Arthritis Health Center

Risks

Blood test

There is very little chance of a problem from having a blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

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. So a titer of 1 to 80 shows more RF in the blood than a titer of 1 to 20.
• Nephelometry units show 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

The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

You can usually get the results within 24 hours.

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.