Use of the Thinprep® 2000 system for Liquid-Based Cytology of urine and body fluids in dogs and cats

Liquid-based cytology (LBC) is a technique that uses hemolytic and mucolytic solutions to preserve cytological samples. Among the available LBC systems, ThinPrep® is widely used in human medicine, often in combination with Papanicolaou staining, but its application in veterinary cytology remains limited. This study aimed to evaluate the performance of the ThinPrep®-2000 system for the cytological assessment of urine and body cavity effusions from dogs and cats. A total of 43 samples (22 effusions and 21 urine samples) were prospectively collected from the Veterinary Teaching Hospital-University of Padua between October 2023 and November 2024. ThinPrep® slides were stained with Papanicolaou and May-Grünwald Giemsa (MGG) and compared to cytospin-prepared slides stained with MGG. Three observers (two ECVCP diplomates and one ECVCP resident) evaluated cellularity, background, monolayer distribution, cytoarchitecture, nuclear and cytoplasmic details, assigning a 0-3 score for each parameter. In addition, each observer provided a morphological diagnosis for every sample. Cohen’s Kappa test was used for intra- and inter-observer agreement, while the Mann-Whitney test was applied for techniques comparison. In effusion samples, sensitivity, specificity, and diagnostic accuracy for detecting neoplasia and inflammation were also calculated. Intra-observer agreement was poor to slight when comparing ThinPrep® and cytospin slides, but substantial to almost perfect when evaluating ThinPrep® slides stained with different protocols. Inter-observer agreement was moderate to substantial. Significant differences were observed between the two techniques, but not between staining protocols in the ThinPrep® slides, moreover one observer assigned significantly higher scores to MGG-stained slides, regardless of the technique. Diagnostic accuracy for neoplasia ranged from 85–100%, with specificity >88% and sensitivity 33–100%. For inflammation, sensitivity exceeded specificity, except in MGG-stained slides for two observers. Overall, ThinPrep® showed different performance compared to the gold standard, possibly due to peculiarities of veterinary cytology, such as training based on a different routine staining method.