Cytochemical stains are an important adjunct to identifying and
confirming a myelocytic leukemia. Blood cells contain various enzymes,
fats, and other substances that can be identified by cytochemical
The widely used FAB (French-American-British) classification of
acute leukemias is based on morphology and cytochemistry.
The most important cytochemical studies in the study of acute leukemia
are myeloperoxidase (MPO), nonspecific esterase (NSE),
PAS, and acid phosphatase (AP).
Myeloperoxidase (MPO)- Myeloperoxidase is an enzyme located
in the granules of myeloid and monocytic cells. Myeloperoxidase
is never found in lymphoid cells. If positive, is the most important
marker distinguishing myeloid from lymphoid blasts.
Nonspecific esterase (NSE) - Alpha naphthyl acetate esterase
is an enzyme found in large amounts in monocytic cells, but in only
minor concentrations in myeloid or lymphoid cells. Useful to identify
PAS - The periodic acid Schiff stain demonstrates glycogen
and related mucopolysaccharides. Myeloid or monocytic blasts are
typically weakly positive or negative. A granular (may be fine,
coarse, or block) PAS pattern with a negative background is characteristic
of lymphoblastic leukemia. PAS staining is positive in the erythroid
population in erythroblastic leukemias.
Acid phosphatase (AP) - Although the acid phosphatase enzyme
is ubiquitous, T-lymphocytes and lymphoblasts have a characteristic
"dot-like" focus of intense positivity, whereas the activity in
most other cells is diffuse.
Tartrate-resistant acid phosphatase (TRAP), one of the AP
isoenzymes, is an important diagnostic feature of hairy cell leukemia.
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