Rheumatoid Factor (RF): what this test reveals

What is Rheumatoid Factor (RF)?

Rheumatoid Factor (RF) is an autoantibody, meaning it is a protein produced by the body's own immune system that is directed against other proteins in the body. Specifically, RF targets the Fc region of IgG-class immunoglobulins (antibodies).

Under normal circumstances, the immune system produces antibodies against external threats such as viruses and bacteria. In the case of autoantibodies, this system begins for some reason to react against the body's own structures. RF was one of the first autoantibodies ever discovered and for a long time was the primary diagnostic marker for rheumatoid arthritis.

RF is measured in IU/mL. It is important to know that a positive RF result alone is not sufficient to diagnose rheumatoid arthritis, and conversely, a negative RF does not rule out the disease.

Reference ranges

In most laboratories, RF is considered within the normal range when its concentration does not exceed 14 IU/mL. Values above this threshold are considered positive.

The degree of elevation also matters. A significantly elevated RF (for example, several times above the upper limit of normal) generally raises more clinical concern than a mildly elevated result.

Reference limits may vary slightly between laboratories, so always compare your result against the range provided by your specific laboratory.

Why it may be elevated

Elevated RF can have various causes, and not all of them are related to a diagnosis of rheumatoid arthritis:

Rheumatoid arthritis is the classic condition associated with RF. However, it is important to understand that RF is positive in only about 70-80% of rheumatoid arthritis cases.

Other autoimmune diseases: Sjogren's syndrome (dry eyes and dry mouth), systemic lupus erythematosus, scleroderma, and several other autoimmune conditions can also produce a positive RF.

Chronic infections: long-standing bacterial, viral, or parasitic infections (for example, endocarditis, hepatitis B and C, tuberculosis) can stimulate RF production.

Other diseases: certain lung diseases, kidney diseases, and malignancies are occasionally associated with a positive RF.

Age: the frequency of positive RF results increases with age even in healthy individuals. Up to 10% of people over 70 may have a mildly positive RF with no obvious disease.

Why it may be low

A negative or normal RF level means that significant quantities of this autoantibody were not found in the blood. This may indicate that autoimmune reactions associated with RF are not active.

It is important to know that a negative RF does not mean rheumatoid arthritis or another autoimmune disease is absent. In some cases, the disease can progress without RF, which is known as seronegative rheumatoid arthritis. In such situations, your doctor may check other markers, for example anti-CCP antibodies.

What to do next

A positive RF result is just one piece of the puzzle. Your doctor will assess it together with your symptoms (joint pain, swelling, morning stiffness), physical examination findings, and results of other tests.

If RF is positive, additional tests are usually recommended: anti-CCP antibodies (a more specific marker for rheumatoid arthritis), inflammatory markers (CRP, ESR), and a complete blood count. In some cases, a rheumatology consultation is needed.

A negative RF in the presence of symptoms also warrants further investigation. Symptoms should not be ignored.

Monitoring over time

If rheumatoid arthritis or another RF-associated condition has been diagnosed, monitoring the marker over time can help assess disease activity and treatment effectiveness. However, RF levels do not always correlate with symptom severity, so RF alone is not sufficient to evaluate disease activity.

For people without a diagnosis whose RF is mildly elevated and who have no symptoms, the doctor will determine whether a repeat test is needed and when.

Questions for your doctor

• What does my RF value mean together with my symptoms and other test results?
• Should anti-CCP antibodies be tested?
• What other inflammatory markers could help clarify the picture?
• Is a rheumatology consultation necessary?
• How often should I repeat this test?

This is not medical advice. Please consult your doctor for health-related decisions.