Does MDS Raise Blasts?
Yes, MDS can raise blast cells, and the blast percentage is a critical diagnostic and prognostic parameter that defines MDS subtypes and distinguishes MDS from acute myeloid leukemia. 1
Blast Percentage Defines MDS Subtypes
The bone marrow blast count is crucial for both diagnosis and prognosis in MDS, with specific thresholds determining disease classification 1:
Lower-Risk MDS (Blasts <5%)
- Refractory Cytopenia with Unilineage Dysplasia (RCUD): Bone marrow blasts <5% with dysplasia in one myeloid lineage 1
- Refractory Anemia with Ring Sideroblasts (RARS): Bone marrow blasts <5% with ≥15% ring sideroblasts 1
- Refractory Cytopenia with Multilineage Dysplasia (RCMD): Bone marrow blasts <5% with dysplasia in ≥2 myeloid lineages 1
Higher-Risk MDS (Blasts 5-19%)
- RAEB-1: Bone marrow blasts 5-9%, with or without peripheral blood blasts <5% 1
- RAEB-2: Bone marrow blasts 10-19%, or peripheral blood blasts 5-19%, with or without Auer rods 1
Critical Diagnostic Threshold
MDS is distinguished from AML by the 20% blast threshold: myeloid neoplasms with >20% blasts in peripheral blood or bone marrow are classified as AML, not MDS 1. This represents a key diagnostic boundary that determines treatment approach and prognosis.
Blast Count Methodology
When evaluating MDS, the blast count must include 1:
- Agranular blasts
- Myeloblasts
- Promonocytes
- Exclude promyelocytes from the blast count
The recommended cell count for accurate assessment is 200 cells for peripheral blood films and up to 500 cells for bone marrow 1.
Prognostic Significance
Bone marrow blast percentage is one of the three most critical prognostic factors in the International Prognostic Scoring System (IPSS), along with cytogenetics and number of cytopenias 1. The IPSS categorizes blast percentages as 1:
- <5%
- 5-10%
- 11-20%
- 21-30%
Higher blast percentages correlate with increased risk of AML transformation and shorter survival 1, 2.
Important Clinical Caveat
Some MDS patients have higher blast percentages in peripheral blood than bone marrow, which is associated with significantly shorter survival and more aggressive disease 3. If marrow blasts are <5% but peripheral blood blasts are 2-4%, the case should be classified as RAEB-1 1. Cases with 1% peripheral blood blasts and <5% marrow blasts should be classified as MDS-U 1.
Disease Progression
Approximately 25-40% of MDS patients develop acute leukemia, which is why MDS has historically been called "preleukemia" 4. The progression from lower-risk to higher-risk MDS involves accumulation of blast cells that have escaped apoptosis and acquired proliferative capacity 4.