Can I order a laboratory test for quantitative serum IgG and IgM levels?

Can You Order Laboratory Tests for IgG and IgM?

Yes, you can and should order quantitative serum IgG and IgM levels when clinically indicated—these are standard, readily available laboratory tests performed by immunonephelometry or turbidimetric immunoassay at virtually all clinical laboratories. 1

When to Order These Tests

Primary Indications for Quantitative Immunoglobulin Testing

Suspected Immunodeficiency:

• Order quantitative IgG, IgA, and IgM levels together when evaluating recurrent or severe infections, particularly sinopulmonary infections resistant to standard therapy 2, 3
• In patients with therapy-refractory recurrent sinusitis, approximately 10% may have common variable immunodeficiency and 6% may have IgA deficiency 3
• Critical threshold: IgG levels typically <5 g/L and IgA <0.1 g/L suggest primary hypogammaglobulinemia 2

Bronchiectasis evaluation:

• Measure serum IgG, IgG subclasses, IgM, and IgA when no obvious cause is identified 2
• Patients with congenital/acquired hypogammaglobulinemia have increased frequency of recurrent respiratory infections and bronchiectasis 2

Important Caveats About IgM Testing

Be cautious with IgM interpretation in infectious disease contexts:

• For H. pylori: IgA and IgM tests are generally NOT FDA-approved and are not recommended due to low specificity and sensitivity 4
• Panels combining IgG, IgA, and IgM provide no added benefit over IgG alone and often consist of non-FDA-approved tests of unclear diagnostic value 4
• For Q fever: IgM antibodies have much lower specificity than IgG and can persist >1 year, limiting diagnostic value as a standalone test 5

How to Order Properly

Standard ordering approach:

1. Request "quantitative serum immunoglobulins" including IgG, IgA, and IgM simultaneously 6, 3
2. Ensure your laboratory uses validated methods (immunonephelometry, turbidimetric immunoassay, or latex agglutination turbidimetric immunoassay) 1, 7
3. When using commercial laboratories, specifically request FDA-approved tests only 4

Age-specific considerations:

• Reference ranges vary significantly by age, particularly in children and adolescents 8
• IgA levels increase with age; IgG and IgM show age-dependent increases with sex differences (girls have higher IgG and IgM at all ages) 8

What NOT to Do

Avoid these common pitfalls:

• Do not order IgM alone for chronic conditions—it lacks specificity 5
• Do not use immunoglobulin panels (IgG/IgA/IgM combinations) for infectious disease serology—stick to pathogen-specific IgG when appropriate 4
• Do not diagnose IgA deficiency unless serum IgA is <7 mg/dL (not just "below normal range") 9
• Do not rely on a single immunoglobulin measurement—if deficiency is suspected, confirm with functional antibody testing (responses to tetanus, diphtheria, pneumococcal vaccines) 6, 3

Clinical Context Matters

For immunodeficiency workup specifically: The most comprehensive approach includes quantitative immunoglobulins PLUS functional assessment through pre- and post-immunization antibody titers to protein antigens (diphtheria, tetanus) and polysaccharide antigens (23-valent pneumococcal vaccine) 6, 3. However, note that antibody response to pneumococcal polysaccharide vaccine is fraught with laboratory variability and reliability issues 6.

For infectious disease diagnosis: Use pathogen-specific serologic tests rather than general immunoglobulin levels. The quantitative IgG/IgM tests discussed here are for assessing immune function, not diagnosing specific infections 10, 4, 5.