In a case where 75% of blasts in a peripheral smear stain positive for Sudan Black B and peroxidase, what is the most likely diagnosis?

The presence of 75% of blasts in a peripheral smear staining positive for Sudan Black B and peroxidase is indicative of acute myelocytic leukemia (AML). Sudan Black B is a dye that stains lipids and is often used in the laboratory to identify myeloid cells. A positive result indicates myeloid differentiation, which is characteristic of AML. Similarly, peroxidase positivity further reinforces the diagnosis of a myeloid lineage, as myeloid cells typically express the enzyme myeloperoxidase.

In acute myeloid leukemia, blasts often demonstrate both Sudan Black B positivity and peroxidase positivity, confirming their myeloid origin. The high percentage of positively stained blasts observed in this scenario supports the diagnosis, as it indicates a significant population of cells exhibiting myeloid characteristics within the bone marrow or peripheral blood.

Other conditions such as chronic myeloid leukemia (CML), acute undifferentiated leukemia (AUL), and acute lymphocytic leukemia (ALL) have different staining profiles. CML is characterized by a proliferation of mature and immature myeloid cells rather than a predominance of blasts, while AUL is marked by a lack of specific lineage markers, making it less likely to show positivity for