Find Information About:

Drugs & Supplements

Get information and reviews on prescription drugs, over-the-counter medications, vitamins, and supplements. Search by name or medical condition.

Pill Identifier

Pill Identifier

Having trouble identifying your pills?

Enter the shape, color, or imprint of your prescription or OTC drug. Our pill identification tool will display pictures that you can compare to your pill.

Get Started

My Medicine

Save your medicine, check interactions, sign up for FDA alerts, create family profiles and more.

Get Started

WebMD Health Experts and Community

Talk to health experts and other people like you in WebMD's Communities. It's a safe forum where you can create or participate in support groups and discussions about health topics that interest you.

  • Second Opinion

    Second Opinion

    Read expert perspectives on popular health topics.

  • Community


    Connect with people like you, and get expert guidance on living a healthy life.

Got a health question? Get answers provided by leading organizations, doctors, and experts.

Get Answers

Sign up to receive WebMD's award-winning content delivered to your inbox.

Sign Up

Arthritis Health Center

Font Size

Rheumatoid Factor (RF)

A rheumatoid factor (RF) blood test measures the amount of the RF antibody present in the blood. Normally, antibodies are produced by the immune system to help destroy and eliminate invading bacteria and viruses that can cause disease. But the RF antibody can attach to normal body tissue, resulting in damage.

A high level of rheumatoid factor can be caused by several autoimmune diseases (including rheumatoid arthritis) and some infections. Occasionally an elevated level of RF is present in healthy people.

The amount of rheumatoid factor in blood can be measured in two ways:

  • Agglutination tests. One test method mixes blood with tiny rubber (latex) beads that are covered with human antibodies. If RF is present, the latex beads clump together (agglutinate). This method is best used as a first-time screening test for rheumatoid arthritis. Another agglutination test mixes the blood being tested with a sheep's red blood cells that have been covered with rabbit antibodies. If RF is present, the red blood cells clump together. This method is often used to confirm the presence of RF.
  • Nephelometry test. This test mixes the blood being tested with antibodies that cause the blood to clump if RF is present. A laser light is shined on the tube containing the mixture, and the amount of light blocked by the blood sample is measured. As levels of RF increase, more clumping occurs, causing a cloudier sample and less light to pass through the tube.

Why It Is Done

A test for rheumatoid factor is done to help support a diagnosis of rheumatoid arthritis.

This test is also done to find out whether a child who has polyarticular juvenile idiopathic arthritis has elevated rheumatoid factor. This can help determine the likely progression of the disease, and the best treatment for it.

How To Prepare

You do not need to do anything before you have this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

1 | 2 | 3 | 4

WebMD Medical Reference from Healthwise

Last Updated: September 09, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

Today on WebMD

Mature woman exercise at home
Hint: Warming up first is crucial.
feet with gout
Quiz yourself.
woman in pain
One idea? Eat fish to curb inflammation.
senior couple walking
Can you keep your RA from progressing?
xray of knees with osteoarthritis
close up of man wearing dress shoes
feet with gout
close up of red shoe in shoebox
two male hands
Woman massaging her neck
5 Lupus Risk Factors