RF is an antibody that attaches to a substance in the body called immunoglobulin G (IgG), forming a molecule known as an immune complex. The immune complex can trigger different types of inflammation-related processes in the body.

This article discusses the test to detect and measure the level of RF in the blood.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

No special preparation is usually necessary.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is most often used to diagnose rheumatoid arthritis. About 80% of patients with rheumatoid arthritis have positive RF tests.

It may also be used to rule out or diagnose other inflammation-related conditions.

Normal values depend on what method the laboratory uses to detect and measure the RF. Some examples are below:

• Less than 60 u/mL (with the nephelometric method)
• Less than 1:80 titer (with the agglutination method)

A positive test may be due to:

• Chronic hepatitis
• Chronic viral infection
• Dermatomyositis
• Infectious mononucleosis
• Leukemia
• Rheumatoid arthritis
• Scleroderma
• Systemic lupus erythematosus

Additional conditions under which the test may be performed:

• Adult Still's disease
• Juvenile rheumatoid arthritis
• Nephrotic syndrome
• Sjogren syndrome

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

The RF test can be positive in some healthy people.