What are the initial steps in testing for infectious mononucleosis (mono)?

Testing for Infectious Mononucleosis

The initial step in testing for infectious mononucleosis should be the heterophile antibody test (Monospot), followed by EBV-specific antibody testing if the Monospot is negative but clinical suspicion remains high. 1

Initial Diagnostic Approach

Step 1: Heterophile Antibody Test (Monospot)

• Recommended as the first-line test by the American Academy of Pediatrics 1
• Widely available, rapid, and cost-effective
• Important limitations to be aware of:
  • False-negative results in approximately 10% of patients, especially common in children younger than 10 years 2
  • False-positive results may occur in patients with leukemia, pancreatic carcinoma, viral hepatitis, and CMV infection 2

Step 2: Complete Blood Count with Differential

• Look for characteristic findings:
  • Elevated white blood cell count
  • Lymphocytosis (lymphocytes ≥50% of white blood cells)
  • Atypical lymphocytes >10% of total lymphocyte count 3
  • These findings strongly support the diagnosis of infectious mononucleosis

When Monospot is Negative but Clinical Suspicion Remains High

Step 3: EBV-Specific Antibody Testing

• Recommended by the Infectious Diseases Society of America when Monospot is negative but clinical suspicion remains high 1
• Panel should include:
  • Viral Capsid Antigen (VCA) IgM
  • Viral Capsid Antigen (VCA) IgG
  • Epstein-Barr Nuclear Antigen (EBNA) IgG

Interpretation of EBV Antibody Results

• Primary (acute) EBV infection: Positive VCA IgM and VCA IgG with negative EBNA IgG 2, 1
• Past infection (>6 weeks): Positive EBNA IgG (with or without VCA IgG) 2, 1
• No prior EBV exposure: All antibodies negative

Clinical Considerations

Clinical Features Supporting Diagnosis

• Classic triad: fever, tonsillar pharyngitis, and lymphadenopathy 3
• Posterior cervical or auricular adenopathy is particularly suggestive 4
• Periorbital/palpebral edema (occurs in one-third of patients) 3
• Splenomegaly (occurs in approximately 50% of cases) 3
• Palatal petechiae 4
• Maculopapular rash (10-45% of cases, more common if ampicillin is given) 3

Common Pitfalls to Avoid

1. Relying solely on Monospot in children: The heterophile antibody test has lower sensitivity in children under 10 years; proceed directly to EBV-specific antibody testing if clinical suspicion is high 2, 5

2. Testing too early: Heterophile antibodies typically become detectable between the sixth and tenth day after symptom onset 2; testing too early may yield false-negative results

3. Misinterpreting negative results: When both heterophile test and initial EBV serology are negative but clinical suspicion remains high, consider:

   • Testing for other causes of mononucleosis-like illness (CMV, adenovirus, toxoplasmosis) 2
   • Repeating EBV serology in 1-2 weeks as antibodies may not have developed yet 5

4. Overlooking special populations: Immunocompromised patients may have atypical presentations and serologic responses, requiring more comprehensive testing including EBV viral load by PCR 1

By following this systematic approach to testing for infectious mononucleosis, clinicians can achieve accurate diagnosis while minimizing unnecessary testing and avoiding diagnostic delays that could impact patient management.