Diagnostic Approach for Selective Antibody Deficiency

Initial Laboratory Assessment

The American Academy of Allergy, Asthma, and Immunology recommends measuring baseline serum immunoglobulin levels, including IgG, IgA, and IgM, to confirm they are within normal limits for age, which distinguishes SAD from other antibody deficiencies like CVID or agammaglobulinemia 1
Measuring total serum protein and albumin is necessary to exclude secondary hypogammaglobulinemia from protein loss, as recommended by the American Academy of Allergy, Asthma, and Immunology 1, 2

The American Academy of Allergy, Asthma, and Immunology recommends administering the 23-valent pneumococcal polysaccharide vaccine (PPSV-23) and measuring serotype-specific antibody responses to diagnose SAD, with abnormal response defined as concentration >1.3 µg/mL for <50% of serotypes tested in children under 6 years, and <70% of serotypes tested in adults and children over 6 years 1
Measuring antibody titers 4-8 weeks after PPSV-23 vaccination is crucial, as recommended by the American Academy of Allergy, Asthma, and Immunology, to assess the patient's response to the vaccine 1, 2

The American Academy of Allergy, Asthma, and Immunology criteria for SAD diagnosis include measuring antibody responses to at least 7-12 pneumococcal serotypes, with abnormal response defined as concentration >1.3 µg/mL for <50% of serotypes tested in children under 6 years, and <70% of serotypes tested in adults and children over 6 years 1

Measuring antibody responses to protein antigens, such as diphtheria, tetanus, pertussis titers, and conjugated pneumococcal or Haemophilus influenzae vaccine responses, is necessary to assess overall B-cell function, as recommended by the American Academy of Allergy, Asthma, and Immunology 1, 2

Performing B-cell subset analysis, including total B-cell count, switched memory B cells, marginal zone B cells, and transitional B cells, is necessary to distinguish SAD from early CVID and provide prognostic information, as recommended by the American Academy of Allergy, Asthma, and Immunology 1, 5

Ruling out IgG subclass deficiency by measuring IgG1, IgG2, IgG3, IgG4 levels is necessary, as IgG2 and IgG3 deficiencies commonly coexist with SAD, as recommended by the American Academy of Allergy, Asthma, and Immunology 1
Assessing for selective IgA deficiency by measuring IgA levels is necessary, as SAD can coexist with selective IgA deficiency, as recommended by the American Academy of Allergy, Asthma, and Immunology 2

The American Academy of Allergy, Asthma, and Immunology recommends recognizing that SAD may represent early or evolving CVID, and patients with SAD should be monitored longitudinally for progression to CVID, with repeat immunoglobulin levels periodically to detect declining IgG or IgA 1, 5
Understanding the limitations of current testing, including the fact that ELISA-measured antibody concentrations do not reflect functional opsonophagocytic activity, is crucial, as recommended by the American Academy of Allergy, Asthma, and Immunology 3, 4
Avoiding overdiagnosis by not relying solely on laboratory values, but also considering infection severity and frequency, is necessary, as recommended by the American Academy of Allergy, Asthma, and Immunology 6, 3