What is the rheumatoid factor?
The rheumatoid factor is a so-called autoantibody. These are defense substances of the immune system that attack the body’s own tissue and can thus trigger a disease (autoimmune disease). As the name suggests, rheumatoid factors play a role primarily in autoimmune rheumatism.
Rheumatoid factors attack certain parts (Fc section) of other antibodies – namely immunoglobulin G. They are therefore practically antibodies against antibodies.
Depending on their structure, rheumatoid factors – like all antibodies (immunoglobulins) – are divided into different classes. These include, for example, immunoglobulin M (IgM), immunoglobulin A (IgA) and immunoglobulin G (IgG). As a rule, the detected rheumatoid factors belong to the IgM class (RF-IgM or RhF-IgM).
When do you determine the rheumatoid factor?
The doctor determines rheumatoid factors when a rheumatic disease is suspected – especially rheumatoid arthritis. However, a positive test result alone is not sufficient for the diagnosis. RF is not a very specific laboratory value – it can be elevated in various rheumatic diseases, but also in non-rheumatic diseases or in healthy individuals.
For the examination, the physician takes a blood sample from the patient. Rheumatoid factor is usually measured in the blood serum. Laboratory physicians can use various techniques for detection (e.g. ELISA, radioimmunoassay). Depending on the measurement method, different threshold values apply, which, when exceeded, are referred to as elevated rheumatoid factor.
When is the rheumatoid factor elevated?
Rheumatoid factor is only one of several parameters used to diagnose the disease.
Rheumatoid factor in rheumatism
In addition to rheumatoid arthritis, the test for rheumatoid factors can also be positive in other rheumatic diseases, i.e. provide elevated readings. These include, for example, the following diseases (the proportion of patients who are rheumatoid factor positive is shown in parentheses):
- Cryoglobulinemia: form of vascular inflammation (50 to 100 percent)
- Sjögren’s syndrome (70 to 95 percent)
- Systemic lupus erythematosus (15 to 35 percent)
- Mixed collagenosis: clinical picture with symptoms of various autoimmune connective tissue diseases such as systemic lupus erythematosus, scleroderma and polymyositis as well as Raynaud’s syndrome (50 to 60 percent)
- Scleroderma (systemic sclerosis): Collective term for autoimmune diseases associated with hardening of the connective tissue (20 to 30 percent)
- Juvenile chronic arthritis (10 to 15 percent)
- Polymyositis and dermatomyositis (5 to 10 percent)
Other causes
- Cirrhosis of the liver
- Chronic inflammation of the liver (chronic hepatitis)
- Chronic inflammatory lung diseases
- Inflammation of the inner lining of the heart (endocarditis)
- Tuberculosis
- Salmonellosis
- Sarcoidosis
- Syphilis
- Acute infections with bacteria, viruses or parasites (e.g. mononucleosis, malaria)
- Malignant tumors
- After blood transfusions
- After vaccinations
- After chemo- or radiotherapy
Last but not least, the rheumatoid factor is detectable in about five percent of healthy people – without any disease value. Especially in older age, many otherwise healthy people are RF-positive (about ten percent of those over 60).
An elevated rheumatoid factor without any symptoms has no significance.
