NAAT — Chlamydia/Gonorrhea
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 5% (sensitivity 95%) |
| False-positive rate | 1.5% (specificity 98.5%) |
| Bayes factor (positive test) | 50× |
| Bayes factor (negative test) | 1/20× |
| Base rate | 5% chlamydia in sexually active women <25; 1% gonorrhea in high-risk populations |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Sexually active woman <25, routine screening | 5% | 50 × 5% ≥ 100%5%× 50100% | 1/20 × 5% = 0.25%5%÷ 200.25% |
| Mucopurulent discharge | 20% | 50 × 20% ≥ 100%20%× 50100% | 1/20 × 20% = 1%20%÷ 201% |
50 × 5% ≥ 100%5%× 50100%: exact posterior is 72%. 50 × 20% ≥ 100%20%× 50100%: exact posterior is 93%. Gold standard for STI detection.
- + result: risk goes from vegetarian (5%) to 72%
- − result: risk drops to near identical twin (0.25%)
Sources: