Rheumatoid Factor (RF)

Rheumatoid factor (RF) is a group of antibodies found in the blood of most people with rheumatoid arthritis, but also in some other diseases and some healthy people. Testing is used for painful, swollen joints, most often alongside anti-CCP, to help a doctor establish a diagnosis. A positive result alone does not mean you have rheumatoid arthritis.

Who the test is for

• people with painful, swollen joints, particularly small joints in both hands and feet;
• people with morning joint stiffness lasting longer than half an hour;
• people with joint symptoms lasting several weeks or recurring;
• people with suspected Sjögren’s syndrome or another autoimmune disease.

What rheumatoid factor is and what it indicates

Rheumatoid factor consists of autoantibodies, most often IgM, directed against part of the body’s own IgG antibodies. In rheumatoid arthritis, a chronic autoimmune inflammatory joint disease, RF is positive in approximately 70 to 80 % of patients, less often early in the disease. High RF is associated with a more severe course, including a greater tendency towards joint damage and rheumatoid nodules under the skin.

RF is not specific to rheumatoid arthritis. It is positive in most people with Sjögren’s syndrome, characterised by dry eyes and mouth, and in cryoglobulinaemia, and less often in systemic lupus erythematosus and other autoimmune diseases. Chronic infections such as hepatitis C, bacterial endocarditis or tuberculosis, and chronic liver and lung disease can also raise it. It is also found in some healthy people, more often at older ages, usually at low levels.

The request may call the test rheumatoid factor, RF or RF latex. Latex refers to the type of laboratory test, not latex allergy.

RF is not a test for back pain or joint wear (osteoarthritis), in which it is generally negative. It is not used in people without symptoms or to monitor treatment. A doctor monitors rheumatoid arthritis activity by joint examination and inflammatory indicators such as CRP and sedimentation rate.

Preparation and sample

Blood is taken from a vein. No fasting or special preparation is required. If other tests that day require fasting, such as glucose or blood lipids, come in the morning before breakfast.

How results are interpreted

Results are expressed in international units per millilitre (IU/mL) and compared with a cut-off that depends on the method and is printed on the report. Below the cut-off is negative and above it positive. The 2010 ACR/EULAR rheumatoid arthritis classification criteria distinguish low-positive results, up to three times the cut-off, from high-positive results, above three times the cut-off. In these criteria, RF and anti-CCP are assessed alongside the number of affected joints, symptom duration and inflammatory markers.

A more reliable conclusion is reached when RF is interpreted alongside anti-CCP, which is considerably more specific for rheumatoid arthritis:

Rheumatoid factor	Anti-CCP	Most common meaning
positive	positive	with typical joint symptoms, rheumatoid arthritis is very likely
negative	positive	rheumatoid arthritis is possible, particularly early in the disease
positive	negative	rheumatoid arthritis is possible, but also another autoimmune disease, chronic infection or a non-specific finding, for example in older age
negative	negative	rheumatoid arthritis is less likely but not excluded

Diagnosis is made by a doctor, most often a rheumatologist, rather than a laboratory result, using joint examination, symptoms, laboratory findings and, if needed, ultrasound or X-rays.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for rheumatoid factor and tests often performed alongside it, such as anti-CCP and CRP, are available in the service price list.

Frequently asked questions

What does high rheumatoid factor mean?

It means antibodies commonly found in rheumatoid arthritis were detected, but they also occur in other diseases and some healthy people. With painful, swollen joints, show the result to a doctor promptly because early treatment slows joint damage. Mildly high RF without symptoms usually has no particular significance; a doctor decides on further tests.

What are the reference values for rheumatoid factor?

There is no universal cut-off for all laboratories because it depends on the method. The cut-off is printed beside the result, so results from different laboratories should not be compared by numbers alone.

Can I have rheumatoid arthritis with negative rheumatoid factor?

Yes. Some patients have no RF, particularly early in the disease, which is then called seronegative rheumatoid arthritis. A doctor therefore requests anti-CCP and inflammatory markers alongside RF, with joint examination and symptom duration being decisive.

What is the difference between rheumatoid factor and anti-CCP?

Both antibodies occur in rheumatoid arthritis, but anti-CCP is rarely positive in other diseases or healthy people, while RF is more often positive without rheumatoid arthritis. Together they provide a more reliable assessment than either alone.