RPR/VDRL — Syphilis
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 18% (sensitivity 82%) |
| False-positive rate | 5% (specificity 95%) |
| Bayes factor (positive test) | 20× |
| Bayes factor (negative test) | 1/5× |
| Base rate | 0.1% in general population |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| General population | 0.1% | 20 × 0.1% = 2%0.1%× 202% | 1/5 × 0.1% = 0.02%0.1%÷ 50.02% |
| MSM at STI clinic | 2% | 20 × 2% = 40%2%× 2040% | 1/5 × 2% = 0.4%2%÷ 50.4% |
Biological false positives occur with pregnancy, autoimmune conditions, IV drug use, TB. Confirmatory treponemal testing always required. In secondary syphilis, sensitivity is ~100% and the Bayes factor is much stronger (100×).
- + result: risk goes from appendicitis this year (0.1%) to red hair (2%)
- − result: risk drops to 0.02%
Sources: