What does a blast look like on a peripheral blood smear or bone‑marrow aspirate?

What Does a Blast Look Like?

Morphologic Definition

Blasts are immature hematopoietic cells characterized by a high nuclear-to-cytoplasmic ratio, easily visible nucleoli, fine nuclear chromatin, and variable cytoplasmic basophilia, with or without granules or Auer rods but no Golgi zone. 1

Key Identifying Features

Nuclear Characteristics

• Fine, delicate chromatin pattern that appears open and lacy, distinctly different from the coarse, clumped chromatin of mature cells 1
• Prominent nucleoli that are easily visible and well-defined 1
• Round to oval nuclear shape with smooth nuclear contours in most cases 1

Cytoplasmic Features

• High nuclear-to-cytoplasmic ratio meaning the nucleus occupies most of the cell with only a thin rim of cytoplasm 1
• Basophilic (blue) cytoplasm of variable intensity on Wright-Giemsa staining 1
• Absence of a Golgi zone (the pale perinuclear clearing seen in more mature cells like promyelocytes) 1
• Variable presence of azurophilic granules or Auer rods (crystallized primary granules pathognomonic for myeloid lineage) 1

Critical Distinctions from Blast Mimics

Common Pitfalls to Avoid

Proerythroblasts can be mistaken for blasts but typically have more intensely basophilic cytoplasm and may show early hemoglobinization 1

Lymphoid cells (particularly hematogones and reactive lymphocytes) may resemble blasts but usually have denser chromatin and less prominent nucleoli 1

Promyelocytes are NOT blasts despite being immature; they have a visible Golgi zone and abundant primary granulation 1

Practical Counting Guidelines

Peripheral Blood Assessment

• Count a minimum of 500 cells on the peripheral blood smear to ensure reliable blast percentage determination 2
• Normal individuals have <1% blasts in peripheral blood, with most showing 0% 3
• Any circulating blasts warrant immediate hematology consultation and bone marrow examination 2

Bone Marrow Assessment

• Count at least 500 nucleated cells in bone marrow aspirate smears, including at least 100 erythroblasts and 30 megakaryocytes 1
• Blasts in normal or reactive marrows are scattered or adjacent to bone trabeculae or blood vessels 1
• Leukemic blasts tend to form aggregates or clusters in abnormal central marrow cavity locations (ALIP - abnormally localized immature precursors) 1

Diagnostic Thresholds

Clinical Significance of Blast Percentages

• <1% peripheral blood blasts: Normal finding 3
• 2-4% peripheral blood blasts: Diagnostic for RAEB-1 (Refractory Anemia with Excess Blasts-1) when other MDS features present 2, 3
• 5-19% peripheral blood blasts: Diagnostic for RAEB-2 2
• ≥20% peripheral blood or bone marrow blasts: Diagnostic threshold for acute myeloid leukemia 2, 3

Ancillary Identification Methods

Immunohistochemistry

CD34 immunostaining is particularly valuable in hypocellular or fibrotic bone marrow specimens where morphologic assessment is inadequate 1, 2

Additional useful markers include CD117, myeloperoxidase, lysozyme, and CD68 for identifying and characterizing blasts in tissue sections 1

Flow Cytometry

Flow cytometry provides immunophenotypic characterization but morphologic blast percentage takes precedence over flow cytometry for prognostic stratification 2

Hemodilution of bone marrow specimens (particularly in fatty or fibrotic marrows) can artificially lower blast percentages by flow cytometry 1

Location and Distribution Patterns

In bone marrow biopsies, blasts forming aggregates of >5 cells or clusters of 3-5 cells in abnormal central locations (ALIP-positive) indicate aggressive MDS subtypes with poor prognosis and increased progression to acute leukemia 1

At least three such aggregates or clusters must be identified in a histology section to classify as ALIP-positive 1