Diagnosis and Evaluation of Blasts in Hematological Disorders
Definition and Classification of Blasts
- The World Health Organization (WHO) defines blasts as immature hematopoietic precursor cells that are normally not found in peripheral blood or appear in minimal quantities (<1%) 1
- The identification and quantification of blasts are performed through visual inspection of peripheral blood or bone marrow smears with Wright-Giemsa staining or similar 3, 4
- Blasts include myeloblasts, monoblasts, promonocytes, and megakaryoblasts (but not dysplastic megakaryocytes) 3
- In acute promyelocytic leukemia (APL), abnormal promyelocytes are counted as equivalent to blasts 3
Diagnostic Significance of Blast Percentage
- 1-4% blasts in peripheral blood: Suggests myelodysplastic syndrome (MDS) or reactive process 5
- 5-19% blasts in peripheral blood: Compatible with refractory anemia with excess blasts type 2 (RAEB-2) 5
- ≥20% blasts in peripheral blood or bone marrow: Diagnostic of acute leukemia (myeloid or lymphoid) 2
- <5% blasts in bone marrow: Normal or low-grade MDS 5
- 5-9% blasts in bone marrow: Refractory anemia with excess blasts type 1 (RAEB-1) 5
- 10-19% blasts in bone marrow: Refractory anemia with excess blasts type 2 (RAEB-2) 5
Characterization of Blasts
- Morphological characterization should be complemented with:
Important Considerations
- The presence of blast aggregates or clusters (ALIP) in bone marrow is associated with aggressive subtypes of MDS and higher risk of progression to acute leukemia 1
- The reliability of blast counting depends on the number of cells counted (a differential count of 500 cells is recommended) 1
- CD34 is a useful marker for identifying blasts, but not all blasts express it 3
- Flow cytometry evaluation should not replace morphological inspection 1