[Diagnostic work-up of body cavity effusions in dogs and cats]

Patients with body cavity effusions are often seen in veterinary practice.However, the underlying causes of effusion formation are numerous and to draw conclusions about the underlying pathogenesis, further laboratory diagnostic work-up is essential.An initial classification of the pathomechanism (transudate versus exudate) may already be achieved using simple tools that are generally available in every practice (e. g. refractometer, hematology device/counting chamber, microscope). The effusion analysis includes macroscopic assessment, evaluation of the protein content, determination of the nucleated cell count and, if necessary, red blood cell count or the hematocrit and the cytological evaluation of direct and enrichment preparations (in the case of cell-poor fluids).In some instances, the cultivation of microorganisms (if an underlying infection is suspected) and/or the analysis of certain biochemical parameters may also be indicated; e. g. measurement of triglycerides when chylous effusion is suspected, measurement of creatinine and/or potassium if a uroperitoneum is suspected or the measurement of bilirubin if biliary peritonitis represents a concern.In addition, some studies focus on markers that may be used to differentiate between transudate and exudate as well as between infectious and sterile inflammatory processes (e. g. glucose, lactate, lactate dehydrogenase).This review article provides systematic instructions for the work-up and examination of body cavity effusions in dogs and cats, in order to narrow down potential differential diagnoses wherever possible.Even when sent to a reference laboratory, the correct preparation of the effusion fluid and the information on the patient (signalment, history, pre-medications, clinical examination, imaging and laboratory diagnostic findings) as well as the results of the in-house effusion analysis are important so that the examiners are able to provide the most accurate diagnosis or underlying pathomechanism.