Rheumatoid Factor
RF· also: Rheumatoid Factor
Clinical Overview
Rheumatoid Factor (RF) is an autoantibody — typically IgM — directed against the Fc portion of IgG immunoglobulin. It is detected in rheumatoid arthritis and several other autoimmune, chronic inflammatory, and infectious conditions. RF is a screening test for rheumatoid arthritis, but it is neither sensitive nor specific enough to diagnose RA alone.
Why This Test Matters
RF is positive in approximately 70–80% of rheumatoid arthritis patients, but also in 5–10% of healthy adults (especially the elderly), and in many non-RA conditions (Sjögren's syndrome, SLE, hepatitis C, bacterial endocarditis, sarcoidosis, tuberculosis). For RA diagnosis, RF should be used alongside anti-CCP antibodies and clinical criteria. Anti-CCP is more specific than RF for RA. A strongly positive RF in RA correlates with more severe disease, extra-articular manifestations (rheumatoid nodules, vasculitis), and poorer prognosis.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 0 – 14 | IU/mL | Negative <14 |
What Causes Abnormal Results?
High RF Causes
- Rheumatoid Arthritis (RA) — most common clinical indication
- Sjögren's syndrome (often very high RF)
- SLE (systemic lupus erythematosus)
- Hepatitis C (associated with cryoglobulinemia and high RF)
- Bacterial endocarditis and other chronic infections
- Sarcoidosis
- Primary biliary cholangitis (PBC)
- Healthy elderly individuals (incidental low-titer RF)
- Cryoglobulinemia
Low RF Causes
- Approximately 20–30% of RA patients are "seronegative" — RF negative — especially early in disease
- Negative RF does not exclude RA (seronegative RA exists)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. RF can be measured from a random blood sample. For initial RA evaluation, order RF and anti-CCP together for best diagnostic accuracy.
Factors That Can Affect Results
- Age (RF positivity increases with age — up to 25% of people over 70 are RF positive without RA)
- Chronic infections (hepatitis C most notably causes very high RF)
- Some assays detect IgM-RF only; others detect IgG- and IgA-RF as well
- Hypergammaglobulinemia can cause false-positive RF
Related Topics
Frequently Asked Questions
If my RF is positive, do I have rheumatoid arthritis?
Not necessarily. A positive RF is not diagnostic of RA on its own. RF is positive in many conditions — including hepatitis C, Sjögren's syndrome, SLE, sarcoidosis, and even in 5–10% of healthy elderly people. RA diagnosis requires at least 6/10 points on the 2010 ACR/EULAR RA criteria — which include joint involvement, serology (RF and/or anti-CCP), inflammatory markers (CRP/ESR), and symptom duration. If your RF is positive and you have joint symptoms, see a rheumatologist for full evaluation.
Is anti-CCP better than RF for diagnosing rheumatoid arthritis?
Yes, in terms of specificity. Anti-CCP (anti-cyclic citrullinated peptide) is approximately 95% specific for RA vs. ~85% for RF, because it is not elevated in the many non-RA conditions that cause false-positive RF. Sensitivity is similar. Anti-CCP also has the advantage of being detectable years before clinical symptoms — making it useful for early diagnosis. Most rheumatologists order both RF and anti-CCP together for initial RA evaluation.