Rheumatoid Factor (RF)
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. But 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 infections.
Occasionally an elevated level of RF is present in healthy people.
The amount of rheumatoid factor in blood can be measured in two
- 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
medical test information form(What is a PDF document?).
How It Is Done
The health professional drawing blood
- 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
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- Apply a gauze pad or cotton ball over the needle site as
the needle is removed.
- Apply pressure to the site and then a
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