What Rheumatoid Factor Does and Doesn't Show

What Is Rheumatoid Factor?
To accurately diagnose rheumatoid arthritis, your doctor may order several tests, including a blood test to determine your levels of an antibody known as rheumatoid factor. If your levels are high, you may have rheumatoid arthritis. Or you may not.

Rheumatoid factor (RF) is a type of antibody produced by your immune system. More specifically, Rfs are auto-antibodies. Normally, antibodies seek out and destroy bacteria or viruses that enter your body and cause disease. What makes auto-antibodies different is that they seek out and destroy healthy tissue, causing disease. If you have high levels of RFs in your blood, as determined by blood testing, you may have rheumatoid arthritis.

The Test
If your doctor suspects you might have rheumatoid arthritis, a blood sample will be taken taken from a vein in your arm and sent to a lab for analysis. Laboratory technicians test for RF by combining your blood with substances that will cause it to clump if the auto-antibody is present. Another lab test, which shines a laser on the blood sample, measures increasing levels of RF. The RF test results are positive in approximately 70 percent of people who have rheumatoid arthritis, according to New York University Langone Medical Center.

The Diagnosis
A positive result from the RF blood test does not automatically mean you have rheumatoid arthritis, because high levels of rheumatoid factors are found in other autoimmune conditions. Healthy people may also have high RF levels and some people who have rheumatoid arthritis do not have elevated RFs. If, however, you have a positive RF test result in combination with specific findings from a physical examination by a rheumatologist (such as swelling and nodules) and other tests (such as X-rays or MRI to view decalcification in bones and damage in joints, and perhaps an analysis of the synovial fluid that fills the spaces between your joints), the diagnosis is likely to be rheumatoid arthritis. And if you have the physical signs and symptoms, you may be diagnosed with rheumatoid arthritis even if you don't have high RF levels.

The Value
Doctors do not routinely order the RF test; it is only used to confirm a strong suspicion that rheumatoid arthritis may be present. Although high levels of RF are often found in people who have active rheumatoid arthritis, the amount can fluctuate and doesn't necessarily reflect the severity of your pain or other symptoms. That is why the test is never used alone as the basis of a diagnosis. It is simply one of several tools a doctor can use when trying to piece together your symptoms and make a clear diagnosis.



Lab Tests Online: Rheumatoid Factor 18 May 2010 Web. August 2012

New York University Langone Medical Center: Rheumatoid Arthritis Web. August 2012