By standard definition, the peroxidase reaction is totally negative in leukemic lymphoblasts. Sudan Black B (SBB) stain is also negative. However, it has been suggested that SBB is less specific than MPO because positive reactions have been reported in ALL.23 It should be remembered that in case of negative MPO or SBB reaction, megakaryocytic, erythroid, some monocytic, and AML MO subtype as well as lymphoid lineage should be considered in the differential, pending the results of flow cytometry.

The periodic acid-Schiff (PAS) reaction is useful in supporting the diagnosis of ALL (Table 14-5), but its importance has declined, as immunophenotyping has become more important for this purpose. Although blocklike PAS positivity is considered characteristic of ALL only 15 percent of cases have this type PAS reaction14 (Plate 14-4). It should be noted that blocklike PAS positivity can be seen in monoblasts in monoblastic leukemia and in erythroblasts in erythroleukemia.24 Moreover, fine granular activity in blocks of PAS material can be identified in all AML cell types (Plate 14-5). PAS stain is therefore a poor discriminator of various cell types, but it may be useful as a part of battery of stains, particularly when many of the reactions are negative or nonspecific.

A strong focal (paranuclear) acid phosphatase (ACP) activity (Plate 14-6) or a strong focal ANAE activity greater than 75 percent of blasts are characteristic of T-ALL.25 However, it should be remembered that a localized ACP and a-naphthyl acetate esterase (ANAE) reaction is also a feature of AML:M7 and M6.

Table 14-5. Reaction of Acute Leukemic Cells to

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