What are the indications for ordering Immunoglobulin A (IgA), Immunoglobulin G (IgG), and Immunoglobulin M (IgM) levels in a patient?

Indications for Ordering Immunoglobulin Levels (IgA, IgG, IgM)

Order serum immunoglobulin levels (IgG, IgA, IgM) in patients with recurrent sinopulmonary infections (≥2 serious bacterial infections per year), chronic respiratory infections, unexplained bronchiectasis, or suspected primary immunodeficiency. 1, 2

Primary Clinical Indications

Recurrent or Severe Infections

• Recurrent sinopulmonary infections including pneumonia, sinusitis, or otitis media (≥2 serious bacterial infections annually) warrant immunoglobulin testing as the initial screening step for antibody deficiency 1, 2
• Bronchiectasis of unknown etiology requires measurement of IgG, IgA, and IgM, as immune deficiency occurs in 5-6% of bronchiectasis patients and recognition of conditions like CVID can significantly improve outcomes with immunoglobulin replacement therapy 2
• Infections with encapsulated organisms (H. influenzae, pneumococci) particularly suggest IgG subclass deficiency or specific antibody deficiency 3
• Chronic or recurrent upper and lower respiratory tract infections that are unusually severe, prolonged, or require frequent antibiotics 1, 3

Suspected Primary Immunodeficiency Syndromes

• Clinical suspicion for CVID (Common Variable Immunodeficiency): recurrent infections in adults with hypogammaglobulinemia affecting at least 2 immunoglobulin isotypes 4
• Selective IgA deficiency evaluation: IgA <7 mg/dL defines selective IgA deficiency, which occurs in approximately 1 in 500 people and is associated with recurrent sinopulmonary infections, autoimmune disease, and atopy 1, 5
• Agammaglobulinemia or severe hypogammaglobulinemia: profound reduction in all serum immunoglobulin isotypes with absent or profoundly decreased circulating B cells 4
• IgG subclass deficiency: consider when total IgG, IgA, and IgM are normal but patient has recurrent respiratory infections; however, measure all 4 IgG subclasses simultaneously only after documenting normal total immunoglobulins 1

Autoimmune Disease Associations

• Unexplained autoimmune phenomena: selective IgA deficiency shows high incidence of autoimmune disease including thyroiditis, with 10 out of 15 patients in one series positive for autoantibodies 6
• IgE deficiency (IgE <2.5 IU/mL) is associated with 46% prevalence of autoimmune disease versus 15% in controls, and warrants checking other immunoglobulin levels 7
• Chronic arthralgias, fatigue, and recurrent infections together suggest possible immunodeficiency with autoimmune overlap 7

Bronchiectasis Workup

• All adult patients with bronchiectasis should have serum immunoglobulins (IgG, IgA, IgM) and serum electrophoresis measured as part of standard etiological investigation 2
• Polyclonal elevation of IgG and IgA commonly occurs in bronchiectasis due to chronic infection and inflammation 2, 8
• Serum electrophoresis is essential to distinguish polyclonal from monoclonal patterns, as monoclonal immunoglobulin (MGUS, myeloma, Waldenstrom's) requires different management and is associated with increased bacterial chest infections 2, 8

Secondary Indications

Hematologic Disorders

• B-cell lymphoproliferative disorders (CLL, lymphoma) to assess for secondary antibody deficiency 2
• Suspected monoclonal gammopathy: order immunoglobulins with serum protein electrophoresis when paraprotein is suspected 2, 8
• Persistent lymphocytosis or lymphopenia on complete blood count 2

Chronic Inflammatory Conditions

• Autoimmune hepatitis: polyclonal hypergammaglobulinemia with IgG >1.5 times upper limit of normal occurs in 85% of cases and is a major diagnostic criterion 8
• Primary sclerosing cholangitis: elevated IgG in 61% and elevated IgM in up to 45% of patients 8
• Chronic inflammatory states with persistent antigenic stimulation 8

Pre-Celiac Disease Testing

• Before ordering celiac serology (tissue transglutaminase IgA, endomysial antibody IgA), measure total IgA to ensure these tests will be reliable, as they are falsely negative in IgA-deficient patients who have 1-3% prevalence in celiac disease versus 0.2% in general population 5

Medication Monitoring

• Patients on immunosuppressive medications including rituximab, antiepileptics (phenytoin, carbamazepine, valproic acid), gold, penicillamine, or chronic corticosteroids that can cause secondary immunoglobulin abnormalities 1, 8

Critical Follow-Up Testing

When immunoglobulin abnormalities are detected:

For Low Immunoglobulin Levels

• Measure pneumococcal antibody levels at baseline, then immunize with 23-valent pneumococcal polysaccharide vaccine and recheck 4-8 weeks post-vaccination to assess functional antibody response 2
• Protective threshold: failure to generate antibody concentration >1.3 μg/mL to >70% of serotypes indicates functional antibody deficiency and is part of diagnostic criteria for primary antibody deficiencies 2
• Confirm abnormal results with repeat measurement at least 1 month apart before establishing diagnosis 1
• B-cell phenotyping with flow cytometry to assess circulating B-cell numbers and memory B cells 4

For Elevated Immunoglobulin Levels

• Serum protein electrophoresis is mandatory to distinguish polyclonal from monoclonal elevation, as this fundamentally changes differential diagnosis and management 2, 8
• Immunofixation electrophoresis if monoclonal protein suspected 8
• Inflammatory markers (CRP, ESR) to assess underlying inflammation 8

Age-Specific Considerations

Pediatric Patients

• IgG levels below 450-500 mg/dL in children >4 years may indicate immunodeficiency requiring further evaluation 9
• Do not diagnose IgG4 deficiency before age 10 years as IgG4 is present in very low concentrations in younger children 1
• Transient hypogammaglobulinemia of infancy spontaneously corrects at mean age 27 months, with all patients reaching normal levels by 59 months; low IgG in children <4 years may represent normal developmental variation 9

Common Pitfalls to Avoid

• Do not order IgG subclasses as first-line testing: measure total IgG, IgA, IgM and specific antibody responses first; IgG subclass measurement adds cost and is frequently unnecessary 1
• Remember that 2.5% of normal population will be "deficient" in at least one IgG subclass by statistical definition (within 2 standard deviations), so isolated low subclass without clinical correlation is not diagnostic 1
• Normal total IgG does not exclude IgG subclass deficiency: IgG1 comprises 60% of total IgG, so selective IgG2, IgG3, or IgG4 deficiency can occur with normal total IgG 1, 3
• Concurrent low total protein and albumin suggests secondary hypogammaglobulinemia from protein loss (nephrotic syndrome, protein-losing enteropathy) rather than primary immunoglobulin disorder 8
• IgA deficiency is NOT a contraindication to IgG replacement therapy: anaphylaxis with IVIG in IgA-deficient patients is extremely rare, and some patients tolerate subcutaneous IgG even after IVIG reactions 1
• Do not rely solely on laboratory values: clinical correlation with infection history is essential, as isolated laboratory abnormalities without recurrent infections may not require intervention 1, 9