - identifies lymphopenia 2, 4
2. Lymphocyte subset enumeration by flow cytometry - quantifies T-cell populations 3
3. Immunoglobulin levels (IgG, IgA, IgM) - hypogammaglobulinemia results from lack of T-cell help 1

Confirmatory Testing When Initial Tests Abnormal

1. T-cell proliferation assays with mitogens and antigens - the gold standard functional test 1
2. TREC quantification - assesses thymic output 3
3. Naive T-cell phenotyping (CD45RA+CD27+) - distinguishes recent thymic emigrants 1, 3

Advanced Diagnostic Testing

• Genetic testing with next-generation sequencing for molecularly undefined cases 3
• Maternal engraftment testing if T cells are present (determines if T cells are of maternal origin) 1

Critical Clinical Caveats

Age-Specific Considerations

• IgG levels can be low or normal in infants because of transplacental transfer of maternal IgG, making immunoglobulin levels less reliable initially 1
• Prematurity (gestational age <37 weeks) and/or low birth weight (<2,500g) can cause transient lymphopenia 3, 6
• Premature infants have lower TREC numbers at birth, which usually increase over time 1

Common Pitfalls to Avoid

• Normal white blood cell counts do not rule out serious infections in immunocompromised patients 7
• Some forms of SCID (such as CD3δ or coronin 1A deficiencies) might show normal thymus on chest imaging 1
• In some SCID forms (e.g., ADA deficiency), T cells can be present at birth but disappear later, potentially missed by TREC screening 1
• Serious infection or metabolic stress in an infant without immunodeficiency can cause the thymus to shrink dramatically 1

Urgent Management Considerations

• Only irradiated, CMV-negative, lymphocyte-depleted cellular blood products should be administered to patients with suspected cellular immunodeficiency to prevent graft-versus-host disease 1
• Live vaccines should not be administered to patients with severely impaired specific immunity 1
• HIV testing is mandatory in all cases of unexplained lymphopenia 3