The ability to distinguish the lineage of canine leukaemia as myeloid or lymphoid has previously relied upon immunohistochemistry as cytomorphology alone can prove misleading. Immunophenotyping by flow cytometry has become established in veterinary medicine, as is the case in the human field.

The aim of this study was to provide the first quantitative assessment of cytomorphological features of acute myeloid and lymphoid leukaemias, which had previously been immunophenotyped by flow cytometry.

Flow cytometry has been offered by the Central Diagnostic Service, University of Cambridge, since 2005. The database of results of immunophenotyping was examined and the blood films from cases of acute (CD34 positive) leukaemias were retrieved. These were examined by a clinical pathologist blinded to the results of the immunophenotyping and scored for a number of morphological features including cell size, nuclear shape, chromatin, cytoplasmic granules etc.

Fourty-three cases were included in the study. Twenty-two acute lymphoblastic leukaemias (ALL), fourteen acute myeloid leukaemias (AML) and seven cases of acute, undifferentiated leukaemia. The blasts of both AML and ALL had a variable appearance and there was significant overlap in their cytomorphological appearance. A selection of the key morphological features examined are summarised in Figure 1 (N.B. as the appearance of the neoplastic population of cells is not always uniform the categories within a particular feature e.g., small or medium cell size are not mutually exclusive).

The data were analysed using the Chi-squared test. Of the features analysed the only one to be statistically significantly associated with a particular lineage was the association of an indented nucleus with AML (p=0.02), but this still did not allow lineage assignment by morphological examination alone.

In conclusion morphological features do not allow reliable differentiation of lineage in acute leukaemia. Immunophenotyping is required for this purpose and flow cytometry is the preferred technique.

Figure 1.