Distinguishing Pleural Effusions in Cats

Pleural effusion has traditionally been classified as a transudate or exudate. In veterinary medicine, the terms modified transudate or long-standing transudate have been used when it is difficult to distinguish between a transudate and exudate. Commonly used diagnostic tests to differentiate the 2 include specific gravity, protein content, and cellularity. In this prospective study, 20 cats with pleural effusion were evaluated. Nine cats had congestive heart failure, 5 had neoplasia, 3 had chylothorax, 2 had pyothorax, and 1 had feline infectious peritonitis. Paired samples of serum and pleural fluid were evaluated using the following 9 tests: pleural fluid lactate dehydrogenase concentration (LDHp), pleural fluid/LDH ratio, pleural fluid/serum total protein ration (TPr), pleural fluid total protein, pleural fluid cholesterol concentration, pleural fluid/serum cholesterol ration (CHOLr), serum-effusion cholesterol gradient (serum cholesterol minus pleural effusion cholesterol concentration [CHOLg]), pleural effusion total nucleated cell count (TNCCp), and pleural effusion glucose. Nine of the samples were classified as effusions and 11 as exudates. When the traditional veterinary classification scheme was used, 12 of 20 exudates were misclassified. LDHp and TPr were the most reliable aids to distinguish between an exudate and transudate. If further clarification was needed or if there was a mismatch between the results and clinical picture, calculation of the CHOLr, CHOLg, and TNCCp was recommended.

COMMENTARY: Pleural effusion can be difficult to characterize because testing requires manipulation of a potentially unstable patient. Thoracocentesis not only is a valuable diagnostic test but also may be an effective therapy. Therefore, any tests that can be performed on effusion fluid to classify it are preferable to other tests that may cause additional stress to the patient. This report demonstrates the limitations of the traditional classification system for effusion, showing that the overlap between a transudate, modified transudate, and exudate may lead to failure to classify or misclassification. The use of the TPr and LDHp may be a more accurate predictor of the type of effusion present, and these tests may be performed in-house. The effusion may be further characterized from traditional tests, such as TNCCp, and other tests, such as CHOLr and CHOLg. Further studies of larger numbers of feline patients are necessary to confirm these findings.—Jennifer Ginn, DVM, Diplomate ACVIM

A new approach to pleural effusion in cats: Markers for distinguishing transudates from exudates. Zoia A, Slater LA, Heller J, et al. J FELINE MED SURG 11:847-855, 2009.