Rheumatoid Factor (RF)
Rheumatoid Factor (RF)Skip to the navigationTest Overview 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
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 DoneA 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 PrepareYou 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 DoneThe 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 FeelsThe 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. RisksBlood testThere is very little chance of a
problem from having a blood sample taken from a vein. - You 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.
ResultsA rheumatoid factor (RF) blood test
measures the amount of the RF
antibody present in the blood. The
results of the rheumatoid factor (RF) test may be reported in titers or units:
- A 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.
NormalThe 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. You can usually get the results within 24 hours. Check with your doctor to see what the normal range is
for the lab that tested your blood. High valuesHigh RF levels may be caused
by: What Affects the TestReasons 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.
- Age. Some people older than age 65 have a high
RF level.
- Having had multiple vaccinations or blood transfusions.
What To Think About- A small 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.
- Cyclic
citrullinated 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.
- Some
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.
- Most children who have
juvenile idiopathic arthritis do not have a high RF level.
ReferencesCitations- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Other Works Consulted- Fischbach 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.
CreditsByHealthwise Staff
Primary Medical ReviewerAnne C. Poinier, MD - Internal Medicine
Martin J. Gabica, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerKathleen Romito, MD - Family Medicine Current as ofOctober 31, 2016 Current as of:
October 31, 2016 Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier. Last modified on: 8 September 2017
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