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

High RF levels can indicate RA, but this 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.

What is RF?

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.

Normal ranges

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.

What do the results mean?

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 and 10%–25% of people aged 70 years may have high RF.

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:

• Pain and stiffness in the joints
• Swelling and inflammation in the joints
• Loss of range of motion
• Nodules under the skin
• General fatigue

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.

Other tests

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 magnetic resonance imaging (MRI), x-rays, and ultrasound to check for signs of inflammation or help distinguish between types of arthritis.

Test procedure

The RF test is a simple blood test. A health-care 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.

RF and other conditions

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:

• Sjogren's syndrome
• Systemic sclerosis
• Lupus
• Connective tissue disease
• Chronic bacterial infections, such as syphilis or tuberculosis
• Viral infections, including HCV infection, HIV, and herpes
• Parasitic infections, such as malaria
• Cancers
• Sarcoidosis
• Liver cirrhosis

Treatment

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.

When to see a doctor

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.

Summary

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.