What labs are recommended to assess for mononucleosis (mono) in a pediatric patient?

Laboratory Assessment for Mononucleosis in Pediatric Patients

Start with a complete blood count with differential and a rapid heterophile antibody test (Monospot) as the initial cost-effective laboratory evaluation for suspected infectious mononucleosis in children. 1, 2

Initial Laboratory Testing Algorithm

First-Line Tests

• Complete Blood Count (CBC) with Differential: Look for >40% lymphocytes and >10% atypical lymphocytes, which are common findings in EBV-associated mononucleosis 1, 2

  • An elevated white blood cell count with increased percentage of atypical lymphocytes supports the diagnosis 1

• Rapid Heterophile Antibody Test (Monospot): This is the classic, cost-effective initial test with 87% sensitivity and 91% specificity in adolescents 2, 3

  • Heterophile antibodies become detectable between days 6-10 after symptom onset and peak during weeks 2-3 of illness 1

Critical Age-Related Caveat

The heterophile antibody test has significantly reduced sensitivity in children younger than 10 years, with false-negative results occurring in approximately 10% of all patients but especially common in young children. 1, 3 The test has low sensitivity and low negative predictive value in children under age 4 and is not useful in this age group 3.

When Heterophile Test is Negative

If the Monospot/heterophile test is negative but clinical suspicion remains high, proceed with:

• EBV-Specific Antibody Testing including:
  • VCA IgM (viral capsid antigen IgM)
  • VCA IgG (viral capsid antigen IgG)
  • EBNA (Epstein-Barr nuclear antigen) 1, 4

Interpretation of EBV Antibody Patterns

• Acute Primary EBV Infection: VCA IgM positive (with or without VCA IgG) AND EBNA antibodies absent 1, 5, 6

  • This pattern indicates recent primary infection with EBV 1

• Past Infection (Not Acute): EBNA antibodies present, indicating infection occurred more than 6 weeks prior and EBV is not the likely cause of current symptoms 1, 6

  • EBNA antibodies develop 1-2 months after primary infection and persist for life 1

Additional Considerations

• Liver Function Tests: The presence of elevated liver enzymes increases clinical suspicion for infectious mononucleosis when the heterophile antibody test is negative 2

• False Positives: Heterophile antibody tests may show false-positive results in patients with leukemia, pancreatic carcinoma, viral hepatitis, CMV infection, autoimmune disease, and haematological malignancies 1, 3

• Alternative Diagnoses: When heterophile testing is negative, consider testing to differentiate from mononucleosis-like illnesses caused by CMV, adenovirus, HIV, or Toxoplasma gondii 1

Practical Testing Strategy by Age

• Children <4 years: Skip heterophile testing and proceed directly to EBV-specific serology (VCA IgM, VCA IgG, EBNA) 3

• Children 4-10 years: Perform heterophile testing but maintain low threshold for EBV-specific serology if negative 1, 3

• Adolescents and young adults: Heterophile testing is highly reliable as first-line test 2, 3