A rheumatoid factor (RF) blood test measures the amount of the RF antibody present in the blood. Normally, antibodies are produced by the immune system to help destroy and eliminate invading bacteria and viruses that can cause disease. However, the RF antibody can attach to normal body tissue, resulting in damage.

A high level of rheumatoid factor can be caused by several autoimmune diseases (including rheumatoid arthritis) and some severe infections. Occasionally an elevated level of RF is present in healthy people.

The amount of rheumatoid factor in blood can be measured in two ways:

• Agglutination tests. One test method mixes blood with tiny rubber (latex) beads that are covered with human antibodies. If RF is present, the latex beads clump together (agglutinate). This method is best used as a first-time screening test for rheumatoid arthritis. Another agglutination test mixes the blood being tested with a sheep's red blood cells that have been covered with rabbit antibodies. If RF is present, the red blood cells clump together. This method is often used to confirm the presence of RF.
• Nephelometry test. This test mixes the blood being tested with antibodies that cause the blood to clump if RF is present. A laser light is shined on the tube containing the mixture and the amount of light blocked by the blood sample is measured. As levels of RF increase, more clumping occurs, causing a cloudier sample and less light to pass through the tube.

Why It Is Done

A test for rheumatoid factor is done to help support a diagnosis of rheumatoid arthritis.

How To Prepare

You do not need to do anything before you have this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Apply a gauze pad or cotton ball over the needle site as the needle is removed.
• Apply pressure to the site and then a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.