Bone marrow neoplastic

Bone marrow - neoplastic myeloid

AML not otherwise specified


Last author update: 1 February 2013
Last staff update: 16 February 2022

Copyright: 2001-2023,, Inc.

PubMed Search: Acute monocytic leukemia (M5b)

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Cite this page: Mihova, D. M5b. website. Accessed March 30th, 2023.
Definition / general
  • Acute monocytic leukemia (M5b)
  • 3 - 6% of AML
  • Affects all ages
  • Mature monocytes or promonocytes predominate in peripheral blood (< 80% of monocyte lineage cells are monoblasts, usually < 20%)
  • Treatment may cause tumor lysis syndrome, DIC and falsely elevated platelet counts (Arch Pathol Lab Med 1999;123:1111)
Case reports
Microscopic (histologic) description
  • Leukemic cells are often promonocytes with less basophilic cytoplasm and more azurophilic granules than monoblasts
  • Have folded or cerebriform nuclei with fine chromatin
  • Erythrophagocytosis is common
Microscopic (histologic) images

AFIP images
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Promonocytes have abundant cytoplasm

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Monocytic cells have range of differentiation

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2 myeloblasts and 3 promonocytes

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Moderate granular cytoplasm

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Nonspecific esterase positive

Electron microscopy images

AFIP images
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Promonocyte has cytoplasm with numerous small cisterna

Positive stains
Negative stains
Molecular / cytogenetics description
  • 30% have cytogenetics abnormalities, including 11q23 in 12% (these cases should be classified as a recurrent genetic abnormality)
  • FLT3 mutations in 30%
  • t(8;16)(p11;p13) fuses MOZ gene at 8p11 with CBP gene at 16p13 and is associated with erythrophagocytosis and coagulopathy
Differential diagnosis
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