Rheumatoid factor

Rheumatoid factor (RF or RhF) is a blood test performed in patients with suspected rheumatoid arthritis (RA).

It is an antibody against the Fc portion of IgG, which is itself an antibody, IgM or IgA type; RF and IgG form immune complexes, which are part of the disease process of various rheumatological diseases. Rheumatoid factor can be either Type 2 (monoclonal IgM to polyclonal IgG) or 3 (polyclonal IgM to polyclonal IgG) cryoglobulin.

These factors are antibodies in the serum of people who suffer from rheumatoid arthritis. Antigenic determinants or immunoglobins.


 * Positive in persons with symptoms

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. But, in combination with signs and symptoms, it can be diagnostic and prognostic. It is part of the disease criteria of RA.

Interpretation
High levels RF (generally above 20 IU/mL, 1:40 or over the 95th percentile there is some variation among labs) are indicative of rheumatoid arthritis (present in 80%) and Sjögren's syndrome (present in almost 100%).

The higher the levels of RF the higher the possibility of a more destructive articular disease.

There is a high rate of false positives due to other causes. These are:

Differential Diagnosis

 * Anthracosis
 * Any chronic viral infection
 * Asbestos
 * Asthma
 * Bacterial endocarditis
 * Behcet's Syndrome
 * Bronchitis
 * Chronic hepatitis
 * Cirrhosis
 * Cryoglobulinemia
 * Dermatomyositis
 * Hepatitis
 * Juvenile Rheumatoid Arthritis
 * Leukemia
 * Mixed connective tissue disease
 * Infectious mononucleosis
 * Myocardial Infarction
 * Neoplasm
 * Osteomyelitis
 * Polymyositis
 * Sarcoidosis
 * Sclederoma
 * Sjogren's Syndrome
 * Syphillis
 * Systemic Lupus Erythmatosus (SLE)
 * Tuberculosis