FAB Classification of Leukemias and Cytochemical Stain Observations in each.

Cytochemical stains are great tools to distinguish the different myelogenous leukemias from each other. There are multiple different stains employed to help this process.

Myeloperoxidase (MPO): MPO is a lysosomal protein that is stored in the azurophilic granules of neutrophils and is released upon degranulation into the extracellular space. During respiratory burst it produces hypochlorous acid from hydrogen peroxide and chloride.

Sudan Black B: SBB is a lysochrome (fat-soluble dye) that is used for staining the lipids and lipoproteins. This is helpful in differentiating between AML and ALL as SBB will only stain myeloblasts and not lymphoblasts.

A-Napthyl-Chloroacetate: Otherwise known as the specific esterase stain will stain red in granulocytes and immature granulocytes.

A-Napthyl-acetate/butyrate: Otherwise known as the non-specific esterase stain will reveal strong staining patterns in monocytic lineage cells and will not stain lymphoblasts or myeloblasts.

PAS: Periodic acid–Schiff (PAS) is a staining method used to detect polysaccharides such as glycogen, and mucosubstances such as glycoproteins, glycolipids and mucins in tissues. Erythroleukemia, a leukemia of immature red blood cells will stain a bright fuchsia.

LAP: Leukocyte alkaline phosphatase (LAP) is the term for alkaline phosphatase that’s found in leukocytes. This can differentiate other WBC disorders from CML. In CML the LAP is decreased.

M0: Acute myeloid leukemia minimally differentiation; No stains positive, It can be possible for a weak MPO or SBB reaction.

M1: Acute myeloid leukemia without maturation; Weakly MPO and SBB positive.

M2: Acute myeloid leukemia with maturation; MPO and SBB positive

M3: Acute promyelocytic leukemia; MPO and SBB strongly positive

M4: Acute myelomonocytic; MPO and SBB positive. Non-specific and specific esterase stain positive.

M5: Acute monocytic/monoblastic leukemia; Non-specific esterase stain positive.

M6: Acute erythroleukemia; PAS positive

M7: Acute megakaryoblastic leukemia. Antibody reactions are positive. Usually anti-VIII, and Anti-GP IIb/IIIa.

None of these stains will yield a definitive diagnosis and are to be used in conjunction with other tests to make a diagnosis of AML.

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