Cytology of Feline Nodal Lymphoma: Low Interobserver Agreement and Variable Accuracy in Immunophenotype Prediction

Nodal lymphomas are less common in cats than in dogs and, consequently, no specific studies have been pub-lished. Cytology is the first step in the diagnosis of nodal lymphoma but is highly subjective. Morphologicalfeatures have been introduced for the cytological classification of canine lymphomas but not for cats. Therefore,the aim of this study was to evaluate interobserver agreement on various cytological features of feline nodallymphomas and to investigate the accuracy in predicting B or T immunophenotypes. Four veterinary cytolo-gists examined 25 feline nodal and mediastinal lymphoma cytological samples by adapting the criteria used forthe evaluation of canine lymphomas and setting histopathology and immunohistochemistry as the gold stan-dard. High interobserver variability was found in the evaluation of most features except for the presence orabsence of cytoplasmic vacuoles, which were more common in B-cell lymphomas. Cytology training centrewas the major factor influencing the extent of agreement among evaluators. Diagnostic accuracy in predictinglymphoma immunophenotype varied from 35% to 75% and did not appear to be correlated with the experi-ence of the evaluators. We conclude that cytological criteria, commonly used to describe canine lymphomas,are not adaptable to the counterpart feline neoplasms. Cytology-based immunophenotyping of feline lym-phomas from different laboratories, and different cytologists within the same laboratory, differ substantiallyand should not be considered reliable. Specific cytological criteria are needed to describe feline lymphoma.