Specialized testing for phenotyping of circulating atypical lymphocytes assists in differentiating leukemias. PCR for antigen receptor rearrangement (PARR) and flow cytometry have become common tests for referral centers. However, cost, time, and availability are concerns with these modalities. Immunocytochemistry (ICC) assists with identification of type of leukemia (B cell vs T cell) but is not considered as conclusive as immunohistochemistry (IHC). In this case series (2 dogs), formalin-fixed paraffin-embedded (FF-PE) blood clots were evaluated via IHC as an alternative to using PARR or flow cytometry for evaluating circulating lymphocyte neoplasia.
In Case 1, ICC on mesenteric lymph node aspirates failed to exhibit reactivity to lymphocyte markers, but IHC on the FF-PE blood clot obtained a few days later suggested a T-cell lymphoproliferative neoplasm, confirmed by PARR. In Case 2, ICC on liver aspirates, IHC on the FF-PE blood clot, and PARR were all in agreement for a T-cell phenotype. This initial data suggests the ability, even with atypical cell counts, to identify a T-cell phenotype, which may be a negative prognostic indicator. IHC on FF-PE blood clots provides a safe, minimally invasive, and inexpensive test alternative to more advanced diagnostic protocols. The stability of the sample allows for easy transport and retrospective evaluation. One potential limitation noted by the authors is potential overlap with nonneoplastic causes of lymphocytosis, such as in ehrlichiosis. Nevertheless, this methodology may provide an important tool in improved cancer therapy and warrants further investigation.