Initial Diagnostic Test for Infectious Mononucleosis
Order a heterophile antibody test (Monospot) as the initial diagnostic test for suspected infectious mononucleosis, combined with a complete blood count with differential. 1, 2
First-Line Testing Approach
The heterophile antibody test (Monospot) is the most widely used and cost-effective initial test, with a sensitivity of 87% and specificity of 91%. 1, 2 This test typically becomes positive between the sixth and tenth day after symptom onset. 1
Simultaneously obtain a complete blood count with differential to look for:
- Lymphocytosis ≥50% of white blood cell differential 1
- Atypical lymphocytosis ≥10% of total lymphocyte count 1
- These findings strongly support the diagnosis when present 1, 3
When to Proceed to EBV-Specific Antibody Testing
Order EBV serologic testing when:
- The heterophile test is negative but clinical suspicion remains high 1, 4
- The patient is a child younger than 10 years (false-negative heterophile tests are common in this age group) 1, 4
- The patient is in the first week of illness (false-negative rate approximately 10% early in infection) 1, 4
- Atypical or severe presentations require confirmation 4
The recommended EBV antibody panel includes:
- VCA IgM antibodies (indicates acute infection when positive) 1, 4
- VCA IgG antibodies 1, 4
- EBNA antibodies (Epstein-Barr nuclear antigen) 1, 4
Interpretation of EBV serology:
- Acute primary infection: VCA IgM positive (with or without VCA IgG) AND EBNA antibodies absent 1, 4
- Past infection (>6 weeks prior): EBNA antibodies present with VCA IgG 1, 5
Critical Pitfalls to Avoid
False-negative heterophile results occur in:
- Children younger than 10 years 1, 4
- Early infection (first week of symptoms) 1, 4
- Overall false-negative rate of approximately 10% 1
False-positive heterophile results may occur in:
Alternative Diagnoses to Consider
When both heterophile and EBV testing are negative, test for other causes of mononucleosis-like illness:
- Cytomegalovirus (CMV) infection 1, 4, 6
- HIV infection (particularly important to rule out) 1, 4, 6
- Toxoplasma gondii infection 1, 4, 6
- Adenovirus infection 1, 4, 6
- Streptococcal pharyngitis 1, 6
Special Population Considerations
For immunocompromised patients:
- Test more aggressively due to increased risk of severe disease and lymphoproliferative disorders 1, 4
- Consider EBV viral load by nucleic acid amplification testing (NAAT) 4
- Quantitative PCR showing >10^2.5 copies/mg DNA in peripheral blood mononuclear cells suggests chronic active EBV 4
Additional supportive finding: Elevated liver enzymes increase clinical suspicion for infectious mononucleosis when the heterophile test is negative. 2