Transudate or exudate: can lactate dehydrogenase activity in canine and feline effusions help to differentiate between the 2?

Background: Lactate dehydrogenase (LDH) activity is often measured in human effusions to help in differentiating between transudates and exudates. Few studies have been performed using effusion samples from animals. Objectives: The purpose of the study was to determine whether LDH can be used to differentiate between transudative and exudative effusions in dogs and cats (including postmortem samples), and whether there is a difference between different laboratory methods of LDH measurement. Methods: Lactate dehydrogenase activity was measured in canine and feline effusions that were submitted to the Murdoch University Veterinary Hospital Clinical Pathology Laboratory over approximately 12 months using 2 wet and one dry chemistry methods, including 10 effusions collected postmortem. Results were compared to classification using traditional methods for effusion types. Results: Lactate dehydrogenase activity was significantly higher in exudates than in transudates, significantly different depending on the method of measurement, and significantly higher in all effusions collected postmortem. An LDH effusion:serum ratio of < 0.5 was associated with transudates. There was no significant difference between samples collected into EDTA or plain serum tubes, in frozen and thawed samples, or after storage at 4°C for 3-7 days. Conclusions: Measurement of LDH activity may be useful in helping to differentiate between transudates and exudates in cats and dogs. The method of measurement must be known and kept consistent if cutoff values are to be used. The LDH activity was increased in all effusions collected from animals after death, potentially invalidating its use postmortem.