CLINICAL QUESTION: How common are discordant pleural fluid exudates, and do they have different diagnostic patterns compared to concordant pleural fluid exudates?
BACKGROUND: Pleural effusions are classified as an exudate if at least one of Light’s Criteria are met: (1) pleural fluid protein: serum protein ratio greater than 0.5; (2) pleural fluid lactate dehydrogenase (LDH): serum LDH ratio greater than 0.6; or (3) pleural fluid LDH is two-thirds the normal serum LDH, per reference range for the laboratory. Previous studies suggest that up to 25% of transudates are misclassified as exudates when using Light’s Criteria. Misclassifying a pleural effusion can delay diagnostic and treatment decisions. A discordant exudative pleural effusion is either when the LDH is elevated or the protein, but not both. The diagnostic patterns of discordant pleural effusions are not known and may represent an opportunity to improve diagnostic accuracy.
STUDY DESIGN: A retrospective cohort study
SETTING: Oxford University Hospitals (U.K.)
SYNOPSIS: 715 exudative pleural fluid samples from Oxford University Hospitals (a tertiary center) from 2015 to 2017 were studied, after excluding subsequent samples from the same patient. Exudative samples were defined as discordant (low protein and high LDH or vice versa) and concordant (high protein and high LDH). A multidisciplinary pleural team used medical records to determine the final diagnosis and cause of each pleural exudate. Chi-squared analysis was used, with adjustments for age and sex.
Discordance occurred in 32% of the exudates analyzed. Discordant exudates had a significantly higher likelihood of diagnosis of fluid overload (10% versus 2%, P <.0001), benign asbestos-related pleural effusion (14% versus 9%, P=.031), and intensive-care-unit–associated effusion (9% versus 3%, P=.001) compared to concordant effusions. Pleural infection (6% versus 16%; P <.0001) and malignant pleural effusion (34% versus 42%; P =.025) were less frequent in discordant than in concordant exudates. Limitations of this study include the retrospective, single-center format and a study population with a potentially higher incidence of asbestos-related effusions than the general population.
BOTTOM LINE: Discordant pleural exudates are common and have a distinct diagnostic pattern that is more likely to represent a fluid overload state or a benign asbestos-related effusion, and less likely to be a pleural infection or malignant effusion when compared to concordant pleural exudates.
CITATION: Addala DN, et al. Incidence of discordant pleural fluid exudates and diagnostic patterns: a retrospective cohort study. Chest. 2025;168(6):1517-1527. doi: 10.1016/j. chest.2025.05.048.
Dr. Shahan
Dr. Shahan is a hospitalist at the Nebraska Medical Center and an associate professor in the department of internal medicine at the University of Nebraska Medical Center, both in Omaha, Neb.