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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 get rid of invading bacteria and viruses that can cause disease. But the RF antibody can attach to normal body tissue and cause damage.
The amount of RF 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 that RF is present.
- 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. Then the amount of light blocked by the blood sample is measured. As levels of RF increase, more clumping occurs. This causes a cloudier sample and allows 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.
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
In general, there's nothing you have to do before this test, unless your doctor tells you to.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How long the test takes
The test will take a few minutes.
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
You can usually get the results within 24 hours.
A doctor always uses the results of an RF test along with information gained from a medical history and a physical examination before diagnosing rheumatoid arthritis.
The results of the rheumatoid factor (RF) test may be reported in titres or units:
- A titre is a measure of how much the blood sample can be diluted before RF can no longer be detected. A titre 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 titre of 1 to 80 shows more RF in the blood than a titre 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.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
High RF levels may be caused by:
- Rheumatoid arthritis.
- Other autoimmune diseases, such as systemic lupus erythematosus (SLE), scleroderma, Sjögren's syndrome, and vasculitis.
- Infectious diseases, such as tuberculosis, mononucleosis, syphilis, and malaria.
- Liver diseases, such as cirrhosis and hepatitis.
- Infection of the heart (endocarditis).
A small number of people have a high RF level but don't have rheumatoid arthritis. A small number of these people will later have rheumatoid arthritis.
Older adults who don't have rheumatoid arthritis sometimes have a slightly high RF level.
Current as of: December 20, 2021
Author: Healthwise Staff
Anne 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 - Family Medicine
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