Rheumatoid Factor (IgG and IgM)

Function and Overview

Rheumatoid Factor (RF) is an autoantibody, an immune system protein that mistakenly attacks the body's own healthy tissues. This test specifically measures the IgG and IgM isotypes of RF. While RF is most famous for its association with Rheumatoid Arthritis (RA), it is essentially a marker of systemic inflammation and immune activation. The IgM isotype is the most commonly measured version in standard screens, but the inclusion of IgG can provide a more comprehensive immunological profile.

Why It Is Ordered

Physicians order this test when a patient presents with symptoms of inflammatory arthritis, such as:

• Symmetrical Joint Pain: Pain affecting the same joints on both sides of the body.
• Morning Stiffness: Stiffness lasting more than 30-60 minutes after waking.
• Nodules: Small bumps under the skin near joints.
• Extra-articular Symptoms: Unexplained fatigue or low-grade fever coupled with joint distress.

Associated Conditions

While a high RF is a hallmark of Rheumatoid Arthritis, it is not exclusive to it. High levels are also found in Sjögren’s Syndrome (often very high levels), Systemic Lupus Erythematosus (SLE), and Mixed Connective Tissue Disease. Interestingly, chronic infections like Hepatitis C, Endocarditis, and Tuberculosis can also trigger a rise in RF. It is important to note that roughly 20% of RA patients are 'seronegative,' meaning they have the disease despite a normal RF test result.