Is the pleural fluid transudate or exudate? A revisit of the diagnostic criteria.

BACKGROUND Pleural effusions are classified into transudates and exudates based on criteria developed in the 1970s. However, their accuracy has not been evaluated. We compared the performance of the pleural fluid absolute lactic dehydrogenase level (FLDH), fluid to serum ratio of LDH (LDHR), and fluid to serum ratio of total protein (TPR). TPR has been used instead of the absolute value of fluid protein based on the observation that fluid protein is influenced by changes in the serum protein concentration. However, the rationale for using LDHR remains unexplored. METHODS Of 212 consecutive patients with pleural effusions, four with multiple causes and eight with an uncertain diagnosis were excluded. ROC curves were generated using sensitivity and 1-specificity values for TPR, FLDH, and LDHR and positive likelihood ratios (LR+ve) were computed using the optimum cut off values. The correlation between pleural fluid and serum concentrations of total protein and LDH was also estimated. RESULTS Of 200 effusions studied, 156 were exudates and 44 were transudates. The optimum cut off levels were: FLDH 163 IU/l, TPR 0.5, LDHR 0.6, and the FLDH-TPR combination 163 and 0.4, respectively. The area under the curve (AUC) with 95% confidence interval (CI) was: 0.89 (0.86 to 0.96) for FLDH, 0.86 (0.80 to 0.91) for TPR, 0.82 (0.77 to 0.89) for LDHR, and 0.90 (0.86 to 95) for FLDH-TPR. A significant correlation was observed between serum and pleural fluid protein levels in transudates and exudates (r=0.5 and 0.6, respectively), but the correlation between serum and pleural fluid LDH levels was insignificant. CONCLUSION FLDH is the most accurate marker for the diagnostic separation of transudates and exudates and LDHR has no role in this process. Combining TPR with FLDH appears to improve the diagnostic accuracy slightly.