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. Rapid Heterophile Antibody Test (Monospot)

   • Becomes detectable 6-10 days after symptom onset
   • Sensitivity: 87%, Specificity: 91% 2
   • Important limitations:
     • False-negative results in children <10 years (approximately 10% of EBV cases may be missed)
     • False-negative results 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 (gold standard)

• More sensitive and specific but more expensive and time-consuming than Monospot 2
• 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 Testing When Indicated

1. Liver Function Tests

   • Elevated liver enzymes increase clinical suspicion for infectious mononucleosis when heterophile antibody 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 1

3. Quantitative EBV Viral Load by PCR

   • Consider for:
     • Persistent or severe symptoms
     • Immunocompromised patients
     • Suspected chronic active EBV infection 1

Special Considerations

Diagnostic Challenges

• Seronegative window period: A patient may present with clinical features of mono but be both heterophile negative and seronegative if tested very early in the course of infection 3
• Repeat testing may be necessary if initial tests are negative but clinical suspicion remains high

High-Risk Populations

• Immunocompromised patients require more vigilant monitoring due to increased risk of:
  • Severe disease
  • Lymphoproliferative disorders
  • Complications 2, 1

Differential Diagnosis

Consider testing for other causes of mononucleosis-like illness when EBV tests are negative:

• Cytomegalovirus (CMV)
• Human Immunodeficiency Virus (HIV) 4

Practical Approach

1. Start with CBC with differential and Monospot test
2. If Monospot is positive → diagnosis confirmed
3. If Monospot is negative but clinical suspicion remains high (especially in children <10 years or early in illness) → proceed to EBV-specific antibody panel
4. Consider liver function tests to support diagnosis when heterophile test is negative

This algorithmic approach ensures accurate diagnosis while maintaining cost-effectiveness in most clinical scenarios 1, 2.