COVID-19 Rapid Antigen Test — COVID-19
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 27% (sensitivity 73% symptomatic; 45% asymptomatic) |
| False-positive rate | 0.7% (specificity 99.3%) |
| Bayes factor (positive test) | 100× |
| Bayes factor (negative test) | 1/3× |
| Base rate | 5–20% depending on community transmission |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Symptomatic during surge | 15% | 100 × 15% ≥ 100%15%× 100100% | 1/3 × 15% = 5%15%÷ 35% |
| Asymptomatic, low prevalence | 2% | 100 × 2% ≥ 100%2%× 100100% | 1/3 × 2% = 0.7%2%÷ 30.7% |
100 × 15% ≥ 100%15%× 100100%: exact posterior is 95%. 100 × 2% ≥ 100%2%× 100100%: exact posterior is 67%. Like rapid flu but more extreme: massive positive Bayes factor makes a positive near-certain, but the negative is weak (1/3×). Repeat in 24–48 hours for symptomatic patients with negative results.
- + result: risk goes from 15% to 95% during a surge
- − result: only drops to vegetarian (5%) — repeat if suspicious
Sources:
- Meta-analysis of 166,943 samples, 2022. PMC 9461282.
- Cochrane review, 2023. AAFP summary.