Can infectious mononucleosis (mono) cause the presence of myelocytes in the peripheral blood?

Infectious Mononucleosis and Myelocytes in Peripheral Blood

Infectious mononucleosis (mono) typically does not cause the presence of myelocytes in peripheral blood, but in rare severe cases with hematological complications, immature granulocytes including myelocytes may appear in circulation. 1

Typical Hematological Findings in Mono

The classic hematological findings in infectious mononucleosis include:

• Elevated white blood cell count with lymphocytosis
• Increased percentage of atypical lymphocytes (>10%)
• Heterophile antibodies (detectable between 6-10 days after symptom onset) 1
• Mild neutropenia (which is considered a normal finding during acute disease) 2

Severe Hematological Complications

In rare cases, infectious mononucleosis can cause more severe hematological abnormalities:

1. Agranulocytosis/Severe Neutropenia: Extremely rare but documented complication that can lead to left shift with immature granulocytes in circulation 2

2. Hemophagocytic Syndrome: A potentially fatal complication characterized by:

   • Peripheral leukopenia and thrombocytopenia
   • Increased histiocytes with hemophagocytosis in bone marrow
   • Can lead to abnormal cell populations in peripheral blood 3

3. Bone Marrow Response: In severe cases with hematological complications, the bone marrow may respond with hyperplasia of granulocyte progenitor cells, which can lead to release of immature forms including myelocytes 2

Diagnostic Considerations

When myelocytes are found in peripheral blood during suspected mono:

• Rule out other causes: Myelocytes in peripheral blood are more commonly associated with myeloproliferative disorders, leukemias, or severe infections rather than typical mono 1

• Consider bone marrow examination: If persistent or significant numbers of myelocytes are present, bone marrow aspiration may be warranted to exclude myelodysplastic syndrome or other myeloid neoplasms 1, 4

• Complete diagnostic workup: Include EBV-specific antibody testing (VCA IgM, VCA IgG, EBNA) rather than relying solely on heterophile antibody tests, which can be negative in approximately 10% of patients 1, 5

Clinical Implications

The presence of myelocytes in peripheral blood during mono should prompt:

• More careful monitoring of hematological parameters
• Consideration of severe complications like virus-associated hemophagocytic syndrome
• Evaluation for possible concurrent hematological disorders

Pitfalls to Avoid

• Don't assume myelocytes are a typical finding in uncomplicated mono - they suggest a more severe or complicated course
• Don't overlook the possibility of other infections or conditions that can mimic mono but cause more severe hematological abnormalities
• Remember that approximately 10% of clinical mono syndromes are not caused by EBV but by other pathogens like CMV, which may have different hematological manifestations 6, 7

The appearance of myelocytes in peripheral blood during infectious mononucleosis should be considered an unusual finding warranting further investigation rather than an expected feature of the disease.