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Myelodysplastic syndromes (MDS) are classified using the World Health Organization (WHO) classification system, which was most recently updated in 2016. It divides MDS into types based mainly on how the cells in the bone marrow look under the microscope, as well as some other factors:

• How many early forms of cell types in the bone marrow (red blood cells, white blood cells, or platelets) show dysplasia (look abnormal under the microscope).
• How many types of low blood cell counts (cytopenias) a person has. 
• What portion of early red blood cells are ring sideroblasts (cells that contain rings of iron deposits around the nucleus).
• The portion of blasts (very early forms of blood cells) in the bone marrow or blood.
• Certain chromosome changes in the bone marrow cells. 

Based on these factors, the WHO system recognizes 6 main types of MDS:

• MDS with multilineage dysplasia (MDS-MLD)
• MDS with single lineage dysplasia (MDS-SLD)
• MDS with ring sideroblasts (MDS-RS)
• MDS with excess blasts (MDS-EB)
• MDS with isolated del(5q)
• MDS, unclassifiable (MDS-U)

Because small differences in the way the cells look can change the diagnosis, doctors may sometimes disagree on a patient’s exact type of MDS.

MDS with multilineage dysplasia (MDS-MLD)

In MDS-MLD:

• Dysplasia is seen in at least 10% of the early cells of 2 or 3 cell types (red blood cells, white blood cells, and/or megakaryocytes [the cells that make platelets]) in the bone marrow. 
• The person has low numbers of at least 1 type of blood cell. 
• There is a normal number (less than 5%) of very early cells called blasts in the bone marrow, and blasts are rare (or absent) in the blood.

This is the most common type of MDS. In the past, it was referred to as refractory cytopenia with multilineage dysplasia (RCMD).

MDS with single lineage dysplasia (MDS-SLD)

In MDS-SLD: 

• Dysplasia is seen in at least 10% of the early cells of 1 cell type (either red blood cells, white blood cells, or megakaryocytes [the cells that make platelets]) in the bone marrow. 
• The person has low numbers of 1 or 2 types of blood cells, but normal numbers of the other type(s). 
• There is a normal number (less than 5%) of very early cells called blasts in the bone marrow, and blasts are rare (or absent) in the blood.

This type of MDS is not common. It seldom, if ever, progresses to acute myeloid leukemia (AML). Patients with this type of MDS can often live a long time, even without treatment.

This was referred to as refractory cytopenia with unilineage dysplasia (RCUD) in the previous classification system. It includes refractory anemia (RA), refractory neutropenia (RN), and refractory thrombocytopenia (RT), depending on which cell type is affected.

MDS with ring sideroblasts (MDS-RS)

In this type of MDS, many of the early red blood cells are ring sideroblasts. For this diagnosis, at least 15% of the early red blood cells must be ring sideroblasts (or at least 5% if the cells also have a mutation in the SF3B1 gene).

This condition is further divided into 2 types, based on how many of the cell types in the bone marrow are affected by dysplasia:

• MDS-RS with single lineage dysplasia (MDS-RS-SLD): dysplasia in only one cell type 
• MDS-RS with multilineage dysplasia (MDS-RS-MLD): dysplasia in more than one cell type

This type of MDS is not common. It rarely turns into AML, and the outcome for people with this type is generally better than for some other types of MDS. This was previously referred to as refractory anemia with ring sideroblasts (RARS).

MDS with excess blasts (MDS-EB)

In this type of MDS, there are more blasts than normal in the bone marrow and/or blood. The person also has low numbers of at least one type of blood cell. There may or may not be severe dysplasia in the bone marrow.

This condition is further divided into 2 types, based on how many of the cells in the bone marrow or blood are blasts:

• MDS-EB1: blasts make up 5% to 9% of the cells in the bone marrow, or 2% to 4% of the cells in the blood 
• MDS-EB2: blasts make up 10% to 19% of the cells in the bone marrow, or 5% to 19% of the cells in the blood

This type accounts for about 1 in 4 cases of MDS. It is one of the types most likely to turn into AML, with the risk being higher for MDS-EB2 than for MDS-EB1. This was previously referred to as refractory anemia with excess blasts (RAEB).

MDS with isolated del(5q)

In this type of MDS, the chromosomes of the bone marrow cells are missing part of chromosome number 5. (There may also be one other chromosome abnormality, as long as it isn’t a loss of part or all of chromosome 7.) The person also has low numbers of 1 or 2 types of blood cells (usually red blood cells), and there is dysplasia in at least 1 cell type in the bone marrow.

This type of MDS is not common. It occurs most often in older women. For reasons that aren’t clear, patients with this type of MDS tend to have a good prognosis (outlook). They often live a long time and rarely go on to develop AML.

MDS, unclassifiable (MDS-U)

This type of MDS is uncommon. For MDS-U, the findings in the blood and bone marrow don’t fit any other type of MDS. For example, the numbers of any one of the cell types may be low in the blood, but less than 10% of that type of cell looks abnormal in the bone marrow. Or the cells in the bone marrow have at least one certain chromosome abnormality that is only seen in MDS or leukemia.

This type is rare, so it has not been studied well enough to predict prognosis (outlook).

Clinical classification of MDS

Along with the WHO classification, MDS can also be classified based on the underlying cause. This is known as a clinical classification.

• If no cause can be identified, it's called primary MDS. (This type is more common.)
• When the cause of the disease is known, it's called secondary MDS.

Secondary MDS is often related to prior cancer treatment, or it develops in someone who already had a different bone marrow disease. This is discussed further in Risk Factors for Myelodysplastic Syndrome.

Identifying MDS as primary or secondary is important because the secondary type is much less likely to respond to treatment.