Rheumatoid factor (RF) is an antibody present in blood. The rheumatoid factor test checks for the presence of this antibody.
The test is performed using a reagent kit. A blood sample is drawn from the patient.
When is the rheumatoid factor test performed?
The RF test is indicated in patients suspected with rheumatoid arthritis or Sjogren’s syndrome. This test helps to distinguish between rheumatoid and inflammatory arthritis from other forms of arthritis. This test may be advised in other forms of inflammatory conditions as well.
Principle of the Rheumatoid Factor test
The rheumatoid factor is an anti-antibody. This can be detected in the laboratory by its ability to bind and form clumps with latex particles or red blood cells that contain human Immunoglobulin G (IgG).
If the rheumatoid factor is present in the patient’s blood it attaches to the IgG coating the latex particles causing clumps. This is called agglutination. Agglutination is considered a positive reaction that indicates the presence of rheumatoid factor at a detectable level.
The results of rheumatoid factor may be reported in two manners – less than 40 to 60 units per millilitres or less than 1:80 titre or 1 to 80 titre. Values may vary according to the laboratory and the kit that is used to test for the factor.
Lower numbers are indicative of absence of conditions like rheumatoid arthritis or Sjogren syndrome but there are several normal healthy individuals who test positive for rheumatoid factor. This makes RF tests a non-reliable test especially in absence of symptoms.
Meaning of abnormal results
Those with a positive rheumatoid factor have high levels or titres in blood. Most patients with rheumatoid arthritis, and almost all patients with Sjogren syndrome, have positive rheumatoid factor test results. If the levels are higher, the disease can be more certainly confirmed.
There are several other conditions that show high levels and titres of rheumatoid factor. These include Systemic lupus erythematosus or SLE, Scleroderma, Sarcoidosis, Dermatomyositis, Adult Still's disease etc. Those with viral infections such as hepatitis, AIDS, influenza, infectious mononucleosis etc may also test positive for rheumatoid factor. Other infections such as parasitic infestations, bacterial infections, tuberculosis may also test positive. Kidney disorders, endocarditis, cancers of blood such as leukaemia and multiple myelomas, lung and liver diseases also cause raise Rheumatoid Factor levels.
Risks associated with the RF test
There is a small risk of bleeding, infection and pain at the site from where the blood sample is taken. This is usually a minor risk unless the person has bleeding tendencies and bleeding disorders.
Limitations of the RF test
This not a confirmatory test as many normal healthy persons may tests positive. In addition several conditions may show positive titres for this test. Procedure must be followed in a detailed manner. Reading for agglutination after a fixed time has elapse may give false readings. Contamination of the agents, samples or handling equipment leads to erroneous results.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)