Rheumatoid Factor (RF)

Function and Overview

Rheumatoid Factor (RF) is an autoantibody—an antibody directed against the body's own tissues. Specifically, RF is an antibody (usually IgM) that binds to the Fc portion of IgG. It is one of the primary biomarkers used in the diagnosis and management of autoimmune disorders, particularly those affecting the joints.

Why it is Ordered

Physicians order an RF test when a patient presents with symptoms such as joint pain, stiffness (especially in the morning), swelling, and warmth in the small joints of the hands and feet. It helps distinguish Rheumatoid Arthritis (RA) from other types of arthritis, such as osteoarthritis or gout. It is also used in the evaluation of Sjögren’s syndrome.

Associated Conditions

• Rheumatoid Arthritis (RA): Approximately 70-80% of RA patients are RF-positive.
• Sjögren's Syndrome: Frequently associated with very high RF levels.
• Systemic Lupus Erythematosus (SLE): Some lupus patients will test positive for RF.
• Chronic Infections: Conditions like Hepatitis C, endocarditis, and tuberculosis can cause transient elevations in RF.
• Cryoglobulinemia: RF is often part of the cryoglobulin complex.

Clinical Interpretation

While highly useful, a positive RF test is not a definitive 'yes' for RA. Some healthy individuals, especially the elderly, may have low-level positive RF results without any underlying disease. Conversely, a negative result does not completely rule out RA (Seronegative RA). Clinicians usually combine RF results with Anti-CCP antibody tests for higher diagnostic accuracy.