What are the types of MDS?

Before 1976, the term "MDS" didn't exist. Doctors and scientists used other names for what we now call MDS. At that time, there was no standard way to split MDS into subtypes.

In 1976, scientists came out with the first system for classifying MDS into subtypes. This system is called the French-American-British or FAB system. It is based on how blood and bone marrow cells look.

In 1997, the International Prognostic Scoring System or IPSS was launched. This system turns patient data into a score. The score tells how quickly an MDS case is likely to progress and helps predict what may happen with the patient's MDS in the future.

In 1999, the World Health Organization, or WHO, published a new classification system. This classification system was then revised in 2008. Its goal was to be more specific than the FAB in describing subtypes and in predicting what will happen to patients. The system is based on patient data from around the world and on the most up-to-date knowledge of MDS.

French American British (FAB) Classification System

Even though the newer WHO system updated the FAB system, the FAB system is still used by some doctors today.

The FAB system is based on four main factors:

  1. The percentage of blast cells in the bone marrow
  2. The percentage of peripheral blood blasts
  3. The percentage of red blood cell precursors with abnormal iron deposits called ring sideroblasts
  4. The percentage of monocytes in the blood

The FAB system divides MDS into five subtypes. These subtypes include:

  1. Refractory anemia, also called RA
  2. Refractory anemia with ring sideroblasts, also called RARS
  3. Refractory anemia with excess blasts, also called RAEB
  4. Refractory anemia with excess blasts in transition, also called RAEB-t
  5. Chronic Myelomonocytic Leukemia, or CMML
MDS Subtype
(FAB Classification System)
Blood Findings Bone Marrow Findings
RA (Refractory Anemia) Less than 1% blasts Less than 5% blasts
RARS (Refractory Anemia with Ring Sideroblasts Less than 1% blasts Less than 5% blastsMore than 15% of young red blood cells in marrow contain ring-shaped iron deposits called ring sideroblasts
RAEB (Refractory Anemia with Excess Blasts in Transmission) Note: Now called AML (acute myelogenous leukemia) Less than 5% blasts
At least 2 cytopenias (low blood cell counts)
5% to 20% blasts
RAEB-t (Refractory Anemia with Excess Blasts in Transformation) More than 5%
Note: At least 2 cytopenias (low blood cell counts)
21% - 30% blasts
CMMA (chronic myelomonocytic Leukemia) Less than 5%More than 1,000 monocytes per cubic millimeter of blood 5% - 20% blasts


WHO (World Health Organization) Classification System

The WHO classification system builds and expands on the older FAB system. It divides MDS into seven subtypes based on tests of the blood and bone marrow. These seven subtypes include:

  1. Refractory cytopenia with unilineage dysplasia or RCUD; there are three subtypes of RCUD:
    a.    refractory anemia or RA
    b.    refractory neutropenia or RN
    c.    refractory thrombocytopenia or RT
  2. Refractory anemia with ring sideroblasts or RARS
  3. Refractory cytopenia with multilineage dysplasia or RCMD
  4. Refractory anemia with excess blasts type 1 or RAEB-1
  5. Refractory anemia with excess blasts type 2 or RAEB-2
  6. MDS with isolated deletion 5q 
  7. MDS unclassified
MDS Subtype Blood Findings Bone Marrow Findings
RCUD (Refractory Cytopenia with Unilineage Dysplasia) 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) Low red blood cell count that can't be treated with iron or vitamins More than 15% red blood cells that contain ring-shaped iron deposits called ring sideroblasts
RCMD (Refractory Cytopenia with Multilineage Dysplasia) 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)
The same as refractory anemia 5% tp 9% blasts in marrow

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

The older FAB system is a bit different than the newer WHO classification system. The WHO system made the following changes to the FAB system:

  • Added isolated deletion 5q Syndrome and Unclassified MDS
  • Created the category of RCUD (refractory cytopenia with unilineage dysplasia)
  • Created a category called RCMD (refractory cytopenia with multilineage cytopenia)
  • Split RAEB-t into 2 subtypes depending on the number of blast cells present in the bone marrow
  • Reclassified Chronic Myelomonocytic Leukemia or CMML into a new group of diseases known as MDS/MPD. CMML used to be considered a kind of MDS

International Prognostic Scoring System (IPSS)

The IPSS gives each patient a score to help their doctor understand how quickly their MDS is progressing and what is likely to happen to their disease over time. The score is based on several factors that are linked to MDS. They are:

  • Percentage of blasts in the bone marrow
  • Changes in cell chromosomes, also called cytogenetics 
  • Low blood cell counts or cytopenias

Each factor gets a score. Together, the scores tell which risk groups you fall into.

Your IPSS score helps your doctor to answer the following questions:

  • How severe is your case of MDS?
  • How likely is your case to become acute myeloid leukemia or AML if treated only with supportive care?
  • How long are you likely to live if you are treated only with supportive care?
Factor Value IPSS Score
Blasts in bone marrow less than 5%
5% to 10%
11% - 20%
21% - 30%
Cell DNA changes (cytogenics) Good
Low blood counts or cytopenias 0 or 1 cytopenia
2 or 3 cytopenias

Using the IPSS, a patient is given a score between 0 and 3.0. A patient can be put into one of four risk groups, two lower-risk and two higher-risk. Scores of less than 1.5 are considered lower-risk.

This score tells you what risk group you fall into. For example:

  • If your IPSS score is 0, you are in the low-risk group
  • If your IPSS score is 0.5 to 1, you are in the intermediate-1 risk group
  • If your IPSS score is 1.5 to 2, you are in the intermediate-2 risk group
  • If your IPSS score is more than 2.0, you are in the high-risk group