Comparative Diagnostic Accuracy of Pleural Effusion Classification Methods in Cats: An Analysis of Naturally Occurring Cases

Background Pleural effusion (PE) in cats can result from transudative or exudative processes. Transudates are caused by decreased colloid osmotic pressure (↓COP) or elevated hydrostatic pressure (↑HP) gradient, while exudates arise from increased pleural capillary permeability. Diagnostic classification approaches traditionally rely on pleural effusion total protein (TP PE ) and total nucleated cell counts (TNCC PE ). In contrast, Light's criteria employing pleural effusion lactate dehydrogenase (LDH PE ), LDH PE divided by serum LDH (LDH ratio ), and pleural effusion to serum protein ratio (TP ratio ) are more accurate than classification based on TP PE /TNCC PE in humans and show promise in cats. Objectives To assess the diagnostic accuracy of TNCC PE , LDH PE , LDH ratio , TP PE , and TP ratio in classifying feline PEs in ↑HP‐transudates, ↓COP‐transudates, or exudates and to compare simplified Light's criteria (which relies solely on LDH PE ) with Light's criteria and existing veterinary classification schemes based on TP PE and TNCC PE (named Vet‐A and Vet‐B). Methods Cross‐sectional study including 83 client‐owned cats with PE. Results There were 55 exudates, 28 ↑HP‐transudates, and 0 ↓COP‐transudates. All the variables analyzed were significantly different between exudates and transudates. Simplified Light's criteria correctly classified 50/55 exudates and 26/28 ↑HP‐transudates (sensitivity = 91%, specificity = 93%, accuracy = 92%). Light's criteria correctly identified 55/55 exudates but misclassified 15/28 ↑HP‐transudates as exudates (accuracy = 82%). Traditional veterinary schemes showed lower accuracies: Vet‐A (57%) and Vet‐B (74%). Simplified Light's criteria outperformed Vet‐A ( p < 0.001) and Vet‐B ( p = 0.007) and trended higher than Light's criteria ( p = 0.096). Conclusions Simplified Light's criteria demonstrate excellent diagnostic accuracy, outperforming traditional veterinary classification schemes.