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 ways:
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.
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
Why It Is Done
A test for rheumatoid factor is done
to help support a diagnosis of rheumatoid arthritis.
This test is also done
to find out whether a child who has polyarticular juvenile idiopathic arthritis has
elevated rheumatoid factor. This can help determine the likely progression of the
disease, and the best treatment for it.
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
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.
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.
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.
is very little chance of a problem from having a blood sample taken from a vein.
may get a small bruise at the site. You can lower the chance of bruising by keeping
pressure on the site for several minutes.
In rare cases, the vein may become
swollen after the blood sample is taken. This problem is called phlebitis. A warm
compress can be used several times a day to treat this.
factor (RF) blood test measures the amount of the RF antibody present in the blood.
of the rheumatoid factor (RF) test may be reported in titers or units:
titer is a measure of how much the blood sample can be diluted before RF can no longer
be detected. A titer of 1 to 20 (1:20) means that RF can be detected when 1 part of
the blood sample is diluted by up to 20 parts of a salt solution (saline). A larger
second number means there is more RF in the blood. So a titer of 1 to 80 shows more
RF in the blood than a titer of 1 to 20.
Nephelometry units show how much
light is blocked by the blood sample in the tube. A high level of RF causes the sample
to be cloudy, so less light passes through the tube than when the RF level is low.
So an RF level of 100 units is higher than one of 40 units.
The normal values listed here-called a reference range-are just a
guide. These ranges vary from lab to lab, and your lab may have a different range
for what's normal. Your lab report should contain the range your lab uses. Also, your
doctor will evaluate your results based on your health and other factors. This means
that a value that falls outside the normal values listed here may still be normal
for you or your lab.
Reasons you may not be able to have the test or why the results may
not be helpful include:
Blood that is very high in fats.
people older than age 65 have a high RF level.
Having had multiple vaccinations
or blood transfusions.
What To Think About
number of people have a high RF level but do not have rheumatoid arthritis. A small
number of these people will later have rheumatoid arthritis.
A doctor always
uses the results of an RF test along with information gained from a medical history
and physical examination before diagnosing rheumatoid arthritis.
peptide (CCP) is an antibody that is found in the blood of more than half of all people
who have rheumatoid arthritis. It is used to confirm a diagnosis of rheumatoid arthritis.
people have rheumatoid arthritis and have a normal level of RF. The RF test may need
to be repeated if rheumatoid arthritis is suspected and the first RF level was normal.
children who have juvenile idiopathic arthritis do not have a high RF level.
Pagana KD, Pagana
TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis:
Other Works Consulted
FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010).
Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
ByHealthwise Staff Primary
Medical ReviewerAnne C. Poinier, MD - Internal Medicine Martin J. Gabica, MD - Family Medicine E.
Gregory Thompson, MD - Internal Medicine Kathleen
Romito, MD - Family Medicine Adam Husney, MD -
Pagana KD, Pagana TJ
(2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis:
This information does not replace the advice of a doctor.
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