Myelodysplastic syndromes (MDS) were originally classified more than 20 years ago at an international conference attended mostly by doctors from France, the United States, and Great Britain. This system was known as the French-American-British (FAB) classification.
The system used today is the World Health Organization (WHO) classification. This system seems to be more helpful than the FAB classification in predicting prognosis (outlook). The WHO system recognizes 7 types of MDS:
- Refractory cytopenia with unilineage dysplasia (RCUD)
- Refractory anemia with ringed sideroblasts (RARS)
- Refractory cytopenia with multilineage dysplasia (RCMD)
- Refractory anemia with excess blasts-1 (RAEB-1)
- Refractory anemia with excess blasts-2 (RAEB-2)
- Myelodysplastic syndrome, unclassified (MDS-U)
- Myelodysplastic syndrome associated with isolated del(5q)
Most of these types are determined by the the blood and the bone marrow look under the microscope. One type is defined by a certain chromosome change in the bone marrow cells. 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.
Chronic myelomonocytic leukemia (CMML) was considered a type of MDS in the FAB classification, but the WHO classification includes it in another group of diseases. Information about CMML can be found in our document, Leukemia: Chronic Myelomonocytic.
Refractory cytopenia with unilineage dysplasia (RCUD)
About 5% to 10% of all MDS patients have RCUD.
People with RCUD have low numbers of one type of blood cell, but normal numbers of the other 2 types. Examples of RCUD include refractory anemia (RA), refractory neutropenia (RN), and refractory thrombocytopenia (RT). Refractory anemia (RA) is the most common type of RCUD. People with RA have low numbers of red blood cells (anemia) but have normal numbers of white blood cells and platelets. In the bone marrow of RA patients, only the cells that grow to become red blood cells look abnormal. In the bone marrow of RCUD patients, at least 10% of the early cells of the affected cell type look abnormal (show dysplasia), but the other types of cells in the bone marrow look normal. 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 seldom, if ever, progresses to acute myeloid leukemia. Patients with this type of MDS can live a long time.
Refractory anemia with ringed sideroblasts (RARS)
About 10% to 15% of all people with MDS have this type.
This condition is similar to refractory anemia- the patient has low numbers of red blood cells but normal numbers of white blood cells and platelets. In the bone marrow, at least 10% of the early cells look abnormal. In RARS, though, 15% or more of the early red blood cells in the bone marrow contain circles of iron deposits (rings) around the nucleus (these cells are called ringed sideroblasts).
This type rarely turns into leukemia, and the outcome for people with this type is generally the same as for those with refractory anemia.
Refractory cytopenia with multilineage dysplasia (RCMD)
About 40% of people with MDS have this type.
In this condition, the counts of at least 2 types of blood cells are low. In the bone marrow, those same types of cells look abnormal under the microscope (dysplasia). Ringed sideroblasts may or may not be present. The number of blasts (very early cells) in the bone marrow is less than 5% and none of the blasts contain Auer rods (an abnormality seen in some leukemia cells). Blasts are rare or absent in the blood.
RCMD changes into leukemia in about 10% of patients. Having this type of MDS will shorten a person’s life. One estimate is that half of patients will die within 2 years of diagnosis.
Refractory anemia with excess blasts-1 (RAEB-1)
One or more cell types are low in the blood and look abnormal in the bone marrow. The number of blasts in the bone marrow is increased; but is still less than 10%. The blasts do not contain Auer rods. Blasts may be present in the blood, but they make up less than 5% of the white blood cells.
The chance of RAEB-1 turning into acute myeloid leukemia is about 25%. This type of MDS has a poor outlook and most patients die within 2 years.
Refractory anemia with excess blasts-2 (RAEB-2)
This type of MDS is similar to RAEB-1 except the bone marrow contains more blasts – between 10% and 20% of the bone marrow cells are blasts. The blood also contains more blasts: between 5 and 19% of the white blood cells in the blood are blasts. The blasts may contain Auer rods. Any one (or more) of the cell types can be low in the blood and look abnormal in the bone marrow.
The chance of RAEB-2 turning into acute myeloid leukemia may be as high as 50%.
Myelodysplastic syndrome, unclassified (MDS-U)
This type of MDS is uncommon. For a case to be considered MDS-U, the findings in the blood and bone marrow can’t fit any other type of MDS. 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. The cells in the bone marrow have at least one certain chromosome abnormality that is only seen in MDS or leukemia. The number of blasts in the bone marrow is less than 5%.
Because this type is so rare, it has not been studied well enough to predict prognosis (outlook).
Myelodysplastic syndrome associated with isolated del(5q)
In this type of MDS, the chromosomes of the bone marrow cells are normal except they show that a part of chromosome number 5 is missing. In the blood, the red cell counts are low, but the white blood cell counts are normal. Often the platelet count is increased. The number of blasts in the bone marrow is less than 5%.
For unknown reasons, patients with this type of MDS have a very good prognosis (outlook). They often live a long-time and rarely go on to develop leukemia.
Clinical classification of MDS
The WHO system defines types of MDS based on the cells in the blood and bone marrow. This is called a cellular classification system. Cases of MDS can also be classified based on the underlying cause. This is known as a clinical classification. If no cause can be identified, it is called primary MDS. When the cause of the disease is known, it is called secondary MDS. Secondary MDS is often called treatment-related MDS, because the most common cause is prior cancer treatment. This is discussed further in the section, “What are the 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.
Last Revised: 07/02/2015