Pap Smear — Cervical Cancer
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 45% (sensitivity 55%) |
| False-positive rate | 3% (specificity 97%) at HSIL threshold |
| Bayes factor (positive test) | 20× |
| Bayes factor (negative test) | 1/2× |
| Base rate | 0.1% for high-grade lesions or cancer in screened women |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Average screened woman | 0.1% | 20 × 0.1% = 2%0.1%× 202% | 1/2 × 0.1% = 0.05%0.1%÷ 20.05% |
| Persistent HPV infection | 2% | 20 × 2% = 40%2%× 2040% | 1/2 × 2% = 1%2%÷ 21% |
One-directional test: positive is informative (20×), but negative barely moves the needle (1/2×) because it misses 45% of cases. This is why HPV co-testing has supplemented Pap-only screening.
- + result: risk goes from appendicitis this year (0.1%) to red hair (2%)
- − result: barely changes it — this is why co-testing exists
Sources: