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Background: An important part of the investigation of pleural effusion is the identification of markers that help separate exudate from transudate. Objectives: The purposes of this study were to compare the accuracy of published and new sets of criteria to distinguish between exudative and transudative pleural effusions, and to determine whether serum biochemical analysis is necessary. Methods: An externally validated cohort study was performed. Pleural effusions were determined to be transudative or exudative on the basis of an assessment of the medical record by two clinicians blinded to biochemical results. Sensitivity, specificity, positive likelihood ratio, negative likelihood ratio, and area under the receiver operating characteristic curve were determined for each proposed combination of criteria. Results: Pleural fluid analysis was available for 311 thoracenteses in the main cohort and for 112 thoracenteses in the validation cohort. The best sensitivity (97% [95% CI 94-99]) and negative likelihood ratio (0.04 [95% CI 0.02-0.08]) for identifying exudative effusions were observed with criteria combining pleural fluid lactate dehydrogenase greater than 0.6 the upper limit of normal serum lactate dehydrogenase and pleural fluid cholesterol greater than 1.04 mmol/L (40 mg/dL). The overall diagnostic accuracy was similar to Light's criteria. Findings were similar in the validation cohort. Conclusions: Our proposed criteria using simultaneously pleural fluid lactate dehydrogenase and pleural fluid cholesterol can identify an exudate with a sensitivity and an overall diagnostic accuracy similar to Light's criteria. It avoids simultaneous blood sampling, thus reducing patient discomfort and potential costs.