[Analysis of body cavity effusion - focus on cytological examination]

Body cavity effusion is a common finding in small animal medicine. Fluid analysis is needed in order to determine the underlying pathology. Classification into the categories of protein-poor transudate, protein-rich transudate and exudate is based on the measurement of cell count and protein content. The cytological examination is an important tool for certain effusion variants (e. g. chyle, hemorrhagic effusions, uroperitoneum, biliary effusions, neoplastic effusions) and can complement the biochemical analysis enabling a more precise classification and, in some instances, providing a specific diagnosis. This review article highlights what to look for during the microscopic evaluation, which specific findings can be identified in the different types of effusion, when further diagnostics test are indicated as well as which limitations are to be taken into account.