Leukemias are classified according to their acute and chronic progress as well as according to their lymphoid or myeloid cell origins. Acute myeloid leukemia is further classified into subtypes.

Most cancers and solid tumors are classified according to cell type, aggressiveness and propensity to spread to other organs. The classifications of AML however are based on the appearance of the cells and their genetic abnormality.

Classification systems

There are two major systems that are used to classify AML into subtypes. One of this is the French-American British (FAB) classification that was in use earlier and has been replaced by the newer World Health Organization (WHO) classification.

The French-American-British (FAB) classification

The French-American-British (FAB) classification of AML was developed in the 1970s by a group of French, American, and British leukemia experts.

They classified AMLs into subtypes from M0 to M7. This was based on the type of cell from which the leukemia developed and the level of maturity of the cells. The FAB classification relied on appearance of leukemia cells under the microscope after routine staining.

According to the FAB classification the subtypes M0 to M5 start in precursors of white blood cells. M6 AML originates in very early forms of red blood cells and M7 AML starts in early forms of cells that form platelets.

The FAB classification also defines symptom differences. For example, uncontrolled bleeding is seen in the M3 subtype of AML, also known as acute promyelocytic leukemia (APL).  The subtypes also predict prognosis and identify the best suitable treatment. APL for example is usually responsive to retinoids.

The World Health Organization (WHO) classification

The most used and more modern classification of AML is the World Health Organization (WHO) classification. The FAB classification system does not take into account several factors that may affect the outlook of the leukemia. Thus the WHO proposed a newer system that includes some of these factors to classify AML.

The WHO classification system divides AML into several broad groups. These include:-

• AML with genetic abnormalities:-
  • AML with a translocation between chromosomes 8 and 21
  • AML with a translocation or inversion in chromosome 16
  • AML with changes in chromosome 11
  • APL (M3), which usually has translocation between chromosomes 15 and 17
• AML with multilineage dysplasia meaning involvement of more than one abnormal myeloid cell type
• AML related to previous chemotherapy or radiation
• Unspecified AML including those that do not fall into one of the above groups. This includes:-
  • Undifferentiated AML (M0)
  • AML with minimal maturation (M1)
  • AML with maturation (M2)
  • Acute myelomonocytic leukemia (M4)
  • Acute monocytic leukemia (M5)
  • Acute erythroid leukemia (M6)
  • Acute megakaryoblastic leukemia (M7)
  • Acute basophilic leukemia
  • Acute panmyelosis with fibrosis
  • Myeloid sarcoma (also known as granulocytic sarcoma or chloroma)

Sometimes ALL with myeloid markers may be included under AML called AML with lymphoid markers, or mixed lineage leukemias or undifferentiated or biphenotypic acute leukemias (with both lymphocytic and myeloid features).