Distinguishing Aplastic Anemia from Myelodysplastic Syndromes

Key Diagnostic Features

• Aplastic anemia and Myelodysplastic Syndromes (MDS) are distinct entities that require bone marrow biopsy for accurate diagnosis, with critical differentiating features being the presence of dysplasia, blast percentage, cytogenetic abnormalities, and abnormal localization of immature precursors (ALIP) in MDS versus their absence in aplastic anemia 1, 2
• The European Hematology Association recommends that age correction is mandatory when assessing bone marrow cellularity—failure to correct dramatically overestimates hypocellular cases 2
• The International Working Group for MDS recommends a 500-cell differential count for accurate blast assessment—a 100-cell count has unacceptably wide confidence intervals 2

Morphologic Criteria

• The World Health Organization (WHO) classification system states that aplastic anemia is characterized by a blast percentage below 1%, no moderate-to-severe erythroid dysplasia, no abnormal sideroblasts, and no ALIP 2, 3
• The WHO classification system also states that Hypocellular MDS (H-MDS) presents with moderate to severe erythroid dysplasia, abnormal sideroblasts, blast percentage of 5% or higher, and presence of ALIP 2

Cytogenetic Abnormalities

• The American College of Medical Genetics recommends that standard cytogenetics/interphase FISH is essential to rule out chromosomal abnormalities in aplastic anemia and MDS 1, 3
• The National Comprehensive Cancer Network (NCCN) guidelines state that specific clonal chromosomal abnormalities can establish diagnosis of MDS even when morphological features are inconclusive 4

Clinical Implications and Prognosis

• The NCCN guidelines state that MDS carries a much greater risk of progression to acute leukemia compared to aplastic anemia, making accurate diagnosis essential for treatment planning 2
• The American Society of Hematology recommends that aplastic anemia be treated with immunosuppressive therapy (horse ATG plus cyclosporine) or hematopoietic stem cell transplantation for young patients with HLA-identical sibling donors 5
• The European Society for Medical Oncology (ESMO) guidelines recommend that H-MDS may be offered immunomodulatory treatment (similar to aplastic anemia) with limited success, and hypomethylating agents (5-azacytidine, decitabine) are treatment options 6

Critical Diagnostic Pitfalls

• The WHO recommends that previous genotoxic exposure or therapy must be excluded as chemotherapy, benzene derivatives, and ionizing radiation can cause hypocellular marrows resembling aplastic anemia 2, 3
• The International Society for Laboratory Hematology recommends that review by at least two observers is essential when diagnostic doubt exists, particularly for H-AML cases 2
• The American Society of Clinical Oncology (ASCO) recommends that bone marrow biopsy is critical and necessary to diagnose these variants in all patients, including children, due to paucicellular aspirates typically obtained in severe marrow hypocellularity 1
• The NCCN guidelines state that careful follow-up investigations (morphology and karyotyping) at regular intervals of several months are recommended when morphological and cytogenetic findings are inconclusive 4