What is the diagnostic workup for Infectious Mononucleosis (IM)?

Diagnostic Workup for Infectious Mononucleosis (IM)

The diagnostic workup for Infectious Mononucleosis should include a complete blood count with differential, heterophile antibody testing, and specific Epstein-Barr virus (EBV) antibody tests when clinically indicated.

Initial Laboratory Testing

Complete Blood Count with Differential

• Look for:
  • Lymphocytosis (>40% lymphocytes)
  • Atypical lymphocytes (>10%)
  • Peripheral blood smear may show Rouleaux formation (red cells appearing as stacks of coins) 1

Heterophile Antibody Testing

• Rapid "spot" test (Monospot)
  • Sensitivity: 87%, Specificity: 91%
  • Important limitations:
    • May be negative in the first week of illness
    • Often negative in children younger than 5 years 1
    • False negatives can occur early in the disease course 2

Additional Testing When Indicated

Liver Function Tests

• Elevated liver enzymes increase clinical suspicion for IM, especially when heterophile antibody test is negative 1

Specific EBV Antibody Panel

Indicated when:

• Heterophile test is negative but clinical suspicion remains high
• Atypical or severe clinical presentation
• Immunocompromised patient
• Prolonged symptoms

The EBV antibody panel includes:

1. IgM antibodies to Viral Capsid Antigen (VCA-IgM)

   • Present during acute infection
   • Most valuable serologic finding for diagnosing acute primary EBV infection 3

2. IgG antibodies to Viral Capsid Antigen (VCA-IgG)

   • Appears early in infection
   • Persists indefinitely

3. Antibodies to Early Antigen (EA)

   • Present during acute infection
   • Usually disappears within a few months

4. Antibodies to Epstein-Barr Nuclear Antigen (EBNA)

   • Absent during acute infection
   • Appears 6-12 weeks after onset
   • Persists indefinitely

Diagnostic interpretation:

• Acute IM: Positive VCA-IgM, positive VCA-IgG, positive EA, negative EBNA 4
• Past infection: Positive VCA-IgG, negative VCA-IgM, negative EA, positive EBNA

Testing for Alternative Diagnoses

When IM is suspected but EBV tests are negative, consider testing for other causes of mononucleosis-like illness:

• Cytomegalovirus (CMV)
• HIV
• Human Herpesvirus 6 (HHV-6)
• Toxoplasma gondii
• Adenovirus
• Streptococcus pyogenes (bacterial pharyngitis) 2

Special Considerations

Immunocompromised Patients

• More extensive workup may be needed
• Higher risk for severe disease and complications 1

When to Consider Advanced Testing

• Persistent symptoms beyond 4 weeks
• Severe symptoms (significant hepatosplenomegaly, extreme fatigue)
• Neurological symptoms
• Hematological abnormalities (severe thrombocytopenia, hemolytic anemia)

Common Pitfalls to Avoid

1. Relying solely on heterophile antibody testing - Can be negative early in disease course or in young children
2. Misdiagnosing bacterial pharyngitis as IM - Important to distinguish to avoid unnecessary antibiotic use 5
3. Failing to recognize potential complications - Such as splenic rupture, which is the most common potentially fatal complication 3
4. Overlooking alternative diagnoses - When typical features are absent or laboratory findings are inconsistent

Diagnostic Algorithm

1. Patient presents with fever, pharyngitis, and lymphadenopathy
2. Order CBC with differential and heterophile antibody test
3. If heterophile positive + typical CBC findings → Diagnose IM
4. If heterophile negative but strong clinical suspicion:
   • Check liver enzymes
   • Order specific EBV antibody panel
   • Consider testing for alternative causes
5. If all EBV tests negative → Evaluate for other mononucleosis-like illnesses

By following this systematic approach, clinicians can accurately diagnose infectious mononucleosis and distinguish it from other conditions requiring specific therapy.