Rheumatoid Factor (RF)

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 titres or units:

• A titre is a measure of how much the blood sample can be diluted before RF can no longer be detected. A titre 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 titre of 1 to 80 shows more RF in the blood than a titre 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. Therefore, an RF level of 100 units is higher than one of 40 units.

Normal

Normal values may vary from lab to lab.

Rheumatoid factor (RF)

Titres	1:20–1:40 or less
Units	Less than 43

High values

High RF levels may be caused by:

• Rheumatoid arthritis. This is the most common reason for a rheumatoid factor (RF) level greater than 43 and a titre greater than 1:20–1:40.
• 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.

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Author:	Douglas Dana Sydney Youngerman-Cole, RN, BSN, RNC	Last Updated: August 31, 2006
Medical Review:	Kathleen Romito, MD - Family Medicine Tom Bailey, MD - Family Medicine Stanford M. Shoor, MD - Rheumatology
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