Rheumatoid factor is a type of protein known as an autoantibody. Your immune system produces antibodies to defend against foreign substances like viruses and bacteria, but autoantibodies mistakenly target the body’s own healthy tissues. There are several types of rheumatoid factor, but the IgM version (RF IgM) is the one most frequently measured in routine blood tests. While RF IgM is most commonly associated with certain medical conditions, its detection is one piece of a larger diagnostic puzzle.
The Function of Rheumatoid Factor IgM in the Body
In a healthy immune system, antibodies identify and neutralize threats. In some individuals, the system loses its ability to differentiate between the body’s cells and foreign invaders, leading to the creation of autoantibodies like RF IgM. Specifically, RF IgM targets a different, normal antibody called immunoglobulin G (IgG).
When RF IgM binds to IgG molecules in the bloodstream, they form clusters known as immune complexes. These complexes are created by plasma cells, often within the synovial membrane that lines the joints. The formation of these complexes is a primary event in the inflammatory process.
These immune complexes circulate and can become lodged in tissues, most notably the joints. Once deposited, they trigger a cascade of inflammatory responses by attracting other immune cells. These cells release chemicals that lead to inflammation, swelling, and eventually, damage to surrounding cartilage and bone, contributing to symptoms in certain autoimmune disorders.
The Rheumatoid Factor Test and Results
The rheumatoid factor test is a standard blood draw that requires no special preparation. A phlebotomist collects a small blood sample from an arm vein, which is sent to a lab to measure the quantity of RF IgM autoantibodies in the bloodstream. The test does not diagnose a condition on its own but provides important data for a physician.
Test results are reported as either a “positive” or “negative” finding. A negative result means little to no rheumatoid factor was detected. A positive result indicates the presence of RF IgM above a certain threshold. The specific amount can be reported as a titer or as a concentration.
A titer, such as 1:80, shows how many times the blood had to be diluted before RF could no longer be detected. Alternatively, results may be given in international units per milliliter (IU/mL). While reference ranges can vary between laboratories, a value less than 15 IU/mL is considered negative.
A higher concentration of RF can suggest a more severe disease prognosis. However, the test result must be interpreted in the context of a full clinical evaluation, including symptoms and other diagnostic tests, as the presence of RF IgM is an indicator, not a definitive answer.
Medical Conditions Associated with Elevated RF IgM
Although strongly linked to rheumatoid arthritis (RA), elevated RF IgM is not exclusive to this condition. For patients with RA, a positive RF test classifies them as “seropositive,” which applies to approximately 60-80% of individuals with the disease. In these cases, the presence of RF often correlates with a greater risk of more significant joint damage.
Other autoimmune diseases are also frequently associated with a positive RF IgM test. Sjögren’s syndrome, a condition that causes dry eyes and mouth, often features high levels of RF. It can also be detected in patients with systemic lupus erythematosus (lupus) and scleroderma, which involves the hardening of skin and connective tissues.
Chronic infections can stimulate the immune system to produce rheumatoid factor. Viral infections like hepatitis C can lead to high levels of RF, and other long-term infections like tuberculosis and endocarditis can also cause elevated levels. Successfully treating the underlying infection often leads to the disappearance of the autoantibodies.
Less commonly, certain types of cancer that affect the blood and immune system, such as leukemia or multiple myeloma, may be linked to a positive RF test. A comprehensive medical evaluation is necessary to determine the underlying cause of an elevated result.
Factors Influencing RF IgM Levels
A person can have rheumatoid arthritis while testing negative for rheumatoid factor. This is known as “seronegative” arthritis, and it highlights that the absence of RF IgM does not rule out the disease.
Conversely, a “false positive” result can occur where an individual has elevated RF IgM without any underlying autoimmune disease. A small percentage of the healthy population may test positive, and this likelihood increases with age. Up to 20% of older adults may have a positive RF test without any related health issues, making it important to consider age during diagnosis.
The levels of RF IgM are not always stable and can be influenced by temporary health events. For instance, some vaccinations have been observed to cause a short-term elevation in these autoantibodies as part of the body’s normal immune response.