Rheumatoid Factor (RF) — Rheumatoid Arthritis
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 31% (sensitivity 69%) |
| False-positive rate | 15% (specificity 85%) |
| Bayes factor | 5× |
| Base rate | 0.5% of US adults |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Unselected adult | 0.5% | 5 × 0.5% = 2.5%0.5%× 52.5% | 1/5 × 0.5% = 0.1%0.5%÷ 50.1% |
| Symmetric polyarthritis, morning stiffness >1 hr | 15% | 5 × 15% = 75%15%× 575% | 1/5 × 15% = 3%15%÷ 53% |
Most positive RF results in unselected patients are false positives (Sjögren's, hepatitis C, endocarditis, older age). Best used in combination with anti-CCP.
- + result: risk goes from identical twin (0.5%) to near red hair (2.5%) — most positives in unselected patients are false
- − result: modest reassurance
Sources:
- Meta-analysis, Ann Intern Med, 2007. PubMed 17548411.