Rheumatoid Factor (RF): Results

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Results

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

Normal values vary from lab to lab. Results are usually available in a day or two.

Check with your doctor to see what the normal range is for the lab that tested your blood.

High values

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.

Last updated:	June 12, 2008
Author:	Shannon Erstad, MBA/MPH
Reviewed By:	Kathleen Romito, MD - Family Medicine, Stanford M. Shoor, MD - Rheumatology
Editors:	Susan Van Houten, RN, BSN, MBA, Tracy Landauer