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 means.

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 monocytes.

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.