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What is Rheumatoid Factor (RF)?
Rheumatoid Factor (RF) is an antibody that, when present in the blood, can attack the body's own healthy tissues — particularly the joints. Measuring RF levels is one of the primary tests used in diagnosing Rheumatoid Arthritis (RA), an autoimmune disease that causes painful joint inflammation.
Why is the RF Test Done?
- To help diagnose Rheumatoid Arthritis in patients with joint pain, swelling, and stiffness — especially in the small joints of the hands and feet.
- To investigate other autoimmune conditions such as Sjögren's syndrome, lupus, or scleroderma.
- To assess disease severity and guide treatment decisions in RA patients.
- When a patient has morning stiffness lasting more than 30 minutes, symmetrical joint swelling, or fatigue alongside joint pain.
Normal RF Levels
- Negative (normal): Less than 14–20 IU/mL (varies by lab)
- Positive: Above the lab's reference range
Importantly, a positive RF does not automatically mean Rheumatoid Arthritis — RF can be elevated in other conditions and even in a small percentage of healthy older adults. Conversely, about 20–30% of RA patients have a negative RF (seronegative RA). The test must be interpreted alongside symptoms, physical examination, and other tests like Anti-CCP antibodies and ESR.
How to Prepare for the RF Test
No fasting is required. A simple blood sample is taken. Tell your doctor if you have had any recent infections or vaccinations, as these can temporarily affect immune markers.