Monospot — Infectious Mononucleosis (EBV)
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 15% (sensitivity 85%) |
| False-positive rate | 5% (specificity 95%) |
| Bayes factor (positive test) | 20× |
| Bayes factor (negative test) | 1/5× |
| Base rate | 5% of adolescents/young adults with pharyngitis-like illness |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| College-age, sore throat, fatigue, lymphadenopathy | 5% | 20 × 5% ≥ 100%5%× 20100% | 1/5 × 5% = 1%5%÷ 51% |
| Same + posterior cervical adenopathy, splenomegaly | 20% | 20 × 20% ≥ 100%20%× 20100% | 1/5 × 20% = 4%20%÷ 54% |
20 × 5% ≥ 100%5%× 20100%: exact posterior is 51%. 20 × 20% ≥ 100%20%× 20100%: exact posterior is 83%. Sensitivity is poor in first 1–2 weeks and in children <4. Atypical lymphocytes >20% on CBC have an even higher positive Bayes factor (~20×).
- + result: risk goes from vegetarian (5%) to coin flip (51%)
- − result: risk drops to shellfish allergy (1%)
Sources:
- StatPearls. Monospot test.
- AAFP. Infectious mononucleosis: rapid evidence review, 2023.