Background: The diagnosis of neoplastic cavitary effusions requires the identification of neoplastic cells in effusions, yet the cytologic appearance of neoplastic effusions can be highly variable due to the varied mechanisms of formation. Additional parameters might aid in the interpretation of equivocal cytologic results. Objectives: Our goal was to evaluate whether total protein concentrations can be used to support the diagnosis of neoplasia in the peritoneal and pleural effusions of dogs with lower cellularities (≤5000 nucleated cells/μL). Methods: Pleural and peritoneal fluid analyses from dogs presented to the University of Illinois Veterinary Teaching Hospital between 2014 and 2019 were evaluated retrospectively. Effusions were categorized as neoplastic or non-neoplastic based on histology or cytology. Non-neoplastic effusions were subcategorized according to mechanism: decreased oncotic pressure, increased hydrostatic pressure, increased vascular permeability, leakage of urine, and leakage of lymph. The TP and blood albumin to fluid TP ratio (Albblood:TPfluid) were compared among groups. Results: Twenty-seven neoplastic and 65 non-neoplastic cases were evaluated. TP was higher in the neoplastic group (P =.001) than in the non-neoplastic group. Neoplastic effusions had a lower Albblood:TPfluid than non-neoplastic (P =.001), and effusions with Albblood:TPfluid of ≤0.6 were 5.6 times more likely to be neoplastic (95% CI 1.69–17.36; P =.003). Conclusions: Fluid TP concentrations were significantly greater in neoplastic than non-neoplastic effusions; however, given the considerable overlap between groups, the diagnostic utility of this difference is low. A neoplastic etiology might be more likely in cases with an Albblood:TPfluid ≤0.6.
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