What are the key lab findings in a patient with suspected immune deficiency, particularly one presenting with recurrent infections?

Laboratory Findings in Immune Deficiency

Begin with quantitative serum immunoglobulin levels (IgG, IgA, IgM) as the cornerstone screening test, followed by complete blood count with differential to identify lymphopenia, neutropenia, or thrombocytopenia. 1, 2

Initial Screening Laboratory Tests

Quantitative Immunoglobulin Measurements

• Measure serum IgG, IgA, and IgM concentrations to identify hypogammaglobulinemia, which is the hallmark of most antibody deficiencies 1, 2
• Hypogammaglobulinemia is defined as significant reduction in ≥2 isotypes (less than 50% of the lower limit of normal for age), not simply borderline values 3
• IgA below 7 mg/dL with normal IgG and IgM indicates selective IgA deficiency, the most common primary immunodeficiency 2
• Elevated IgM with severely reduced IgG and IgA suggests immunoglobulin class-switch defects (hyper-IgM syndromes) 3

Complete Blood Count with Differential

• Lymphopenia (absolute lymphocyte count <1500 cells/μL in adults) suggests T-cell deficiency or combined immunodeficiency 1, 4
• Neutropenia indicates possible phagocytic defects or severe combined immunodeficiency 1, 4
• Thrombocytopenia or splenomegaly are associated findings in patients with confirmed immunodeficiency 4

Total Serum Protein and Albumin

• Measure to exclude secondary hypogammaglobulinemia from protein loss (nephrotic syndrome, protein-losing enteropathy), which presents with low albumin alongside low immunoglobulins 2, 5

Functional Antibody Assessment

Specific Antibody Titers to Protein Antigens

• Measure pre-existing antibody titers to diphtheria, tetanus, pertussis, and conjugated pneumococcal or Haemophilus influenzae vaccines to assess functional antibody production 1, 2, 5
• These responses assess B-cell function even when total immunoglobulin levels appear normal 5

Polysaccharide Antigen Response Testing

• Administer 23-valent pneumococcal polysaccharide vaccine (PPSV-23) and measure serotype-specific antibody responses 4-8 weeks post-vaccination to diagnose selective antibody deficiency 5
• Abnormal response is defined as concentration >1.3 µg/mL for <50% of serotypes in children under 6 years, and <70% of serotypes in adults and children over 6 years 5
• However, interpretation of polysaccharide vaccine responses is problematic, as many healthy subjects have been erroneously started on immunoglobulin replacement based solely on absent robust responses 3

Advanced Immunophenotyping

Flow Cytometry for Lymphocyte Subsets

• CD4+ T-cell enumeration is essential in HIV-infected patients, as CD4+ counts <200 cells/μL predict opportunistic infections like Pneumocystis pneumonia 3
• B-cell subset analysis including total B cells, switched memory B cells (CD27+IgM-IgD-), and unswitched memory B cells (CD27+IgD+IgM+) helps distinguish X-linked agammaglobulinemia from common variable immunodeficiency 3, 1
• Reduced class-switched memory B cells with normal unswitched memory B cells is characteristic of immunoglobulin class-switch defects 3
• Low IgM levels and class-switched memory B-cell counts predict permanent immunodeficiency rather than transient hypogammaglobulinemia of infancy 3
• CD8+ T cells, natural killer cells, and B cells should all be enumerated, as reductions in multiple lymphocyte subgroups (not just CD4+ cells) can occur in idiopathic immunodeficiency 6

T-Cell Functional Studies

• Delayed hypersensitivity skin tests and lymphocyte proliferation assays to mitogens assess T-cell function when opportunistic infections suggest combined immunodeficiency 3, 1
• Decreased lymphoproliferative response to mitogens and antigens, altered cytokine production (particularly IL-4 and IL-5), and deficient expression of activation molecules (CD25, CD69, CD40L, CD70) are seen in common variable immunodeficiency 7

Critical Diagnostic Considerations

Distinguishing Primary from Secondary Immunodeficiency

• Measure total serum protein and albumin first to exclude protein-losing conditions before pursuing extensive immunodeficiency workup 2, 5
• In HIV-infected patients, use CD4+ lymphocyte counts to construct differential diagnoses: counts >200 cells/μL make opportunistic infections unlikely, and chronic cough is more likely due to the same disorders as in immunocompetent patients 3

Common Variable Immunodeficiency (CVID) Diagnosis

• CVID requires hypogammaglobulinemia (≥2 isotypes <50% lower limit of normal), defective antibody production, AND abnormal B-cell flow cytometry showing alterations in memory B cells or isotype-switched B cells 3
• Abnormal flow cytometry is particularly important to confirm questionable diagnoses and distinguish true CVID from patients with recurrent infections and borderline IgG levels 3
• Increased susceptibility to apoptosis following lymphocyte activation, decreased T and B lymphocyte counts, and decreased Th2 cytokine synthesis are additional laboratory features 7

Avoiding Diagnostic Pitfalls

• Never rely solely on IgA-based serologic tests (such as IgA-tissue transglutaminase for celiac disease) in patients with suspected or confirmed IgA deficiency, as these will be falsely negative 2
• Do not diagnose immunodeficiency based solely on absent pneumococcal polysaccharide vaccine responses without considering clinical severity, as this leads to inappropriate immunoglobulin replacement therapy 3
• Recognize that selective antibody deficiency may represent early or evolving CVID, requiring longitudinal monitoring with repeat immunoglobulin levels to detect declining IgG or IgA 5
• IgG subclass deficiency diagnosis is controversial, and the clinical significance of abnormal IgG subclass levels in patients with recurrent infections is unclear 3

When to Pursue Genetic Testing

• Consider targeted gene sequencing or whole exome/genome sequencing in familial cases, when progression to CVID is suspected, or when molecular confirmation is needed to distinguish between disorders with similar laboratory findings (e.g., CD40L defect vs. AID deficiency) 3, 1
• Molecular diagnosis is necessary because prognosis and therapy differ substantially between disorders that present with similar immunoglobulin patterns 3