MDS is different from other cancers in that it does not grow in one spot to form a tumor. Because of this, MDS has to be classified differently than many other types of cancers. Specialists have created an MDS classification system based on the different risk factors associated with the disease. There are two main systems currently used to categorize patients based on prognosis: the International Prognostic Scoring System- Revised (IPSS-R) and the WHO Prognostic Scoring System (WPSS).

IPSS-R 

Scores for the IPSS-R are broken into 5 categories:

• Very Low Risk
• Low Risk 
• Intermediate Risk
• High Risk 
• Very High Risk

Patients are categorized based on several factors: red blood cell count, white blood cell count, platelet count, percentage of blasts in the bone marrow, and the number and type of chromosome abnormalities.

WPSS

Scores for the WPSS are also broken into 5 categories:

• Very Low Risk
• Low Risk 
• Intermediate Risk
• High Risk 
• Very High Risk

Patients are categorized based on several factors: MDS type as classified by WHO (World Health Organization), whether or not the patient needs regular blood transfusions, and chromosomal abnormalities. 

Primary and Secondary MDS

MDS risk status can be classified in several ways. One way to classify a patient's risk status is as primary or secondary related to the cause of MDS. MDS is diagnosed as primary when we can’t find a cause and as secondary when we do know the cause. Primary MDS is the most common classification. You can also classify MDS by type.

MDS Types

The WHO has a system for classifying different types of MDS mainly differentiated by the appearance of the cells within the bone marrow. There are other factors that also help to differentiate between the different types of MDS: how many blasts (immature stem cells) look abnormal, how many types of cells have low counts, percentage of blasts in the bone marrow, how many blood cells are ring sideroblasts, and which chromosomes exhibit changes in the bone marrow. 

MDS With Multilineage Dysplasia (MDS-MLD)

This is the most common type of MDS. Someone with MDS-MLD has low counts of at least one blood type, less than 5% of stem cells in the bone marrow are immature stem cells (blasts), blasts are rarely found in the blood, and there is at least 10% of 2 or 3 cell types that look abnormal in the bone marrow. 

MDS With Single Lineage Dysplasia (MDS-SLD)

Someone with MDS-SLD has low counts of at least one blood type, less than 5% of stem cells in the bone marrow are blasts, blasts are rarely found in the blood, and there is at least 10% in only 1 cell type that looks abnormal in the bone marrow. 

MDS With Excess Blasts (MDS-EB)

Someone with MDS-EB has more blasts in their bone marrow and blood than normal and has low counts of at least one blood type. Cells may or may not appear abnormal. This type of MDS is the most likely to turn into AML.

There are two subtypes of MDS-EB. These subtypes are determined by how many blasts are present in the bone marrow or blood. 

MDS-EB1 

5-9% of the cells in the bone marrow are blasts or 2-4% in the blood.

MDS-EB2

10-19% of the cells in the bone marrow are blasts or 5-19% of cells in the blood.

MDS With Ring Sideroblasts (MDS-RS)

Someone with MDS-RS has ring sideroblasts for at least 15% of their immature red blood cells. If there is a mutation in the SF3B1 gene, this percentage is lower at only 5% needed for the diagnosis. This is a rare form of MDS and will rarely turn into AML. Patients with this type of MDS generally have better outcomes than other types of MDS.

There are two subtypes of MDS-RS. These subtypes are determined by how many types of cells in their bone marrow appear abnormal.

MDS-RS with Single Lineage Dysplasia (MDS-RS-SLD) 

Only one cell type appears abnormal

MDS-RS- with Multilineage Dysplasia (MDS-RS-MLD)

More than one cell type appears abnormal

MDS With Isolated del(5q)

Someone with this type of MDS is missing part of chromosome 5 in their bone marrow cells, low counts of 1 or 2 blood cell types, and at least 1 cell type that appears abnormal. This is a rare form of MDS as it is most commonly diagnosed in older women, who make up only a small portion of MDS patients. This type rarely turns into AML and generally has better outcomes than other types of MDS.

MDS, Unclassifiable (MDS-U)

Someone with this type of MDS doesn’t fit in any other MDS type. This type is uncommon, and isn’t very well understood yet.