Best Tests to Ensure Immunocompetency
Begin with a stepwise approach using screening tests first, followed by advanced testing only when indicated by clinical presentation or abnormal screening results. 1
Initial Screening Tests
The most appropriate initial battery includes:
Humoral Immunity Assessment
- Serum immunoglobulin levels (IgG, IgA, IgM, IgE)
- Serum specific antibody titers to vaccine antigens (tetanus, diphtheria, pneumococcus)
- Antibody response to booster immunization if baseline titers are low
- Flow cytometry to enumerate total B cells
Cellular Immunity Assessment
- Flow cytometry to enumerate CD4 and CD8 T cells and NK cells - this is the cornerstone test for T-cell immunocompetence
- Complete blood count with differential
- Cutaneous delayed hypersensitivity testing (though less commonly used now)
Phagocytic Function
- Blood cell count with differential to assess absolute neutrophil count
- Neutrophil morphology on peripheral blood smear
- DHR (dihydrorhodamine) reduction test or nitroblue tetrazolium for oxidative burst function
Complement System
- CH50 assay (total hemolytic complement activity) for classical pathway
- AH50 assay (alternative pathway hemolytic activity)
Advanced Testing (When Screening Abnormal)
Only proceed to these if screening tests are abnormal or clinical suspicion remains high:
For Humoral Defects
- Flow cytometry for B-cell subsets (naive vs. switched memory cells)
- In vitro immunoglobulin production assays
- Antibody response to φX174 bacteriophage (specialized test)
For Cellular Defects
- T-cell subset enumeration (naive, memory, activated cells)
- In vitro proliferative response to mitogens and antigens
- T-cell cytotoxicity assays
- Cytokine production in response to stimuli
For Phagocytic Defects
- Chemotaxis and phagocytosis assays
- Enzyme assays (myeloperoxidase, G6PDH)
- Bacterial or fungal killing assays
Critical Interpretation Points
The clinical presentation must guide which arm of immunity to prioritize. Recurrent sinopulmonary infections suggest humoral defects, while opportunistic infections (fungal, viral, Pneumocystis) point to cellular defects. Severe bacterial infections with encapsulated organisms may indicate complement deficiency. 1
Common Pitfalls to Avoid
Antibody titers must be interpreted in context of immunization history - don't order vaccine titers without knowing what vaccines the patient received and when 1
Timing matters - avoid testing during acute illness when possible, as results may be transiently abnormal
Age-appropriate reference ranges - immunoglobulin levels vary significantly by age; use proper pediatric references
Don't order genetic testing first - reserve microarray, targeted gene sequencing, or whole-exome sequencing for cases where screening/advanced tests suggest specific diagnoses 1
Emerging Functional Tests
Recent evidence supports interferon-γ release assays (IGRA) in response to mitogen as a practical functional test, particularly in critically ill or immunocompromised patients. This fully automated assay can be performed on whole blood with results in 4 hours and may identify severe immunological phenotypes when combined with monocyte HLA-DR expression. 2, 3 However, this remains primarily a research tool and is not yet standard practice for routine immunocompetence assessment.
Special Populations
For immunocompromised patients (transplant recipients, those on immunosuppressants), serological testing for vaccine responses is most reliable for hepatitis B, measles, rabies, rubella, and yellow fever, where established immune correlates of protection exist. 4 In these patients, functional testing becomes more important than simple cell counts.