Alkaline phosphatase is a useful cytochemical marker for the diagnosis of acute myelomonocytic and monocytic leukemia in the dog

Background Immunophenotyping has replaced cytochemical staining as the preferred technique for classifying acute leukemia. However, some acute myeloid leukemias (AML) lack lineage‐associated markers. In our experience, alkaline phosphatase (ALP) is expressed in immature canine monocytes. We hypothesized that ALP is a useful marker for monocytic AML. Objectives The objective was to compare ALP expression in neoplastic cells from dogs with lymphoma, chronic lymphocytic leukemia (CLL), acute lymphoid leukemia (ALL), and AML. Methods Alkaline phosphatase results were retrieved from medical records of dogs with acute leukemia. Smears from dogs with lymphoma or leukemia were also prospectively stained for ALP activity. CLL was based on persistent lymphocytosis (10 × 109/L) and acute leukemia on ≥ 20% blasts in blood or bone marrow. ALL was classified based on positive phenotyping for T‐ or B‐lymphocyte antigens, and AML on positive phenotyping for CD11b, CD11c or CD14, or cytochemical staining for chloroacetate esterase, Sudan Black B, or myeloperoxidase. Results There was no ALP activity in all 49 lymphomas and 7 CLLs. Weak ALP activity was seen in 31% of 14 ALL (all T‐ALL). ALP activity was seen in all 20 AML (P