What is a rheumatoid factor (RF) test?

A rheumatoid factor (RF) test checks a sample of your blood for rheumatoid factor, a protein made by the immune system.

Normally, your immune system makes proteins called antibodies to attack germs that could make you sick. Rheumatoid factor, however, does not attack germs. It attacks healthy cells, usually immunoglobulin G (IgG), one of the most common antibodies made to fight germs. Your immune system makes rheumatoid factor by mistake or when it's overactive fighting an infection.

Many people have small amounts of RF in their blood and are healthy. Even high amounts aren't always a sign of a health condition. But if you have certain symptoms and higher levels of rheumatoid factors, you may have an autoimmune disorder or another health condition related to high RF levels.

Other names: RF Blood Test

What is it used for?

RF testing is mostly used with other tests to help diagnose rheumatoid arthritis. Rheumatoid arthritis (RA) is a type of autoimmune disorder that damages your joints and causes pain, swelling, and stiffness. It is a chronic (long-lasting) condition that can also affect your organs and cause other symptoms.

An RF test can also be used to check for other autoimmune disorders, certain infections, and certain types of cancer.

Why do I need an RF test?

You may need an RF test if you have symptoms of rheumatoid arthritis. These may include:

• Joint pain or stiffness when moving. This is usually worse in the morning or after inactivity. RA usually affects both joints. If one of your hands or knees is affected, so is the other.
• Joint tenderness, redness, and warmth.
• Joint swelling that may make activities hard. These could include things such as combing your hair, buttoning your clothes, or bending your knees.
• Fatigue, fever, and a loss of appetite.
• Hard bumps (rheumatoid nodules) under your skin near the joints.

You may also need an RF test if your health care provider wants to check for other conditions that can cause high RF levels. These may include:

• Other autoimmune disorders, including:
  • Sjögren syndrome.
  • Lupus.
  • Scleroderma.
  • Juvenile idiopathic arthritis in children and teens.
• Chronic infections, including:
  • Hepatitis C (liver).
  • Tuberculosis (mostly affects the lungs).
  • Endocarditis (heart).
• Certain types of cancer, including leukemia.

People with rheumatoid arthritis often develop anemia, so if you have anemia, your provider may want to give you an RF test. It is rare to get tested for high RF levels if you don't have symptoms of any of the above conditions.

What happens during an RF test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

At-home tests for rheumatoid factor are available. The test kit provides everything you need to collect a sample of blood by pricking your finger. You'll mail your sample to a lab for testing. If you do a home test, remember to share your results with your provider.

Will I need to do anything to prepare for the test?

You don't need any special preparations for an RF test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

The results of an RF test alone cannot diagnose a condition. However, your provider can use these results, along with your medical history and the results of other tests, to determine if you have rheumatoid arthritis or any other condition that causes high RF levels. Generally, your results will be either positive or negative.

A negative (normal) result means that you have little or no rheumatoid factor in your blood. But that doesn't rule out rheumatoid arthritis or another health problem. If you have symptoms of rheumatoid arthritis, but your RF test results are normal, your provider may order more tests to make a diagnosis.

A positive (abnormal) result means that a higher level of rheumatoid factor was found in your blood. Your provider will also consider other test results along with your symptoms and medical history to find out if you have:

• Rheumatoid arthritis or another autoimmune disease.
• A chronic infection.
• Certain cancers.

Sometimes, rather than a negative or positive result, your results may tell you how much RF was in the sample. Your results may also give a normal range for RF for that laboratory.

Generally, the higher your RF levels, the more likely it is that you have a condition linked to rheumatoid factors. However, people with rheumatoid arthritis can have little or no rheumatoid factor. Similarly, people without rheumatoid arthritis can have a lot of RF in their blood. Your provider may refer you to a rheumatologist (a provider who specializes in autoimmune disorders, including RF) to get more testing if your results are inconclusive.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about an RF test?

An RF test is not used to diagnose osteoarthritis. Although rheumatoid arthritis and osteoarthritis both affect the joints, they are very different diseases. Osteoarthritis is not an autoimmune disease and does not involve rheumatoid factor.