Concept of Acute Myeloid Leukemia (AML) FAB Classification

There is no need of mnemonics to remember the FAB classification of Acute Myeloid Leukemia (AML); just remember the process myeloid differentiation. A simple schematic diagram with few intermediate processes and stimulating factors eliminated will meet our purpose here.

myeloid differentiation aml

The cells belonging to the myeloid lineage are:

  1. Granulocytes: Neutrophils, Eosinophils and Basophils
  2. Monocytes and Macrophages
  3. Erythrocytes (RBCs)
  4. Megakaryocytes (Platelets)

In French-American-British (FAB) classification of AML, it is classified from M0 to M7. The scheme takes into account:

  • The degree of maturation (M0 to M3)
  • The lineage of leukemic blast (M4 to M7)
  1. M0 – undifferentiated progenitor cells
  2. M1 to M3 – myelocytes (granulocyte precursors)
  3. M4 to M5 – monocyte precursors
  4. M6 – erythrocyte precursors
  5. M7 – platelet precursors

Simplified FAB classification of AML

  1. M0 – Undifferentiated
  2. M1 – Myeloblastic without maturation
  3. M2 – Myeloblastic with maturation (Commonest type)
  4. M3 – Promyelocytic
  5. M4 – Myelomonocytic (Naegeli type)
  6. M5 – Monocytic (Schilling type)
  7. M6 – Erythroleukemia (Di Gulielmo’s disease)
  8. M7 – Megakaryocytic

AML Concepts in Concise

1. FAB used 30% blasts to delineate chronic myeloid leukemia (CML) from Blast crisis and AML. WHO revised classification uses the presence of ≥20% myeloblasts in the bone marrow or peripheral blood for the diagnosis of AML.

2. Mo, M1 and M2:

  • <3% blasts are MPO positive in M0 and ≥3% blasts are MPO positive in M1
  • <10% maturation beyond myeloblasts = M1
  • >10% mauration beyond myeloblasts = M2
  • Auer rods:
    • M0 – None
    • M1 – 50%
    • M2 – 70%
    • M3 – 100%
  • t(8:21) is pressent in M2

3. M3:

  • Most cases have t(15:17) – results in disruption of Retinoic Acid Receptor (RAR) required for myeloblast maturation.
  • All-trans-retinoic acid is hence, used for treatment of Acute Promyelocytic Leukemia (APL).
  • DIC is seen in 5-10% cases due to prothrombotic release of Auer rods.

4. M4: >20% monocytic precursors (<20% in M2)

  • Myeloperoxidase positive (Myeloblastic) + Auer rod positive (Monoblastic) + Non-specific esterase positive (Monoblastic)

5. M5:

  • >80% monocytic precursors
  • Gum infiltration and hyperplasia
  • High lysozyme level
  • Auer rod negative + Non-specific esterase positive

6. M6:

  • ≥50% erythroid precursors and ≥20% blasts in nonerythroid component
  • Also known as Di Gulielmo’s disease or Syndrome

7. M7:

  • Rapid myelofibrosis due to release of PDGF.
  • Resistant to treatment
  • GATA1 mutations is seen in those associated with Down’s Syndrome.
  • GpIIb/IIIa or vWF positive

8. Ara-C (Cytarabine) is used for the treatment of CML except M3, in which all-trans retinoic acid is used.

9. Remission criteria:

  • Complete remission = Bone marrow blasts <5%, No auer rods
  • Complete blood count recovery = ANC >1000/microlitre and Platelets ≥1,00,000/microlitre
  • Partial remission = ≥50% fall in blasts over pretreatment values

10. M8:

  • There is no M8 in FAB classification, but some authors have proposed rare deno-vo Acute Basophilic Leukemia as M8.

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