Testing for isotypes does not help differentiating rheumatoid arthritis from other rheumatoid factor positive...

Area of Science:

• Immunology
• Clinical Chemistry
• Rheumatology

Background:

• Rheumatoid factors (RFs) are crucial biomarkers for rheumatoid arthritis (RA) diagnosis and classification.
• Current routine diagnostic methods like nephelometry detect total RF but lack isotype specificity (IgA, IgG, IgM).
• Advancements in isotype-specific immunoassays present an opportunity to refine RF diagnostics.

Purpose of the Study:

• To assess the utility of isotype-specific RF testing as a second-step diagnostic approach following traditional nephelometry.
• To determine if IgA, IgG, and IgM RF detection can differentiate rheumatoid arthritis (RA) from other RF-positive diseases.

Main Methods:

• Serum samples (n=117) initially positive for RF by nephelometry were analyzed.
• Isotype-specific RFs (IgA, IgG, IgM) were quantified using fluoroimmunoenzymatic assays (FEIA).
• Results were compared between patients with RA (n=55) and non-RA diagnoses (n=62).

Main Results:

• Eighteen samples (15.4%) were positive only by nephelometry; two were positive solely for IgA RF.
• Ninety-seven samples were positive for IgM RF, with or without IgA and IgG RF.
• No significant correlation was found between RF isotypes and RA diagnosis; correlations with total RF were moderate to weak.

Conclusions:

• Isotype-specific RF testing did not effectively differentiate RA from other RF-positive conditions when used as a second-level test after nephelometry.
• Total RF measurement by nephelometry, despite limitations, remains a valuable diagnostic method.
• The clinical utility of IgA, IgG, and IgM RF isotypes as secondary diagnostic markers for RA is currently controversial.