A new approach to pleural effusion in cats: markers for distinguishing tansudates from exudates

The aetiopathogenic classification of pleural effusion (PE) can be challenging.1 In human medicine, PEs are categorised only as transudates, resulting from increased hydrostatic pressure or decreased osmotic pressure, or exudates, resulting from increased vascular permeability. [2] , [3] , [4] and [5] Simultaneous evaluation of pleural fluid and serum protein, lactate dehydrogenase (LDH) and other biochemical parameters has been proven reliable and effective in identifying the pathophysiology of formation of a PE. [2] , [3] , [4] and [6] In the case of a transudate, subsequent evaluation of the serum albumin will then clarify if the effusion formed is due to a decrease in colloid osmotic pressure or the result of an increase in hydrostatic pressure.In veterinary medicine, PEs were originally classified as transudates or exudates and specific gravity, protein content and cellularity of the effusion were used to differentiate them.7 Due to the common overlap in values of these parameters between transudate and exudate Perman introduced the modified transudate group to veterinary medicine8 and defined it as closely resembling an exudate based on protein content and cellularity, but resulting from increased hydrostatic pressure. [8] and [9] Another commonly reported definition of modified transudate is a 'long standing transudate'.10 While the latter definition does not describe a pathophysiological mechanism of fluid formation, but instead an 'in vivo ageing sample artifact', Perman's definition, also called obstructive effusion,11 gives information on the pathophysiological mechanism of the fluid formation. A survey of the literature neither reveals published evidence to support this classification nor studies showing how the cut-off values for the markers conventionally used to classify PEs were derived. Studies assessing the sensitivity and specificity of these markers in classifying PEs correctly are also lacking. The large and variable number of disorders associated with modified transudates and the fact that this category has overlapping protein content and cellularity with transudates and exudates, limit the current veterinary classification scheme of PEs.The aim of this study was to determine whether the simultaneous evaluation of pleural fluid and serum protein, LDH and other biochemical parameters is useful in the classification of feline PE based on the pathophysiological mechanism of formation, as already demonstrated in human literature. [2] , [3] , 4 S.L. Valdes, A. Pose and J. Suarez, et al. Cholesterol: a useful parameters for distinguishing between pleural exudates and transudates. Chest, 99 (1991), pp. 1097'1102. [4] and [5]