## Distinguishing MDS from Aplastic Anemia ### Key Morphologic Discriminator **Key Point:** Dysplastic changes in bone marrow morphology are the hallmark feature that distinguishes MDS from aplastic anemia. Dysplasia refers to abnormal maturation and morphology of hematopoietic cells — including nuclear abnormalities (irregular nuclear membranes, coarse chromatin, abnormal mitotic figures) and cytoplasmic abnormalities (abnormal granulation, Auer rods in blasts). ### Comparative Features | Feature | MDS | Aplastic Anemia | |---------|-----|------------------| | **Bone marrow cellularity** | Hypercellular or normocellular (early MDS) | Hypocellular (<25% cellularity) | | **Dysplastic changes** | **Present** (defining feature) | **Absent** | | **Blast percentage** | 5–19% (by definition) | <5% | | **Cytogenetics** | Abnormal in ~50% (e.g., trisomy 8, del 5q) | Normal | | **Prognosis** | Risk of AML transformation | Depends on severity; may recover | ### Why Dysplasia Is the Key Discriminator **High-Yield:** The WHO classification of MDS is built on the presence of **dysplastic changes** in ≥10% of cells in at least one cell line, combined with cytopenias. Aplastic anemia, by contrast, shows a **hypocellular marrow with normal morphology** of the cells that are present — there is no dysplasia, only reduced production. **Clinical Pearl:** A hypercellular marrow can occur in both MDS (early, or in certain subtypes) and in other conditions like immune-mediated marrow failure; however, the presence of dysplasia narrows the diagnosis to MDS and excludes aplastic anemia. ### Mnemonic: DYSPLASIA = MDS **D**ysplastic changes **Y**ellow marrow (hypocellular) = Aplastic anemia **S**pecific karyotype abnormalities **P**rogenitor maturation defect **L**ow blast count (5–19%) **A**bnormal morphology (not aplastic) **S**econdary AML risk **I**ncreased apoptosis **A**bnormal iron metabolism [cite:Harrison 21e Ch 110]
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