Chest X-Ray — Community-Acquired Pneumonia
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 29% (sensitivity 71%) |
| False-positive rate | 14% (specificity 86%) |
| Bayes factor | 5× |
| Base rate | 5% of primary care patients presenting with cough and fever |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Cough and low-grade fever | 5% | 5 × 5% = 25%5%× 525% | 1/5 × 5% = 1%5%÷ 51% |
| High fever, productive cough, focal crackles | 20% | 5 × 20% ≥ 100%20%× 5100% | 1/5 × 20% = 4%20%÷ 54% |
5 × 20% ≥ 100%20%× 5100%: exact posterior is 56%. CXR misses ~30% of pneumonias, especially early in disease. Lung ultrasound (sensitivity ~93%) increasingly supplements CXR.
- + result: risk goes from vegetarian (5%) to face card from a deck (25%)
- − result: risk drops to shellfish allergy (1%)
Sources: