This reaction stains polysaccharides. Periodic acid oxidizes carbohydrates and similar compounds to aldehydes. The aldehydes can then react with the Schiff reagent (leuko-fuchsin) to release fuchsin and stain the cellular components containing oxidizable compounds. A variety of intracellular compounds react with the PAS reagents, but in the blood and marrow cells glycogen is the most abundant compound, as evidenced by amylase digestion that blocks the staining.
In the blood from normal people, the cytoplasm of polymorphonuclear leukocytes stains intensely pink or red, with granular appearance in some cells. Monocyte cytoplasm stains faintly pink and may contain fine or coarse granules. Erythrocytes do not stain. Platelets stain intensely. In the normal marrow, the earliest granulocyte precursors do not stain. Cytoplasmic staining increases with increasing maturity of the granulocytic cells.

PAS reaction may be helpful in the diagnosis of some cases of acute lymphoblastic leukemia and some subtypes of acute myeloid leukemia.