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.
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.
There 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.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
A 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. Therefore, a titer of 1 to 80 shows more RF in the blood than a titer of 1 to 20.
- Nephelometry units indicate 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.
Normal values vary from lab to lab. Results are usually available in a day or two.
1:20 to 1:80
14 units/mL to 60 units/mL
Check with your doctor to see what the normal range is for the lab that tested your blood.
High RF levels may be caused by:
- Rheumatoid arthritis . This is the most common reason for a positive rheumatoid factor (RF) level.
- Hepatitis C infection.
- 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 chronic active hepatitis.
- Infection of the heart (endocarditis).
- Leukemia .
What Affects the Test
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.
- Age. About 5% to 10% of people older than age 65 have a high RF level.
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 health professional 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.
- Older adults who do not have rheumatoid arthritis sometimes have a slightly high RF level.
- 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.
- Less than one-third of children who have juvenile rheumatoid arthritis have a high RF level.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.
|Author||Shannon Erstad, MBA/MPH|
|Editor||Susan Van Houten, RN, BSN, MBA|
|Associate Editor||Tracy Landauer|
|Primary Medical Reviewer||Kathleen Romito, MD - Family Medicine|
|Specialist Medical Reviewer||Stanford M. Shoor, MD - Rheumatology|
|Last Updated||June 12, 2008|