Rheumatoid Factor (RF) – Serum Immunoglobulin G (IgG) Antibody to IgG

What Is Rheumatoid Factor?

Rheumatoid factor (RF) is a type of antibody (immunoglobulin G, or IgG) that your immune system produces against its own proteins. While it can be found in some healthy people, elevated levels often signal an autoimmune condition, such as rheumatoid arthritis, where the body mistakenly attacks its own joints and tissues. This test is not diagnostic on its own but can support a clinical evaluation.

Why Might You Need This Test?

Your healthcare provider may order this test if you have symptoms like joint pain, swelling, stiffness, or fatigue—common in conditions like rheumatoid arthritis. It may also be used as part of a broader screening for autoimmune diseases or to monitor disease activity in people already diagnosed with such conditions.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The blood is then processed to separate the serum (the liquid part without blood cells), which is tested for rheumatoid factor antibodies using an immunoassay method. This test is typically done in a lab and results are usually available within a few days.

What Do the Results Mean?

Results are not interpreted alone. A positive RF test (meaning antibodies are present) does not definitively diagnose rheumatoid arthritis but may raise suspicion. Doctors will consider symptoms, physical exam findings, and other test results (like X-rays or other blood tests) to make a diagnosis. Some people test positive for RF without having an autoimmune disease.