Rheumatoid factor is an immune system protein that attacks healthy body cells. High levels of rheumatoid factor in the blood can indicate an autoimmune condition, such as rheumatoid arthritis.

A rheumatoid factor (RF) test is a blood test that can help diagnose rheumatoid arthritis (RA).

High RF levels can indicate RA, but it is only one aspect of a full diagnosis. High RF can also occur in people with other conditions or those with no health issues. In some cases, people with an autoimmune condition have normal RF levels.

In this article, we discuss RF testing, normal RF ranges, and what abnormal levels mean.

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A doctor may recommend a blood test to measure RF levels.

RF is a type of protein called an autoantibody. The immune system makes RF. Doctors do not yet fully understand the role RF plays in the body.

In people with an autoimmune condition, RF can attack healthy cells. The immune system mistakes healthy cells for foreign bodies and typically creates higher levels of RF to attack these cells.

An RF test is one test doctors use to help diagnose autoimmune conditions.

Higher levels of RF in the body can be an indication that there is some level of autoimmune activity happening, which can mean the presence of an underlying condition.

According to the Hospital for Special Surgery, the normal RF range is between 0–20 units per milliliter of blood (u/ml).

Having RF levels above this range is not enough to diagnose RA.

High RF levels, or positive results, indicates that there is a lot of RF in the blood. A concentration of RF can be a sign of inflammation and autoimmune disruption.

Around 80% of people with RA have significant concentrations of RF in their blood. In the early stages of arthritis, some 30% of people may have raised levels of RF.

However, higher levels of RF can also occur in other conditions, and a small percentage of people without a medical condition, though the exact reason for this is uncertain.

Autoantibodies can also occur as part of the natural aging process. According to one study, 5% of healthy 50-year-olds may have high RF, and 10–25% of people aged 70 years.

If a person has abnormally high RF levels and also experiences symptoms of RA, it is likely they have RA.

Signs of RA can include:

How high the RF results are may play into a doctor's diagnosis. Very high levels can be more indicative of RA than lower levels, as this indicates more activity in the immune system.

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Imaging tests, such as X-rays, can help a doctor to diagnose rheumatoid arthritis accurately.

If a doctor suspects RA, they will often order other blood tests alongside or after an RF test, including:

  • anti-cyclic citrullinated peptide (anti-CCP) tests
  • C-reactive protein (CRP) test
  • antinuclear antibody (ANA) test
  • erythrocyte sedimentation rate (ESR) test

Learn more about blood tests for RA here.

Additionally, doctors may order imaging tests that can include MRI, X-rays, and ultrasound to check for signs of inflammation or help distinguish between types of arthritis.

The RF test is a simple blood test.

A healthcare professional will take a small sample of blood from a vein in a person's arm. This procedure only takes a few minutes. The doctor then sends the sample to a laboratory where a technician will measure the levels of RF.

It may take several days for the results to come back. A doctor will discuss the results and the next steps with the person.

While high RF may indicate RA in many cases, it is not the only reason a person might have high RF levels.

Many other conditions or diseases may cause higher RF levels, including:

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Treatment for RA will involve managing symptoms.

Treatment for high RF depends on treating the underlying cause.

In people with RA, the goal of treatment is to slow the progression of the disorder and help prevent or reduce symptoms.

There is no outright cure for RA. However, effective, early treatment will stop or slow down disease progression and prevent permanent joint damage.

Anyone who suspects they have RA or another autoimmune condition should see a doctor. Even if an RF test shows a negative result, the person should always check back in with their doctor for a full diagnosis.

Anyone showing new signs or symptoms during their diagnosis or treatment should also visit their doctor to discuss the changes.

RF is a protein that the immune system produces. Some autoimmune conditions, notably RA, cause high levels of RF in the blood. Doctors measure a person's RF levels to help diagnose RA or another disease.

A positive or high RF test result can indicate RA, though this is not the only cause. In some cases, a person with high RF may have no underlying issues. Some people without an autoimmune condition have high RF levels. Doctors need to use various tests to diagnose RA.

After having an RF test, people continue to work with their doctor to gain an accurate diagnosis and explore treatment options.