The World Health Organization (WHO) developed a system to classify the different types of myelodysplastic syndromes (MDS). The classification system is based on the appearance of the cells in the bone marrow, as well as circulating blood cell counts. The system is also used for predicting prognosis (outlook for recovery) based on the type of syndrome. There are eight types of this disease with different causes, symptoms and prognosis. Types of myelodysplastic syndromes include:

• Refractory anemia.
  
  
• • Patients have only anemia.
    
    
  • Only the early (immature) cells that develop into red blood cells have an abnormal appearance (dysplasia) under a microscope.
    
    
  • The number of blasts (early precursor cells) is normal, measuring less than 5 percent.
    
    
  • Some of the red blood cells in the bone marrow may contain detectable iron deposits that form a ring structure (ring sideroblasts), but the amount of ring sideroblasts in the bone marrow is less than 15 percent.
    
    
  • Approximately 5 to 10 percent of all MDS patients have refractory (unresponsive to treatment) anemia.
    
    
  • This type rarely progresses to acute myeloid leukemia (AML).
    
    
• Refractory anemia with ringed sideroblasts.
  
  
• • This condition is similar to refractory anemia except the amount of ring sideroblasts in the bone marrow is greater than 15 percent.
    
    
  • Approximately 10 to 15 percent of all MDS patients have this type.
    
    
  • This type rarely progresses to AML.
    
    
• Refractory cytopenia (low blood count) with multilineage dysplasia.
  
  
• • At least two types of  blood cell counts are low.
    
    
  • Two of the cell types in the bone marrow have an abnormal appearance (dysplasia) under the microscope.
    
    
  • The number of blasts is normal, measuring less than 5 percent.
    
    
  • The number of ringed sideroblasts in the bone marrow is less than 15 percent.
    
    
  • Approximately 25 percent of people with MDS have this type.
    
    
  • This type progresses to AML in approximately 10 percent of patients.
    
    
• Refractory cytopenia with multilineage dysplasia and ringed sideroblasts.
  
  
• • At least two of the three types of blood cell counts are low.
    
    
  • Two of the cell types in the bone marrow have an abnormal appearance (dysplasia) under a microscope.
    
    
  • The number of blasts is normal, measuring less than 5 percent.
    
    
  • The number of ringed sideroblasts in the bone marrow is greater than 15 percent.
    
    
  • Approximately 15 percent of people with MDS have this type.
    
    
  • This type progresses to AML in approximately 10 percent of patients.
    
    
• Refractory anemia with excess blasts-1 (RAEB-1).
  
  
• • Any one of the three cell types can be low and look abnormal under a microscope.
    
    
  • The number of blasts is abnormal, and is greater than 5 percent but less than 10 percent.
    
    
  • Approximately 40 percent of people with MDS have type 1 or type 2 refractory anemia with excess blasts.
    
    
  • This type progresses to AML in approximately 25 percent of patients.
    
    
• Refractory anemia with excess blasts-2 (RAEB-2).
  
  
• • Any one of the three cell types can be low and look abnormal under a microscope.
    
    
  • The number of blasts is abnormal, and is greater than 10 percent but less 20 percent.
    
    
  • Approximately 40 percent of people with MDS have type 1 or type 2 refractory anemia with excess blasts.
    
    
  • This type progresses to AML in approximately 33 percent of patients.
    
    
• Myelodysplastic syndrome associated with isolated del(5q).
  
  
• • Patients have only anemia.
    
    
  • There is a deletion (the loss of part of a chromosome) in part of chromosome 5.
    
    
  • This type has a very good prognosis (outlook for recovery) and rarely progresses to AML.
    
    
• Myelodysplastic syndrome, unclassified (MDS-U).
  
  
• • Any one of the three cell types can be low and either the white blood cell or the megakaryocyte (platelet forming) cell series look abnormal under a microscope.
    
    
  • The number of blasts is normal, measuring less than 5 percent.
    
    
  • The group does not fall into one of the other myelodysplastic syndrome categories.
    
    
  • This type is uncommon and rarely progresses to AML.

Myelodysplastic syndromes can be further classified as primary or secondary. Primary myelodysplastic syndromes are diseases in which the patient has no apparent risk factor. Secondary myelodysplastic syndromes are diseases which can be linked to several risk factors. Secondary myelodysplastic syndromes are much less likely to respond to treatment.

Chronic myelomonocytic leukemia, atypical chronic myeloid leukemia and juvenile myelomonocytic leukemia are three diseases that were once classified as MDS. Now known as myelodysplastic myeloproliferative diseases (MDS MPD), the conditions were given their own category because they do not fit into the MDS category. Unlike MDS, these diseases resemble myeloproliferative diseases (e.g., leukemia) and produce too many white blood cells.