Rheumatoid factor (RF) is an abnormal protein that is looked for when rheumatoid arthritis is suspected. This assay is practically no longer done, in favor of the search for anti-CCP antibodies.
Rheumatoid factor (RF) is an autoantibody which attacks IgG gammaglobulins, cells present in our body. Unreliable, unspecific and inaccurate, this blood test, originally useful for detecting rheumatoid arthritis, is almost no longer practiced today. What test replaces it? What does a positive RF mean? Negative ?
What is rheumatoid factor?
Rheumatoid factor is a abnormal protein possessing autoantibody activitythat is to say that it will attack against the own cells of our organism, the IgG gammaglobulins.
Rheumatoid factor is measured with a blood test that does not require fasting, unless the workup includes blood sugar or other tests that require fasting.
Why is a rheumatoid factor assay requested?
“We look for rheumatoid factors during a suspicion of rheumatoid arthritis, but today, this test is almost no longer practiced“, explains Professor Francis Berenbaum, Head of the Rheumatology Department at Saint-Antoine Hospital in Paris. “There are still a few requests, but we are not getting any more information from them.” There are two reasons for this : “Rheumatoid factor is not always present early in the disease, and only 80% of people with rheumatoid arthritis have it.” In addition, it may be present in other pathologies such as chronic infections or other inflammatory diseases such as the so-called dry syndrome (Gougerot-Sjögren’s disease). “Finally, the test is too inaccurate : it therefore has no diagnostic value, 20% of patients having no rheumatoid factor. Today, anti-CCP antibodies are much more disease specific.” Rheumatoid arthritis is an autoimmune disease, which causes joint pain and inflammation of several joints at once. It most often affects the hands, wrists, knees. These joints swell and become painful. Without treatment, these joints tend to deform gradually over time. The blood test also makes it possible to estimate the intensity of the inflammation found during rheumatoid arthritis and to monitor its evolution. Indeed, the evolution of this disease is very variable from one person to another.
What is the normal rheumatoid factor value?
We assume that below 20 IU/ml, rheumatoid factor is normal.
What does a positive rheumatoid factor mean?
The threshold for positivity is approximately 20 IU/L, but it also depends on the techniques used by the laboratory. It should be remembered that rheumatoid factors are absent in 20% of rheumatoid arthritis cases, “this is why we are looking for anti-CCP antibodies, which are much more systematic for the disease”, insists the teacher. Moreover, “there are polyarthritis with this negative test, but it also works the other way around: you can have a false positive”. Rheumatoid factors are present in other pathologies:
- Leukemia
- Lymphoma
- Mononucleosis
- Tuberculosis
- Inflammatory lung diseases (sarcoidosis)
- Lupus
- Juvenile arthritis
- Scleroderma
- Hepatitis C
What does a negative rheumatoid factor mean?
A negative result cannot rule out a diagnosis. Remember that there are cases of polyarthritis which nevertheless show a negative test.
What is the anti-CCP antibody assay, which replaces rheumatoid factor?
“Anti-CCP antibodies, for anti-cyclic citrullinated peptides, are other autoantibodies present in 90% of people with rheumatoid arthritis. They are very specific: there is almost no other disease that generates these antibodies”, says Francis Berenbaum. Thus, this test helps to differentiate rheumatoid arthritis from other types of arthritis. In addition, anti-CCP antibodies appear earlier: they are detectable a few years before the appearance of the first manifestations. For this reason, a positive anti-CCP test makes it possible to implement a therapeutic strategy that is as effective as possible and as soon as possible.
Thanks to Professor Francis Berenbaum, Head of the Rheumatology Department at Saint-Antoine Hospital in Paris.