What are the diagnostic tests for mononucleosis?

Diagnostic Tests for Mononucleosis

Start with a complete blood count with differential and a rapid heterophile antibody test (Monospot), but proceed directly to EBV-specific antibody testing if the heterophile test is negative and clinical suspicion remains high, or if the patient is a child under 10 years old. 1, 2

Initial Laboratory Testing Approach

First-Line Tests

• Complete blood count with differential looking for:

  • Lymphocytosis ≥50% of white blood cell differential 1
  • Atypical lymphocytosis ≥10% of total lymphocyte count 1
  • The combination of >50% lymphocytes AND >10% atypical lymphocytes is highly specific (specificity 0.99, positive LR 54) 3

• Heterophile antibody test (Monospot) as the most widely used initial test 1

  • Sensitivity: 87%, Specificity: 91% 1, 4
  • Usually becomes positive between days 6-10 after symptom onset, peaking during weeks 2-3 1, 2
  • Critical limitation: False-negative rate of approximately 10% overall, especially common in children younger than 10 years and during the first week of illness 1, 2, 4

When to Proceed to EBV-Specific Antibody Testing

Do not rely solely on heterophile testing in the following situations - proceed directly to EBV-specific antibodies 2, 5:

• Children under 10 years of age 2, 5
• Patients in the first week of illness 5
• Negative heterophile test with persistent high clinical suspicion 1, 2
• Atypical or severe presentations requiring confirmation 5

EBV-Specific Antibody Panel

The recommended panel includes 1, 5:

• Viral Capsid Antigen (VCA) IgM antibodies - indicates acute/recent infection 2
• Viral Capsid Antigen (VCA) IgG antibodies - develops rapidly in acute infection 2
• Epstein-Barr Nuclear Antigen (EBNA) antibodies - critical for timing the infection 2

Interpreting EBV Antibody Results

• Primary acute EBV infection: VCA IgM positive (with or without VCA IgG) AND EBNA antibodies absent 1, 2, 5
• Past infection (>6 weeks prior): EBNA antibodies present with VCA IgG 1, 2
• Important context: Over 90% of normal adults have IgG antibodies to VCA and EBNA from past infection 1, 2
• EBNA antibodies develop 1-2 months after primary infection and persist for life 2

Common Pitfalls to Avoid

False-Positive Heterophile Results

May occur in patients with 1, 2, 5:

• Leukemia
• Pancreatic carcinoma
• Viral hepatitis
• CMV infection

False-Negative Heterophile Results

Common in 1, 2:

• Children younger than 10 years
• Early in the course of infection (first week)
• Approximately 10% false-negative rate overall

Testing Errors to Avoid

• Do not order EBV testing from throat swabs - EBV can persist in throat secretions for weeks to months after infection and does not confirm acute infection 2
• Do not skip EBV-specific testing in young children - heterophile tests are unreliable in this population 2, 5

Differential Diagnosis Testing

When heterophile and EBV testing are negative, consider testing for other causes of mononucleosis-like illness 1, 2, 5:

• Cytomegalovirus (CMV) infection
• HIV infection
• Toxoplasma gondii infection
• Adenovirus infection
• Streptococcal pharyngitis

Special Population Considerations

Immunocompromised Patients

Require more extensive testing 2, 5:

• Quantitative EBV viral load by nucleic acid amplification testing (NAAT) 2, 5
• Quantitative PCR showing >10^2.5 copies/mg DNA in peripheral blood mononuclear cells suggests chronic active EBV 5
• Post-transplant patients require EBV DNA surveillance due to high risk of lymphoproliferative disease 2

Supportive Laboratory Findings

• Elevated liver enzymes (transaminases) increase clinical suspicion for infectious mononucleosis when heterophile test is negative 6, 4