What are the diagnostic tests for infectious mononucleosis (mono)?

Diagnostic Testing for Infectious Mononucleosis (Mono)

The most effective diagnostic approach for infectious mononucleosis is serological testing, including VCA IgM, VCA IgG, and EBNA IgG antibodies, with heterophile antibody tests (Monospot) serving as a rapid initial screening tool. 1

Primary Diagnostic Tests

First-Line Testing

1. Complete Blood Count (CBC) with differential

   • Look for:

     • 40% lymphocytes

     • 10% atypical lymphocytes 2

   • This is a cost-effective initial screening test

2. Heterophile Antibody Test (Monospot)

   • Becomes detectable 6-10 days after symptom onset
   • Sensitivity: 87%, Specificity: 91% 2
   • Important limitations:
     • False negatives in children under 10 years (approximately 10% of EBV cases missed)
     • False negatives in adults during the first week of illness 1, 2

Confirmatory Testing

When Monospot is negative but clinical suspicion remains high, proceed to:

EBV-Specific Antibody Panel (standard panel includes):

• Viral Capsid Antigen (VCA) IgM
• Viral Capsid Antigen (VCA) IgG
• Epstein-Barr Nuclear Antigen (EBNA) IgG 1

Interpretation of Serological Patterns

Pattern	Interpretation
VCA IgM (+), VCA IgG (+), EBNA IgG (-)	Acute primary infection (within 6 weeks)
VCA IgM (-), VCA IgG (+), EBNA IgG (+)	Past infection (>6 weeks)
VCA IgM (-), VCA IgG (-), EBNA IgG (-)	No previous EBV infection

Additional Tests to Consider

1. Liver Function Tests

   • Elevated liver enzymes increase clinical suspicion for mono when heterophile test is negative 2

2. IgG Avidity Testing

   • Helps distinguish between recent and past infection
   • Low avidity indicates recent infection
   • High avidity suggests past infection or reactivation
   • Consider for ambiguous serological patterns 1

3. Quantitative EBV Viral Load by PCR

   • Consider for:
     • Persistent or severe symptoms
     • Immunocompromised patients
     • Evaluating for chronic active EBV infection (symptoms >6 months) 1

Clinical Pearls and Pitfalls

• Diagnostic Dilemma: Some patients with clinical features of mono may be both heterophile negative and initially seronegative for EBV, requiring repeat testing as antibodies develop 3

• Age Considerations: Clinical manifestations vary according to age at presentation, which may affect test selection and interpretation 4

• Differential Diagnosis: Consider other causes of mononucleosis-like illness when EBV testing is negative:

  • Cytomegalovirus (CMV)
  • HIV
  • Other viral infections 4, 3

• Test Selection Strategy: For typical presentations, heterophile antibody testing is sufficient. For atypical or severe presentations, proceed directly to specific EBV antibody testing 5

• Timing Matters: Be aware that serological patterns evolve over time, and testing too early may yield false-negative results 3

• Immunocompromised Patients: These individuals require more extensive testing as they are at higher risk for severe disease and complications 2, 6