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

An RF test is a blood test that can help diagnose rheumatoid arthritis.

While high RF levels can indicate rheumatoid arthritis, this is only one aspect of a full diagnosis. High RF levels can also occur in people with other conditions and those without diagnosable health issues.

This article discusses RF testing, normal RF ranges, and what abnormal levels mean.

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RF is an autoantibody, which is a type of protein that the immune system produces.

In people with autoimmune conditions, the immune system mistakes healthy cells for foreign bodies and typically creates higher levels of antibodies, such as RF, to attack these cells.

Higher levels of RF in the body can indicate that there is some level of autoimmune activity happening, which in turn, can point to the presence of an underlying condition.

As a result, an RF test is one test that doctors use to help diagnose autoimmune conditions.

The onset of rheumatoid arthritis and other autoimmune conditions, cancers, and bacterial infections can all affect RF levels. As a result, these levels may be difficult to assess in isolation.

Some doctors define normal RF levels as 0–20 units per milliliter (U/ml) of blood. On the other hand, one 2012 study designated the upper limit of regular levels as 25 U/ml.

A person’s risk of developing rheumatoid arthritis typically increases with their RF level.

The authors of the 2012 study report that a person with an RF level of 100 U/ml or higher may be up to 26 times more likely to develop rheumatoid arthritis than a person with an RF level under 25 U/ml.

A 2017 review of rheumatoid arthritis biomarkers describes these figures as important diagnostic and prognostic information.

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

Higher levels of RF can also occur in other conditions. For instance, autoantibodies can occur as part of the natural aging process.

How high a person’s RF levels are will factor into a doctor’s diagnosis. Very high levels can be more indicative of rheumatoid arthritis than lower levels, as this points to more activity in the immune system.

A high concentration of RF can be a sign of inflammation and autoimmune disruption. A person’s symptoms will depend on what condition accompanies the high RF levels.

For example, a person with high RF levels accompanying rheumatoid arthritis may experience:

The RF test is a simple blood test.

A doctor takes a small sample of blood from a vein in a person’s arm. Then, they send the sample to a laboratory, where a technician measures the levels of RF.

It may take several days for the results to come back. When they do come back, the doctor discusses the results and next steps with the person.

If a doctor suspects rheumatoid arthritis, they often order other blood tests alongside or after an RF test. These may include:

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

Learn more about blood tests for rheumatoid arthritis here.

While high RF levels may indicate rheumatoid arthritis, in many instances, this condition is not the only reason that a person might have high RF levels.

Many other conditions can cause higher RF levels, including:

When a doctor tests for RF levels, they may also perform an anti-CCP test. Anti-CCPs are antibodies the immune system also produces.

If the results of both tests are negative, but a person is still showing other symptoms of arthritis, they may have seronegative arthritis.

People with seronegative arthritis are typically at lower risk of serious disease progression and developing other complications. However, seronegative arthritis is still a serious condition that can require medical management and treatment.

People with seronegative arthritis may also develop other serious forms of arthritis, such as gout.

Learn more about seronegative rheumatoid arthritis here.

Treatment for high RF levels depends on the underlying cause.

For example, in people with rheumatoid arthritis, Sjögren’s disease, and other chronic conditions, treatment aims to slow the progression of the condition and help prevent or reduce symptoms.

On the other hand, if a bacterial infection is causing an increase in RF levels, a doctor may prescribe medication to treat the infection. This may result in RF levels decreasing.

Rheumatoid factor is a protein that the immune system produces. Some autoimmune conditions, notably rheumatoid arthritis, cause high levels of RF in the blood. Doctors measure a person’s RF levels to help them diagnose rheumatoid arthritis or another condition.

A high RF test result can indicate the presence of rheumatoid arthritis. However, this is not the only cause, and some people without an autoimmune condition can also have high RF levels. Doctors need to see various test results, consider a person’s clinical history, and perform a physical examination to diagnose rheumatoid arthritis.

Anyone who suspects they have rheumatoid arthritis or another autoimmune condition should contact a doctor.