Rheumatoid factor
Rheumatoid factor (RF) is a blood test performed in patients with suspected rheumatoid arthritis (RA). It is an antibody against IgG, which is itself an antibody; RF and IgG form immune complexes, which are part of the disease process of various rheumatological diseases.
Indications
RF is often determined in patients suspected in any form of arthritis. It has relatively little use there, as positive results can be due to other causes, and negative results do not rule out disease. Nevertheless, in combination with symptoms, it can be of help in distinguishing the causes. In addition, it is part of the disease criteria of RA and Sjögren's syndrome.
Interpretation
High levels RF (over the 95th percentile) are indicative of rheumatoid arthritis (present in 80%) and Sjögren's syndrome (present in 90%). There is a high rate of false positives due to other causes. These are:
