What are the laboratory findings characteristic of a mono flare?

Laboratory Findings in a Mononucleosis Flare

A mononucleosis flare is characterized by an elevated white blood cell count with an increased percentage of atypical lymphocytes, which is the hallmark laboratory finding of EBV-associated mononucleosis. 1

Key Laboratory Findings

• Complete Blood Count (CBC) abnormalities:

  • Leukocytosis with lymphocytes making up at least 50% of the white blood cell differential count 2, 3
  • Atypical lymphocytes constituting more than 10% of the total lymphocyte count (the most characteristic finding) 1, 4
  • Mild anemia may be present in some cases 5

• Serologic markers:

  • Positive heterophile antibody test (Monospot test) - the classic diagnostic test for infectious mononucleosis 1, 6
  • Positive EBV-specific antibodies:
    • Presence of VCA IgM (with or without VCA IgG) antibodies in the absence of EBNA antibodies indicates recent primary EBV infection 1
    • EBNA antibodies develop 1-2 months after primary infection and remain detectable for life 1

• Liver function tests:

  • Elevated hepatic transaminases (AST/ALT) are common 3
  • These elevations typically resolve as the infection resolves 7

Diagnostic Algorithm

1. Initial laboratory evaluation:

   • CBC with differential to assess for lymphocytosis and atypical lymphocytes 5, 3
   • Heterophile antibody test (Monospot) 1

2. If Monospot is negative but clinical suspicion remains high:

   • EBV-specific antibody testing for IgG and IgM to viral capsid antigen (VCA) and antibodies to Epstein-Barr nuclear antigen (EBNA) 1
   • Liver function tests to check for elevated transaminases 3

3. Interpretation of EBV-specific antibodies:

   • VCA IgM positive, EBNA negative = recent primary infection 1
   • VCA IgM negative, VCA IgG positive, EBNA positive = past infection 1

Important Clinical Considerations

• False-negative heterophile antibody tests occur in approximately 10% of patients, especially in children younger than 10 years and during the first week of illness 1, 3

• When evaluating a potential mono flare, it's important to consider other causes of mononucleosis-like illness such as CMV, adenovirus, HIV, and Toxoplasma gondii 1

• In immunocompromised patients, monitoring EBV viral load may be necessary as they are at risk for EBV-associated lymphoproliferative disease 1, 5

Potential Complications to Monitor

• Splenic rupture is a rare but potentially life-threatening complication (0.1-0.5% of cases) 2

• Hematologic complications may include hemolytic anemia, thrombocytopenia, and rarely, conditions resembling hemophagocytic syndromes 1

• Neurologic, cardiac, and respiratory complications can occur and may require additional laboratory testing 2

By recognizing these characteristic laboratory findings, clinicians can accurately diagnose mononucleosis flares and distinguish them from other conditions with similar presentations.