Discriminating transudates and exudates in dogs with pleural effusion: diagnostic utility of simplified Light’s criteria compared with traditional veterinary classification

What is it about?

Simplified Light’s criteria vs traditional veterinary classification in discriminating transudates and exudates: a cross-sectional study of 100 dogs with pleural effusion
A. Zoia DVM, Cert SAM, Dip ECVIM-CA; M. Petini, DVM; D. Righetti, DVM, M. Caldin,DVM, PhD, Dip ECVCP; M. Drigo PhD, Dip ECVCP.

Key findings
Simplified Light’s criteria have an accuracy of 98% in discriminating exudates from transudates. The accuracy of the traditional veterinary classification is significantly lower and varies from 57% to 71% according to TPp and TNCCp cut-off values applied. 

Introduction
Variation in Starling forces, due to increased hydrostatic pressure gradient (HP) or decreased colloid osmotic pressure (COP), causes the formation of a transudative pleural effusion. An increased pleural membrane or pulmonary capillary permeability and the obstruction of lymphatic flow causes the formation of an exudative pleural effusion. Therefore, in human medicine, pleural effusions are divided into transudate and exudate based on the pathophysiology of its formation and Light’s criteria are the best method to correctly classify pleural effusion into these two categories. According to Light’s criteria, a pleural effusion is an exudate if one or more of the following criteria is met: pleural fluid/serum total protein (TP) ratio >0.5, or lactate dehydrogenase (LDH) concentration of the pleural fluid >200 UI/L, later modified to 2/3 of the upper limit of the serum LDH reference interval. 
The aim of this study was to determine whether a simplified Light’s criteria (i.e., pleural effusion LDH concentration and serum TP) can identify the pathophysiology of pleural effusion formation in dogs, and to assess whether these criteria were more accurate than the traditional veterinary classification based on pleural effusion TP (TPp) and nucleated cell count (TNCCp).


Approach
Records of dogs presented to the San Marco Veterinary Clinic (November 2010 to June 2016) with pleural effusion were reviewed and the diagnosis was recorded. All included dogs had at the time of presentation seven milliliters of pleural effusion collected by thoracocentesis, which were preserved in plain and K3-EDTA tubes. From all pleural K3-EDTA, TNCCp and a cytological examination were performed. The serum and pleural effusion biochemistry parameters were measured by validated canine assays. 
	Pleural effusions were then classified based on the pathophysiology of their formation in 3 groups: pleural effusions due to decreased COP, pleural effusions due to increased HP, and pleural effusion due to exudate. The cause of the pleural effusion was used as the gold standard for establishing the pathophysiology of its formation. 
For the simplified Light’s criteria, the LDH cut-off value to discriminate an exudate from a transudate was set at > 100 IU/L (i.e., > 2/3 of the serum upper reference interval; serum reference interval: 50–150 IU/L). A serum TP cut-off value of = 4.0 g/dL was used to discriminate an increased HP transudate from a decreased COP transudate. 
For the traditional veterinary classification scheme, two previously published cut-off values for TPp and TNCCp were used. Classification A) transudate: TPp <2.5 g/dL, TNCCp <1500 µL; modified transudate: TPp = 2.5–7.5 g/dL, TNCCp = 1000–7000 µL; exudate TPp >3.0 g/dL, TNNCp >7000 µL. Classification B) transudate: TPp <2.5 g/dL, TNCCp <1000 µL; modified transudate: TPp >2.5 g/dL, TNCCp <5000 µL; exudate TPp >2.5 g/dL, TNNCp >5000 µL. In accordance with Perman and Osborne’s definition, a modified transudate was regarded as an effusion resulting from an increase in the HP gradient, and a transudate as an effusion resulting from a decreased COP. 

Results
One-hundred dogs entered the study. Based on the pathophysiology of pleural effusion formation, there were seven transudates due to decreased COP, 18 transudates due to increased HP gradient, and 75 exudates. 
Simplified Light’s criteria caused a misclassification of only two exudates, and correctly classified all 18 transudates due to an increased HP gradient and all seven transudates due to decreased COP. 
The TPp-TNCCp classification scheme A misclassified 31 exudates and 12 increased HP gradient transudates, while all seven decreased COP transudates were correctly classified. The TPp-TNCCp classification scheme B misclassified 21 exudates and eight increased HP gradient transudates, while all seven decreased COP transudates were correctly classified.

Interpretation
The frequency of agreement between the simplified Light’s criteria and the traditional veterinary classifications based on TPp and TNCCp with the true nature of the pleural effusion was significantly different (P < 0.001), with simplified Light’s criteria having an overall accuracy of 98%, while the TPp-TNCCp classification A and B having an overall accuracy of 57% and 71%, respectively. 

Significance of findings
The utilization of the simplified Light’s criteria is clinically important because it is indicative of the underlying pathophysiological process involved. Such a distinction allows appropriate investigations to be instigated, enabling better patient management. 


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