What are the steps to diagnose immunodeficiency?

Diagnostic Approach to Immunodeficiency

A stepwise laboratory evaluation is essential for diagnosing immunodeficiency, beginning with screening tests and progressing to advanced tests based on clinical presentation and initial findings. 1

Initial Evaluation Based on Clinical Presentation

• Suspect immunodeficiency when patients present with recurrent, persistent, or severe bacterial, viral, or fungal infections; failure to thrive; or other characteristic presentations like autoimmunity or lymphoproliferation 1, 2
• The pattern of infections often provides clues to the type of immunodeficiency:
  • Recurrent sinopulmonary infections with encapsulated bacteria suggest antibody deficiency 1
  • Deep-seated infections with abscess formation suggest phagocytic defects like Chronic Granulomatous Disease 1
  • Severe or disseminated infections with opportunistic pathogens suggest T-cell defects like SCID 1

Screening Laboratory Tests

Humoral Immunity Assessment

• Measure serum immunoglobulin levels (IgG, IgA, IgM, IgE) 1
• Determine specific antibody titers to both protein antigens (tetanus, diphtheria) and polysaccharide antigens (pneumococcal) 1
• Evaluate antibody response to booster immunization if titers are low 1
• Enumerate B cells by flow cytometry 1

Cellular Immunity Assessment

• Complete blood count with differential to identify lymphopenia 1
• Flow cytometry to enumerate CD4 and CD8 T cells and NK cells 1, 2
• T-cell receptor excision circle (TREC) screening in newborns for SCID 2
• Cutaneous delayed hypersensitivity testing 1

Phagocytic Cell Assessment

• Complete blood count with differential to identify neutropenia or neutrophilia 1
• Neutrophil morphology examination on peripheral blood smear 1
• Dihydrorhodamine (DHR) reduction or nitroblue tetrazolium test for neutrophil oxidative burst 1

Complement Assessment

• CH50 assay (total hemolytic complement activity) 1
• AH50 assay (alternative pathway hemolytic activity) 1
• Lectin pathway function assessment 1

Advanced Testing

Advanced Humoral Immunity Tests

• Flow cytometry to enumerate B-cell subsets (naive and switched memory cells) 1
• In vitro immunoglobulin production in response to mitogens or other stimuli 1

Advanced Cellular Immunity Tests

• Flow cytometry to enumerate T-cell subsets (naive, memory, activated cells) 1, 2
• In vitro proliferative response to mitogens and antigens 1, 2
• T-cell cytotoxicity assays 1
• Cytokine production assessment in response to stimuli 1

Advanced Phagocytic Cell Tests

• Chemotaxis and/or phagocytosis assays 1
• Enzyme assays (myeloperoxidase, G6PDH) 1
• Bacterial or fungal killing assays 1

Genetic Testing

• Microarray for copy number variation as initial genetic screening 1
• Targeted gene sequencing for suspected specific disorders 1, 3
• Whole-exome/genome sequencing for complex or undiagnosed cases 1, 3

Diagnostic Patterns in SCID

• T cells are always affected in SCID with very low or absent counts and function 2
• Different patterns of affected cells help determine the genetic defect:
  • T-B-NK- phenotype: seen in adenosine deaminase deficiency 2
  • T-B+NK- phenotype: common in X-linked SCID and JAK3 deficiency 2
  • T-B-NK+ phenotype: seen in RAG1/2 deficiencies 2
  • T-B+NK+ phenotype: observed in IL-7 receptor deficiency 2

Important Considerations

• A molecular diagnosis is highly desirable to establish unequivocal diagnosis, permit accurate genetic counseling, and identify candidates for specific therapies 1
• When uncertainty exists regarding evaluation or management, consultation with physicians experienced with immunodeficiencies is essential 1
• Internet resources are available for healthcare providers and patients (see Table V in guidelines) 1
• Early diagnosis is critical as it offers the best opportunity for reduced morbidity and survival 4
• For SCID specifically, diagnosis is an urgent medical condition requiring immediate intervention 1

Pitfalls to Avoid

• Relying solely on global immune parameters (like total immunoglobulin levels) without assessing specific immune responses 1
• Failing to consider secondary causes of immunodeficiency before diagnosing primary immunodeficiency 1
• Delaying evaluation when characteristic clinical presentations are present 1
• Not considering the possibility of combined immunodeficiencies when only one component appears affected 1
• Overlooking the need for genetic diagnosis, which is essential for definitive treatment planning 1