The Rheumatoid Factor Test

The rheumatoid factor blood test (RF) helps diagnose Rheumatoid Arthritis (RA). It measures rheumatoid factor, a chemical in the blood of many, but not all, people with RA. By itself, it does NOT show whether you have rheumatoid arthritis or not.
The Rheumatoid Factor Test gives your doctor more information to consider when diagnosing your condition, it does not necessarily mean you have RA. It is used along with your physical exam, other tests, and your symptoms to help diagnose your condition.  

People with other autoimmune conditions, such as lupus and Sjogren's syndrome, can also have a positive Rheumatoid Factor. So can people with infections such as hepatitis and mononucleosis. 

False positives occur in 5% of healthy individuals and in up to 20% of people over 65 years of age. False positives also occur in inflammatory condition such as Sjögren's Syndrome, Lupus and Systemic Sclerosis.

Some healthy people can have rheumatoid factor in their blood too. Also some people with RA don’t have a positive RA result. That’s why this test will not,

The higher the level in rheumatoid disease the worse the joint destruction and the greater the chance of systemic involvement.

What Abnormal Results Mean

An abnormal result means the test is positive, which means higher levels of rheumatoid factor have been detected in your blood. The higher the level, the more likely one of these conditions is present. There are also other tests for these disorders that help make the diagnosis.
People with the following diseases may also have high levels of rheumatoid factor:

• Rheumatoid arthritis 
• Juvenile rheumatoid arthritis.
• Sjögren's Syndrome 
• Scleroderma
• Systemic lupus erythematosus
• Adult Still's disease
• Dermatomyositis
• Sarcoidosis
• Felty's syndrome
• Systemic sclerosis 
• Infective endocarditis.
• Infectious mononucleosis.
• Hepatitis.
• Tuberculosis.
• Syphilis.
• HIV.
• Influenza.
• Malignancy.
• Leukaemia.

Other blood tests for rheumatoid arthritis:

• erythrocyte sedimentation rate (ESR)
• C-reactive protein (CRP)
• anti-CCP (anit-cyclic citrullinated peptide) 

SOURCES:

1. Wilson D; Rheumatoid factors in patients with rheumatoid arthritis. Can Family Physician. 
2. Longmore M, Wilkinson IB & Rajagopalan SR; Oxford Handbook of Clinical Medicine, 2004
3. MedlinePlus 
4. WebMD