Rheumatoid Factor IgG (ELISA)

Function

Rheumatoid Factor (RF) is an autoantibody—an antibody directed against the body's own tissues. Specifically, RF targets the Fc portion of IgG molecules. While the standard RF test usually measures the IgM isotype, the IgG isotype (measured here via ELISA) can provide additional diagnostic depth in specific clinical scenarios.

Why it is Ordered

It is ordered when Rheumatoid Arthritis (RA) is suspected, particularly in patients who have joint pain and swelling but may have tested negative for standard IgM RF (seronegative RA). The IgG variant of RF is sometimes associated with more severe, systemic manifestations of the disease, such as vasculitis.

Associated Conditions

• Rheumatoid Arthritis (RA): The primary association, especially in erosive disease.
• Sjogren’s Syndrome: Often presents with very high RF levels.
• Chronic Infections: Hepatitis C, endocarditis, and tuberculosis can cause 'false positive' RF elevations.
• Cryoglobulinemia: IgG RF is often a component of mixed cryoglobulins.

Clinical Insight

RF is not specific to RA. About 5% of healthy individuals and up to 20% of the elderly may have low-level positive RF without any autoimmune disease. Therefore, it must be interpreted alongside Anti-CCP antibodies and clinical symptoms.