What Is the Normal Range for RA Factor?

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What Is the Normal Range for RA Factor?
Rheumatoid arthritis pain can be crippling. (weathered hands image by robert mobley from Fotolia.com)

Faced with a patient exhibiting the common symptoms of rheumatoid arthritis (RA), such as joint pain, stiffness or swelling, a doctor may order a rheumatoid factor test to determine the levels of RF present in the patient's system. The rheumatoid factor can signify potential health concerns, so a patient should know a few facts about it in order to better understand test results.

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Definition of Rheumatoid Arthritis

RA is an autoimmune disorder causing chronic inflammation. RA often affects the small joints of the hands and feet because it attacks the joint lining, resulting in painful swelling. RA can eventually lead to joint deformation and the erosion of bone. RA can also affect the entire body, producing both fevers and fatigue.

The Rheumatoid Factor

The rheumatoid factor (RF) is an antibody in the blood linked to RA and other autoimmune diseases. While high levels of RF in a patient's system may suggest that the patient has or will develop RA in the future, the RF test is not a sole indicator. Approximately 10 per cent of people exhibit an elevated level of the RF antibody without symptoms of RA, and some 20 per cent of patients with RA will test negative or present very low levels of RF. Dr. Joan Bathon, with the Johns Hopkins Arthritis Center, says, "The blood tests are used to confirm what is found by medical history and physical examination."

RF Test

Testing the RF requires a blood sample, which then goes for analysis. Doctors use the test to help diagnose rheumatoid arthritis and aid in distinguishing it from other conditions that cause similar symptoms of joint pain, inflammation and stiffness, such as Sjögren's syndrome.

RF test results come in two forms: units or titres. Because high RF levels make a blood sample cloudy, a Nephelometry unit test measures how much light the blood sample blocks. Using this measurement, a RF level of 100 units is higher than one of 40 units. A test measured in titres determines how diluted a blood sample can be before RF is not detectable. The higher the second number, the more RF is present in the diluted blood sample. So a titre of 1:80 indicates more detectable RF than a titre of 1:20.

Test Results

Reviewed by Dr. Shoor, an internist specialising in rheumatology, Quest Diagnostics gives the normal range of RF in the blood as 1:20 to 1:80 titres, or 14 units/ml to 60 units/ml. Levels smaller than or within this range are considered "normal" and are not strong factors in diagnosis.

Causes of High RF

A high RF does not only indicate the possibility of RA. The rheumatoid factor may also be elevated in people with lupus, Sjögren's syndrome, or people who suffer from infections such as hepatitis, mononucleosis and tuberculosis. Cancer patients may also test high for RF. "However, a lab test by itself does not make a diagnosis," Dr. Bathon says. "It has to be considered along with the individual patient's signs and symptoms. . . .The best thing to do in cases of persistent joint pain is to see a rheumatologist to see if there is physical evidence of joint inflammation."

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