RF (Rheumatoid Factor)
Definition
Detects a protein produced by the immune system that mistakenly attacks healthy antibodies (IgG) instead of germs. It is an autoimmune marker, not an indicator of infection.
Purpose
Together with other tests, it helps diagnose rheumatoid arthritis (RA). It is also used in screening for other autoimmune conditions (lupus, Sjögren's syndrome), chronic infections (hepatitis C, tuberculosis) and certain cancers. Symptoms that prompt the test: joint pain/stiffness, swelling, fatigue, rheumatoid nodules.
Results
A positive result means that a higher level of rheumatoid factor is present in the blood. The physician evaluates this together with symptoms and additional tests to decide whether RA, another autoimmune disease, a chronic infection or cancer is present.
Normal Values
Laboratory specific; the source notes that the result may also give the normal RF range for that laboratory.
Risks and Limitations
This test has a high margin of error in both directions, and this must be stated in the glossary:
A healthy person can test positive: Many people have small amounts of RF in their blood and are healthy. An RA patient can test negative: People with rheumatoid arthritis may have very little or no rheumatoid factor.
In other words, a positive result does not mean RA, and a negative result does not rule out RA. (Seronegative RA is a real clinical entity.)