Diagnostic Approach for Selective IgA Deficiency

Primary Diagnostic Tests

The American Academy of Allergy, Asthma, and Immunology recommends measuring serum immunoglobulin levels, including IgG, IgA, and IgM concentrations, as the cornerstone of diagnosis 1, 3
IgA must be below 7 mg/dL (or below the age-dependent reference range) while IgG and IgM remain within normal limits to confirm selective IgA deficiency 2, 4
Total serum protein and albumin levels should be measured to exclude secondary hypogammaglobulinemia from protein loss (nephrotic syndrome, protein-losing enteropathy), which would present with low albumin alongside low immunoglobulins 2, 4

Specific antibody titers to protein antigens (diphtheria, tetanus, pertussis, conjugated pneumococcal or Haemophilus influenzae vaccines) should be measured to assess functional antibody production 1, 3
Antibody response to polysaccharide antigens using the 23-valent pneumococcal polysaccharide vaccine is essential to identify coexisting specific antibody deficiency 1, 2, 5
Measure pre-immunization and post-immunization titers (4-8 weeks after vaccination) to assess functional antibody production 1, 4

Celiac disease screening with tissue transglutaminase (tTG) IgG is recommended, as IgA-based tests are unreliable in IgA deficiency 6, 7
Total IgA level measurement is critical before interpreting celiac serology, as 2-2.6% of celiac patients have IgA deficiency compared to 0.2% of the general population 6, 7

Complete blood count with differential to evaluate for cytopenias or lymphoproliferation is recommended 1, 3
T-cell enumeration (CD4, CD8, NK cells) by flow cytometry if there are opportunistic infections suggesting combined immunodeficiency 1, 2
Genetic testing (microarray for copy number variation, targeted gene sequencing) may be considered in familial cases or when progression to common variable immunodeficiency is suspected 1, 3

Never rely solely on IgA-based serologic tests (such as IgA-tTG for celiac disease) in patients with suspected or confirmed IgA deficiency, as these will be falsely negative 6, 7, 8
Distinguish selective IgA deficiency from early common variable immunodeficiency (CVID) by confirming normal IgG and IgM levels; patients may progress from selective IgA deficiency to CVID over time, requiring ongoing monitoring 2, 4