Rheumatoid factor (RF)
Rheumatoid factor (RF) is a blood test that measures the amount of the RF antibody in the blood.
How the test is performed
Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
How to prepare for the test
No special preparation is usually necessary.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
This test is most often used to diagnose rheumatoid arthritis or Sjogren syndrome. About 80% of patients with rheumatoid arthritis, and almost all patients with Sjogren syndrome have positive RF tests.
It may also be used to rule out or diagnose other inflammation-related conditions.
Results are usually reported in one of two ways:
- Less than 40-60 u/mL
- Less than 1:80 (1 to 80) titer
A low number (normal result) usually means you do not have rheumatoid arthritis or Sjogren syndrome. However, some people who do have these conditions still have a "normal" or low rheumatoid factor (RF).
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
High levels of rheumatoid factor, especially in people who have symptoms, can help diagnose rheumatoid arthritis or Sjogren syndrome. The higher the level reported, the more likely one of these conditions is present.
People with these autoimmune diseases may also have higher levels of rheumatoid factor:
- Systemic lupus erythematosus
- Adult Still's disease
- Juvenile rheumatoid arthritis
Many other medical problems and infections may cause higher-than-normal levels of RF. However, RF is not used to diagnose these problems:
- AIDS, hepatitis, influenza, infectious mononucleosis, and other viral infections
- Endocarditis, tuberculosis, and other bacterial infections
- Parasite infections
- Leukemia, multiple myeloma, and other cancers
- Chronic lung disease
- Chronic liver diseases
- Nephrotic syndrome
Sometimes, people who are healthy and have no other medical problem will have a higher-than-normal RF level.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
The RF test can be positive in some healthy people.
Goodyear CS, Tighe H, McInnes IB. Rheumatoid factors and other autoantibodies in rheumatoid arthritis. In: Firestein GS, Budd RC, Harris Jr. ED, McInnes IB, Ruddy S, eds. Kelley's Textbook of Rheumatology. 8th ed. Philadelphia, Pa: W.B. Saunders Company;2008:chap 51.
Reviewed By: Mark James Borigini, MD, Rheumatologist in the Washington, DC Metro area. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.