Lymphocyte phenotyping uses flow cytometry and labeled monoclonal antibodies directed against a variety of lymphocyte CD antigens. Generally, lymphocyte phenotyping can give more detailed information about the nature of the white blood cell populations, because multiple markers can be examined simultaneously. Phenotyping by flow cytometry can be used to evaluate persistent lymphocytosis to determine whether the cells are neoplastic versus reactive.

Samples can include either: Blood in LTT; lymph node or bone marrow aspirates (inject aspirates into ~ 200 uL of saline containing ~ 10% serum ); or body cavity effusions. Bone marrow from cats is unabled to be evaluated at this time.

This test is useful for evaluating peripheral blood lymphocytosis in dogs with mature lymphocytosis. Chronic lymphocytic leukemia (CLL) is almost always of T-cell origin. Cells tend to have an aberrant phenotype (have lost certain cell surface markers) and are thus able to be identified as neoplastic.

This may help with prognostication for lymphosacoma cases. Dogs with B-cell origin LSA tend to respond better to treatment (average survival ~ 12 m) than dogs with T-cell origin LSA (GI and cutaneous LSA in dogs tend to be T-cell and are associated with a poorer prognosis). Approximately 80% of canine LSA cases are of B-cell origin.

Lymphocyte phenotyping can help in evaluating pleural effusions in cats. In LSA, the vast majority of cells will typically be B lymphocytes, whereas with chylothorax there is a mixed population of lymphocytes, and with thymoma, the cells have cells have unique phenotypic characteristics.

This type of evaluation is not considered to be very helpful in evaluating immunosuppression or prognosis in FIV-infected cats.

Contributed by Dr. Anne Avery, CSU Clinical Immunopathology Diagnostic Service, Ft.Collins, CO.