Purpose of review: The separation of pleural transudates from exudates, as the first step in the study of pleural effusions of unknown cause, is generally accepted as a useful practice. However, the optimal way to do this remains moot.
Recent findings: New and more sophisticated biochemical markers have been proposed together, with new approaches to the interpretation of the results. Nevertheless, new studies have consolidated the criteria of Light et al. as those with a better accuracy. Effective diuresis increases the concentration of most pleural biochemical parameters used to differentiate transudates from exudates and appears as the main cause of the failures of this dichotomic approach. Among the alternative criteria proposed for identifying transudates in the setting of diuresis, the total protein gradient between serum and pleural fluid seems to be the most cost effective.
Summary: Together with clinical judgment, the use of biochemical criteria seems mandatory. The criteria of Light et al. remain those of election. In the setting of effective diuresis, the use of the protein gradient is recommended. Although new and more sophisticated markers have been tested, it seems that looking for the causes of misclassification, when applying the criteria that to date have shown better efficiency, deserves preferential investigation.