Simple SummaryAnalysis and classification of effusions is helpful in guiding diagnostic tests to identify an underlying disease. The traditional veterinary classification of effusions does not adequately reflect the disease mechanism causing the effusion in cats and might not sufficiently inform diagnostic work-up. The aim of this study was to assess whether other parameters might aid classification. Sixty-five cats with body cavity effusions (within the abdomen, chest and pericardial sac) were included. Effusions were classified as transudates (e.g., heart failure [n= 18]) or exudates (e.g., inflammation, cancer [n= 47]) based on the disease mechanism causing the effusion, and using the traditional scheme. Several parameters (activity of enzyme lactate dehydrogenase in the effusion; effusion/serum ratio of the enzymatic activity of lactate dehydrogenase; effusion/serum total protein ratio; serum–effusion albumin gradient; acute phase proteins in serum and effusion) were analysed in their ability to differentiate exudates from transudates. All tested parameters performed better in classifying effusions in comparison to the traditional scheme. Acute phase proteins were helpful in classifying effusions based on their disease mechanism but could not separate effusion types when using the traditional scheme. This latter finding further supports the classification of effusions based on their disease mechanism. Better classification of effusions might improve disease diagnosis in the future.AbstractThe traditional veterinary classification (TVC) of effusions based on cell count and total protein (TP) does not adequately reflect the aetiology. Light’s criteria (LC) (activity of lactate dehydrogenase [LDH] in the effusion [LDHef], effusion/serum LDH ratio [LDHr], effusion/serum TP ratio [TPr]), serum–effusion albumin gradient (ALBg), acute phase proteins (APPs) [serum amyloid A (SAA), α1-acid glycoprotein (AGP), haptoglobin] might aid classification. The aim was to evaluate the utility of these parameters except LDHr in differentiating exudates from transudates. Sixty-five cats with effusions (33 peritoneal, 31 pleural, 1 pericardial), with 18 transudates and 47 exudates based on aetiological classification (AC), were included. The sensitivity, specificity and accuracy of several parameters to identify exudates (based on AC) was assessed. APPs were compared between exudates and transudates based on AC and TVC, with receiver operating characteristics analysis identifying the best APP to recognise exudates. Simplified LC (LDHef, TPr) had an accuracy of 79% and TVC of 48%. ALBg had the highest sensitivity (98%) and LDHef the highest specificity (83%) in identifying exudates in cats. All APPs but effusion SAA could differentiate exudates from transudates based on AC (effusion AGP had the largest area under the curve 0.79) but not TVC. All parameters were better than TVC in identifying exudates. The conformity of APPs with AC but not TVC favours the use of AC to classify effusions.