The WHO Classification of MDS

Previously, the World Health Organization (WHO) used a risk-based grouping system for the classification of MDS based on blast percentage, ring sideroblasts, and number of lineages with dysplasia. The current WHO classification groups MDS entities as those having defining genetic abnormalities and those that are morphologically defined. See Table 1 for an overview of the classification and defining features of MDS.²

Table 1. Classification and Defining Features of MDS.

	Blasts	Cytogenetics	Mutations
MDS with defining genetic abnormalities
MDS with low blasts and isolated 5q deletion (MDS-5q)	<5% BM and<2% PB	5q deletion alone, or with 1 other abnormality other than monosomy 7 or 7q deletion
MDS with low blasts and SF3B1 mutationa (MDS-SF3B1)	<5% BM and<2% PB	Absence of 5q deletion, monosomy 7, or complex karyotype	SF3B1
MDS with biallelic TP53 inactivation (MDS-biTP53)	<20% BM and PB	Usually complex	Two or more TP53 mutations, or 1 mutation with evidence of TP53 copy number loss or cnLOH
MDS, morphologically defined
MDS with low blasts (MDS-LB)	<5% BM and<2% PB
MDS, hypoplasticb (MDS-h)	<5% BM and<2% PB
MDS with increased blasts (MDS-IB)
MDS-IB1	5–9% BM or 2–4% PB
MDS-IB2	10–19% BM or 5–19% PB or Auer rods
MDS with fibrosis (MDS-f)	5–19% BM; 2–19% PB

^aDetection of ≥15% ring sideroblasts may substitute for SF3B1 mutation. Acceptable related terminology: MDS with low blasts and ring sideroblasts.
^bBy definition, ≤25% bone marrow cellularity, age adjusted.
BM bone marrow, PB peripheral blood, cnLOH copy neutral loss of heterozygosity.