Concordant and Discordant Exudates and Their Effect on the Accuracy of Light's Criteria to Diagnose Exudative Pleural Effusions.

INTRODUCTION: To describe the incidence of discordant exudate (DE) effusions, their underlying etiologies and their effect on the accuracy of the Light's criteria to diagnose exudate effusions.

METHODS: A retrospective review of pleural fluid analysis (PFA) from a cohort of patients with pleural effusion (PE) who underwent thoracentesis. PEs were defined as exudative based on the Light's criteria. The effusions were further classified in concordant or DE.

RESULTS: From 847 PE samples, 611 (72.1%) were diagnosed as an exudate and 236 (27.9%) as a transudate. In 10.3% of cases (n = 87), there was discordancy between the final pleural fluid diagnosis and the PFA defined by Light's criteria. 281 (33.2%) of the 632 effusions classified as an exudate by Light's criteria were DE (52 transudates and 229 exudates). 182 (65%) of the 281 DE were found to be protein discordant (37 transudates and 145 exudates), and 99 (35.2%) were lactate dehydrogenase discordant (15 transudates and 84 exudates). The positive predictive value and positive likelihood ratio of Light's criteria for the diagnosis of an exudate effusion decreased from 99.4% and 67.4%, respectively, when the exudates were concordant to 81.5% and 1.7, respectively, if they were discordant.

CONCLUSIONS: In a significant percentage of patients, there is discordancy between the results of the PFA and the final clinical diagnosis. DE decreased the accuracy of Light's criteria to identify exudate PE, increasing the risk of misclassifying a transudate as an exudate. Concordant exudates almost universally established the presence of an exudative PE.