AML M5 – Acute Monoblastic Leukaemia

Acute monoblastic and monocytic leukaemia is characterized by leukemic cells being predominately  that of monocytic differentiation. It is often associated with extramedullary masses, cutaneous and gingival infiltration and CNS involvement.

Diagnostic Criteria (WHO 2016)

• ≥ 20% blasts (including promonocytes) in the peripheral blood or bone marrow
• ≥ 80% of the leukemic cells are of monocytic lineage

• Acute monoblastic leu­kemia: ≥ 80% of monocytes are monoblasts
• Acute monocytic leukemia <80% monoblasts with predominant monocytic cells being promonocytes or monocytes

Blood Film Features:

• Monoblasts: large cells with large nuclei with prominent nucleoli and abundant basophilic cytoplasm, some with fine az­urophilic granules and vacuoles
• Promonocytes: large cells with folded nuclei and nucleoli (which may be quite indistinct), basophilic cytoplasm with larger azurophilic granules and vacuoles
• Mature and immature monocytes

Bone Marrow Features:

• Monoblasts
• Promonocytes
• Granulocytic component less than 20%
• Cytochemistry: myeloperoxidase – negative, nonspecific esterase – positive

Other features to look for:

• Haemophagocytosis – associated with t(8;16)(p11.2;p13.3)

Other Resources

• ASH image bank:  Acute monoblastic/monocytic leukemia
• Clinical Flow Wiki: AML-M5