Rapid Influenza Diagnostic Test (RIDT) — Influenza
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 38% (sensitivity 62%; adults ~54%, children ~67%) |
| False-positive rate | 1.8% (specificity 98.2%) |
| Bayes factor (positive test) | 50× |
| Bayes factor (negative test) | 1/3× |
| Base rate | 15% during flu season among patients with influenza-like illness |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| ILI symptoms during flu season | 15% | 50 × 15% ≥ 100%15%× 50100% | 1/3 × 15% = 5%15%÷ 35% |
| ILI symptoms outside flu season | 2% | 50 × 2% ≥ 100%2%× 50100% | 1/3 × 2% = 0.7%2%÷ 30.7% |
50 × 15% ≥ 100%15%× 50100%: exact posterior is 90%. 50 × 2% ≥ 100%2%× 50100%: exact posterior is 51%. The asymmetry is the teaching point. Positive: 50× (trust it). Negative: 1/3× (don't trust it). A negative rapid flu during flu season still leaves 1-in-20 odds. Mirror image of D-dimer.
- + result: risk goes from 15% to 90% during flu season
- − result: only drops to vegetarian (5%) — don't trust it
Sources:
- Chartrand et al. Ann Intern Med, 2012. PubMed 22371850.
- CDC. Rapid influenza diagnostic tests.