What is rheumatoid factor?
Rheumatoid factor (RF) is an autoantibody — an antibody that mistakenly attacks the body's own proteins, specifically the Fc region of IgG immunoglobulin. It is produced by the immune system in rheumatoid arthritis (RA) and several other conditions. The RF test is one of the oldest and most widely used blood tests in rheumatology, though it has important limitations in specificity. It is almost always ordered alongside anti-CCP antibodies, CRP and ESR when evaluating joint pain.
Rheumatoid factor normal range
| Result | RF (IU/mL) | Interpretation |
|---|---|---|
| Negative | < 14 IU/mL | Normal |
| Low positive | 14 – 60 IU/mL | Weakly positive — clinical correlation needed |
| Moderately positive | 60 – 100 IU/mL | Moderately elevated |
| High positive | > 100 IU/mL | High — associated with more severe RA |
What does a POSITIVE RF mean?
A positive RF in the setting of symmetrical small joint pain, morning stiffness lasting >1 hour, and swollen joints strongly supports rheumatoid arthritis. About 70-80% of people with RA are RF-positive (seropositive RA). RF-positive RA tends to be more severe, with greater joint destruction and systemic complications than seronegative RA. However, RF is not diagnostic alone — about 5% of healthy people are RF-positive, and the rate increases with age.
Other causes of positive RF
| Condition | Notes |
|---|---|
| Rheumatoid arthritis | 70-80% of cases are RF-positive |
| Sjogren syndrome | RF commonly positive |
| Hepatitis C | Very common cause of false-positive RF |
| Lupus (SLE) | Positive in ~20-30% of cases |
| Subacute bacterial endocarditis | Chronic antigen stimulation raises RF |
| Cryoglobulinaemia | Often associated with Hep C |
| Healthy elderly | RF positive in ~10-25% of people over 75 (no disease) |
RF vs Anti-CCP — which is better?
Anti-CCP antibodies (anti-cyclic citrullinated peptide) are more specific for rheumatoid arthritis than RF. Anti-CCP is positive in about 95% of RA cases but rarely positive without RA. Anti-CCP also appears earlier in RA — sometimes years before symptoms — and is not raised by hepatitis C or other conditions that falsely raise RF. If both RF and anti-CCP are positive, the diagnosis of RA is highly likely. If only RF is positive, other causes should be considered.
Questions to ask your doctor
- Should I also test anti-CCP antibodies?
- Do I need to see a rheumatologist?
- How does my RF level correlate with disease severity?
- Are CRP and ESR also elevated?
- Do I need X-rays of my hands and feet?