What causes myelocytes to be present in the blood?

Causes of Myelocytes in Peripheral Blood

The presence of myelocytes in peripheral blood most commonly indicates either a myeloproliferative neoplasm, myelodysplastic syndrome, or a reactive process such as infection or inflammation causing a "left shift" in granulocyte maturation. 1

Pathological Causes

Myeloid Neoplasms

1. Chronic Myelogenous Leukemia (CML)

   • Most characteristic finding: increased granulocytes with a peak in myelocytes
   • Associated with Philadelphia chromosome t(9;22) and BCR-ABL fusion gene
   • Typically presents with markedly elevated WBC (>25,000/μL)
   • Other findings include increased basophils and absence of morphologic dysplasia 1, 2

2. Myelodysplastic Syndromes (MDS)

   • Characterized by peripheral cytopenias and dysplastic changes
   • Myelocytes may appear as part of abnormal myeloid maturation
   • Associated with various cytogenetic abnormalities
   • Bone marrow shows dysplasia in ≥10% of cells in one or more myeloid lineages 1

3. Chronic Myelomonocytic Leukemia (CMML)

   • Features of both MDS and myeloproliferative neoplasms
   • Persistent monocytosis >1 × 10^9/L
   • May show myelocytes in peripheral blood as part of abnormal myeloid maturation 1

4. Acute Myeloid Leukemia (AML)

   • Particularly during early stages or transformation from MDS
   • Associated with ≥20% blasts in bone marrow or peripheral blood
   • Myelocytes may be seen alongside blasts and promyelocytes 1

Non-Neoplastic Causes

Reactive Processes

1. Infections

   • Severe bacterial infections trigger increased granulocyte production
   • "Left shift" occurs when bone marrow releases immature granulocytes (including myelocytes)
   • COVID-19 infection has been associated with increased immature granulocytes 3

2. Inflammatory Conditions

   • Severe inflammation can trigger bone marrow to release immature granulocytes
   • Systemic inflammatory response syndrome (SIRS)
   • Severe acute pancreatitis 3

3. Bone Marrow Recovery

   • Following chemotherapy or bone marrow transplantation
   • During recovery from bone marrow suppression

4. Myelofibrosis

   • Can be primary or secondary to other conditions
   • Fibrosis in bone marrow forces immature cells into peripheral circulation 4

Diagnostic Approach

When myelocytes are found in peripheral blood, the following evaluations should be performed:

1. Complete blood count with differential

   • Assess for other abnormalities (cytopenias, leukocytosis, other immature cells)
   • Look for characteristic patterns (e.g., basophilia in CML) 1

2. Peripheral blood smear examination

   • Evaluate for dysplastic changes
   • Assess for blasts and other immature cells
   • Look for specific morphologic features of myelocytes 1

3. Bone marrow aspiration and biopsy

   • Essential for diagnosis of myeloid neoplasms
   • Assess cellularity, blast percentage, and dysplasia
   • Perform iron staining to evaluate for ring sideroblasts 1

4. Cytogenetic analysis

   • Detect chromosomal abnormalities (e.g., Philadelphia chromosome)
   • Important for diagnosis and prognosis 1

5. Molecular studies

   • BCR-ABL testing for CML
   • Mutation analysis for MDS and other myeloid neoplasms 1

Clinical Significance

The presence of myelocytes in peripheral blood is never normal and always warrants further investigation. In the context of myeloid neoplasms, it may indicate disease progression or transformation. In reactive conditions, the number of myelocytes typically correlates with the severity of the underlying process.

In some cases, such as in acute lymphoblastic leukemia, the presence of a left shift (including myelocytes) at diagnosis has been associated with better prognosis and longer remission duration, possibly reflecting the body's immune response to the leukemia 5.