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WHO Classification

World Health Organization Classification System

The WHO system for classifying MDS is a refinement of the earlier French-American-British (FAB) system and includes the mandatory inclusion of cytogenetics in the diagnostic work-up. Other primary differences between the WHO and FAB systems are:1

  • Morphological reclassification1
    • The FAB categories of refractory anemia (RA) and refractory anemia with ringed sideroblasts (RARS) are more precisely defined in the WHO system, and the FAB category of refractory anemia with excess blasts (RAEB) is broken down into 2 subtypes, depending on BM blast percentage.
  • Redefinition of blast excess levels1
    • The threshold of BM blast percentage for a diagnosis of AML was reduced from 30% in the FAB system to 20% in the WHO system, which resulted in the elimination of the FAB category of refractory anemia with excess blasts in transformation (RAEB-t).

Additionally, the WHO system reclassifies chronic myelomonocytic leukemia (CMML) as a myelodysplastic/myeloproliferative disease and adds 2 new categories: unclassified MDS (MDS-U) and MDS with 5q- syndrome.1

MDS SubtypeBlood FindingsBone Marrow Findings
RCUD (Refractory Cytopenia with Unilineage Dysplasia)2 One low blood cell count (cytopenia) has
3 subtypes:
  • low red cell counts is called refractory anemia or RA
  • low white cell count is called refractory neutropenia or RN
  • low platelet count is called refractory neutropenia or RN
Less than 5% young white blood cells
called blasts

Little or no dysplasia
RARS (Refractory Anemia with Ring Sideroblasts)2 Low red blood cell count that cannot be treated with iron or vitamins More than 15% red blood cells that contain ring-shaped iron deposits called ringed sideroblasts
RCMD (Refractory Cytopenia with Multilineage Dysplasia)2 Low white blood cell count (neutropenia) or low platelet count (thrombocytopenia)

Dysplasia in more than 1 cell type

Less than 5% blasts, or less than 15% ringed sideroblasts

Note: In patients with more than 15% ringed sideroblasts, the subtype is called RCMD-RS

RAEB-1 (Refractory Anemia with Excess Blasts)2 The same as refractory anemia 5% to 9% blasts in marrow

Normal number of blasts in blood
RAEB-2 (Refractory Anemia with Excess Blasts 2)2 Similar to refractory anemia and RAEB-1 10% to 19% blasts in marrow
MDS with isolated deletion 5q2 The same as refractory anemia, plus normal or high platelet count Deletion of chromosome 5q, with no other chromosome abnormality
Unclassified MDS2 Low count for either platelets or white
blood cells

Unusual features, such as scarring (fibrosis) of the bone marrow

Note: Just 1% to 2% of MDS patients have this subtype

EXJADE reduces LIC and serum ferritin in transfused patients with lower-risk myelodyspastic syndrome.

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