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Updated 25 July 2012

Rheumatoid factor test

Rheumatoid factor is a commonly used blood test to assist in the diagnosis of rheumatoid arthritis and to help differentiate this from other forms of arthritis.

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p> Also known as: RF

Related tests : Anti -CCP (Cyclic Citrullinated Peptide), ANA (Antinuclear Antibody)

The Laboratory Test

What is Rheumatoid Factor test?

Rheumatoid factor (RF) is a commonly used blood test to assist in the diagnosis of rheumatoid arthritis (RA) and to help differentiate this from other forms of arthritis which are characterized by swelling, pain and restricted movement of joints. RA is an autoimmune disease, where the body's own antibodies (or immunoglobulins) - normally involved in fighting infections as part of the body's immune defence - cause damage. In the case of RA, this results in inflammation and destruction of the body's joints by these self-directed antibodies and the subsequent immune reactions. Inflammatory markers such as CRP (C - reactive protein), ESR (erythrocyte sedimentation rate), a FBC (full blood count) and ANA (antinuclear antibody) are also ordered as indicated. Anti CCP (Cyclic Citrullinated Peptide) antibody is requested when the RF is negative, or in the very early stages of arthritis.

When is the test done?

This test may be requested to confirm the diagnosis of RA when suspected in a patient with inflamed, swollen and painful joins. These symptoms may be part of RA or other autoimmune diseases.

In RA abnormal antibodies which are not usually present in our blood, and which are directed at the body's own antibodies (immunoglobulins) are measured by this test. The test is requested as part of a full clinical evaluation and where indicated in conjunction with X-rays or other imaging of joints.

Early investigation and diagnosis with appropriate treatment can prevent or slow down damage to joints.

How is the test performed?

A small blood sample, less than a half teaspoon, is drawn from a vein in the arm. No preparation is required for this test. Apart from some minor bruising or bleeding at the puncture site, there should be no discomfort. One of the laboratory test methods is nephelometry - a light detection method for the antibodies. In this method the patient's blood is mixed with manufactured antibodies that cause clumping if rheumatoid factor is present. A light passes through a sample containing the mixture, and an instrument then measures how much light is blocked by it. High levels of rheumatoid factor in the blood will cause the blood to be cloudier and result in a greater blocking of light. This is measured in units.

How is the test interpreted?

A significant elevation of RF in a patient with typical complaints and clinical findings makes the diagnosis of RA likely. The test may however be positive under a number of other conditions. These include other auto-immune diseases such as SLE (Systemic Lupus Erythematosus), where an ANA will also be requested and Sjoegren's syndrome, but also infections such as tuberculosis and viral infections and other conditions such as liver cirrhosis and cancers. With increasing age a positive RF test is commonly found too, without RA actually being present. Above age sixty years up to 25% of all people may have a positive RA.

This means that the test is not highly specific for RA. Furthermore, up to 50% of patients with clinically confirmed RA may test negative for RF at the onset of the disease, meaning that it is also not very sensitive. Repeat testing for RF may be required, but there is no value in monitoring the activity of the disease with repeat testing.

What about other tests?

Anti-CCP (Cyclic Citrullinated Peptide) antibody testing has been developed recently for early and more accurate diagnosis of RA, which may be more valuable in confirming the early diagnosis of RA when the clinical picture is still unclear and the RF test negative. Anti-CCP testing may also predict more aggressive RA. However currently the RF is still better known and cheaper for screening patients with suspected RA.

Investigations should always be interpreted against the background of the patient's relevant history and clinical findings.

 
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