Why Get Tested?
To help diagnose the presence of an autoimmune disease such as rheumatoid arthritis (RA)
When To Get Tested?
When you have joint pain and fatigue that your healthcare practitioner suspects may be due to RA
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
What is being tested?
Rheumatoid factor (RF) is an autoantibody, an immunoglobulin M (IgM) protein that is produced by the body's immune system. Autoantibodies attack a person's own tissues, mistakenly identifying the tissue as "foreign." While the biologic role of RF is not well understood, its presence is useful as an indicator of inflammatory and autoimmune activity. This test detects and measures RF in the blood and may be used, along with other tests, to help in the diagnose of rheumatoid arthritis (RA).
RA is a chronic, systemic autoimmune disease that causes inflammation, pain, stiffness, and destructive changes in the hands, feet, and other joints throughout the body. Some patients may show signs of fatigue, low-grade fevers, and weight loss.
It can affect anyone at any age but usually develops in the late child-bearing years in women and between the ages of 60 to 80 in men. Over 70% of those affected with RA are women. The course of RA and its prognosis are variable. It may develop and progress slowly or rapidly. It may go into remission in some people and, in a few, it may go away. Left untreated, RA can shorten a person's lifespan and can, within a few years, leave many of those affected too disabled to work.
There are a variety of treatments available to minimize the complications of RA, but they depend on making an accurate diagnosis and on beginning treatment before the development of significant joint damage. 
Rheumatoid factor has been used to detect RA. Because the sensitivity and specificity of RF are not ideal, other laboratory tests are often performed in conjunction with RF testing. About 80% of those with RA will have a positive RF test, but it can be negative in people who have clinical signs of RA.
A test for cyclic citrullinated peptide antibodies (CCP) may be done along with an RF test as studies have shown that the CCP antibody test has a sensitivity and specificity that is considered better than RF and is more likely to be positive with early RA. Other tests that may be performed are:
Complete blood count (CBC, for anemia & thrombocytosis)
Erythrocyte sedimentation rate (ESR)
C-reactive protein (CRP)
Anti-nuclear antibodies (ANA)
Antineutrophil cytoplasmic antibodies (ANCA)
Synovial fluid analysis (inflammatory joints)
Elevated RF can be found in a small percentage (5-10%) of healthy people. RF may also be elevated in the elderly, though they may not demonstrate clinical signs. In addition, elevated levels of RF may be detected in people who do not have RA (false positive) but may have another disorder:
Sjögren syndrome
Systemic lupus erythematosus
Bacterial, viral, and parasitic infections (hepatitis, TB, syphilis, leprosy)
Certain cancers
Lung disease, liver disease, and kidney disease
When is it ordered?
The test for RF may be ordered when a person has signs and symptoms of RA. Symptoms may include:
Pain, warmth, swelling, and morning stiffness in the joints 
Nodules under the skin 
If the disease has progressed, evidence on X-rays of swollen joint capsules and loss of cartilage and bone 
 
An RF test may be repeated when the first test is negative and symptoms persist.