Rheumatoid Factor Explained
Rheumatoid factor is an immunoglobulin (antibody) which can bind to other antibodies. Antibodies are normal proteins found in the blood which function within the immune system. Rheumatoid factor though is not normally found in the general population (only found in about 1-2% of healthy people). The incidence of rheumatoid factor increases with age and about 20% of people over 65 years old have an elevated rheumatoid factor.
A blood test is used to detect the presence of rheumatoid factor. The blood test is commonly ordered to diagnose rheumatoid arthritis. Rheumatoid factor is present in 80% 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%, while at one year it is 75%. Up to 20% of rheumatoid arthritis patients remain negative for rheumatoid factor (also known as "seronegative rheumatoid arthritis") throughout the course of their disease.
Other Conditions Can Test Positive For Rheumatoid Factor
Other autoimmune diseases can also be positive for rheumatoid factor including:
- Sjogren's syndrome
- Systemic lupus erythematosus
- Mixed Connective Tissue Disease
Other infections or conditions which can be associated with positive rheumatoid factor include:
- Bacterial endocarditis
- Diffuse interstitial pulmonary fibrosis
- Liver cirrhosis
Conditions Not Associated With Rheumatoid Factor
Rheumatic conditions NOT associated with elevated rheumatoid factor include:
- Ankylosing spondylitis
- Psoriatic arthritis
- Reiter's syndrome / Reactive arthritis
High levels or titers of rheumatoid factor are associated with more severe rheumatoid arthritis. The factor also has been associated with a higher tendency to develop the non-joint complications of the disease such as rheumatoid nodules and rheumatoid lung disease.
How Is Rheumatoid Factor Measured?
The amount of rheumatoid factor in blood can be measured by:
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. The lab value for rheumatoid factor of 1:20 or less is considered normal.
This method mixes the patient's blood with antibodies that cause the blood 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. The lab value for rheumatoid factor of 23 or less units is considered normal.
When analyzing lab results the following should be considered:
- A rheumatoid factor more than 23 units and a titer more than 1:80 is indicative of rheumatoid arthritis but may also occur in other conditions.
- False positive results can occur when the blood is high in fats.
- Inaccurate results can be caused by improper handling of the blood specimen.
- A negative test result for rheumatoid factor does not exclude the diagnosis of rheumatoid arthritis.
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