Rheumatoid Factor Blood Test: What Does It Detect?

Rheumatoid factor is an immunoglobulin (antibody) which can bind to other antibodies. Typically, antibodies are normal proteins found in the blood which function within the immune system. Rheumatoid factor, however, is not normally found in the general population. While it is only found in about 1-2 percent of healthy people, the incidence of rheumatoid factor increases with age and about 20 percent of people over 65 years old have an elevated rheumatoid factor.

The presence of rheumatoid factor is detectable by a blood test. The blood test is commonly ordered when rheumatoid arthritis is suspected. Rheumatoid factor is present in 80 percent of adults who have rheumatoid arthritis, but there is a much lower prevalence in juvenile rheumatoid arthritis. The incidence of rheumatoid factor increases with duration of disease in rheumatoid arthritis: at 3 months the incidence is 33 percent, while at one year it is 75 percent. Up to 20 percent of rheumatoid arthritis patients remain negative for rheumatoid factor throughout the course of their disease. People who are negative for rheumatoid factor are historically classified as having seronegative rheumatoid arthritis.

• Seronegative and Seropositive Rheumatoid Arthritis

Other autoimmune diseases that can be positive for rheumatoid factor include:

• Sjogren's syndrome
• Systemic lupus erythematosus
• Scleroderma
• Polymyositis
• Dermatomyositis
• Mixed Connective Tissue Disease

Other infections or conditions which can be associated with positive rheumatoid factor test results include:

• Bacterial endocarditis
• Osteomyelitis
• Tuberculosis
• Syphilis
• Hepatitis
• Mononucleosis
• Diffuse interstitial pulmonary fibrosis
• Liver cirrhosis
• Sarcoidosis

Rheumatic conditions that are not associated with elevated rheumatoid factor include:

• Osteoarthritis
• Ankylosing spondylitis
• Gout
• Psoriatic arthritis
• Reiter's syndrome / Reactive arthritis

High levels or titers of rheumatoid factor are typically associated with severe rheumatoid arthritis. Rheumatoid factor also has been associated with a higher tendency to develop the non-joint (extra-articular) manifestations of the disease, such as rheumatoid nodules and rheumatoid lung disease.

The amount of rheumatoid factor in blood can be measured by:

Agglutination tests

One method mixes the patient's blood with tiny latex beads covered with human antibodies (IgG). The latex beads clump or agglutinate if rheumatoid factor (IgM RF) is present. Another method mixes the patient's blood with sheep red blood cells that have been covered with rabbit antibodies. The red blood cells clump if rheumatoid factor is present.

A titer is an indicator of how much the agglutination test blood sample can be diluted before rheumatoid factor is undetectable. A titer of 1:20 indicates that rheumatoid factor can be detected when 1 part of blood is diluted by up to 20 parts saline. Depending on the lab, a value for rheumatoid factor of 1:20 or less is considered normal.

Nephelometry test

Patient's blood is mixed with antibodies causing it to clump if rheumatoid factor is present. A light is passed through the tube containing the mixture and an instrument measures how much light is blocked by the mixture. Higher levels of rheumatoid factor create a more cloudy sample and allow less light to pass through, measured in units. Depending on the lab, a value for rheumatoid factor of 23 or less units is considered normal.

When analyzing lab results, remember that a rheumatoid factor more than 23 units and a titer more than 1:80 is indicative of rheumatoid arthritis but may also occur with other conditions. False positive results can occur when the level of fats in blood is high. Inaccurate results can be caused by improper handling of the blood specimen. And finally, a negative test result for rheumatoid factor does not preclude the diagnosis of rheumatoid arthritis.

• Rheumatoid Factor - Understanding the Test Results