Technical falsely positive rheumatoid factor by ELISA in sera with elevated IgM levels.

Area of Science:

• Immunology
• Clinical Chemistry
• Serology

Background:

• Elevated immunoglobulin M (IgM) levels can be associated with various autoimmune and lymphoproliferative disorders.
• The rheumatoid factor (RF) assay is commonly used in diagnosing rheumatoid arthritis, but non-specific binding can lead to false positives.
• Understanding the behavior of monoclonal and polyclonal IgM is crucial for accurate serological testing.

Purpose of the Study:

• To investigate the adsorption characteristics of raised IgM levels onto different antigens.
• To determine if non-specific IgM adsorption can mimic the rheumatoid factor phenomenon.
• To assess the specificity of IgM binding in sera with elevated IgM levels.

Main Methods:

• Sera with monoclonally or polyclonally raised IgM levels were tested for IgM adsorption.
• Adsorption was assessed on polystyrene plates coated with tetanus toxoid-human antitoxin complexes, tetanus toxoid, and human serum albumin.
• Inhibition assays using heat-aggregated human immunoglobulin were performed to assess specificity.

Main Results:

• Approximately 50% of sera with elevated IgM showed significant IgM adsorption to coated plates.
• In some cases, this non-specific IgM adsorption exceeded rheumatoid factor binding in rheumatoid sera.
• The adsorption was not inhibited by heat-aggregated human immunoglobulin, confirming its non-specific nature.

Conclusions:

• Raised IgM levels, both monoclonal and polyclonal, can cause significant non-specific adsorption to various antigens.
• This non-specific IgM binding can lead to falsely positive rheumatoid factor results.
• The phenomenon is largely observed in sera with elevated IgM levels, highlighting the need for careful interpretation of RF assays in such cases.