Diagnostic Performance of the Cancer Ratio in Malignant Pleural Effusion in Combination with the Light’s Criteria

The cancer ratio (CR), defined as serum lactate dehydrogenase to pleural fluid adenosine deaminase ratio (sLDH/pADA), has emerged as a diagnostic marker for malignant pleural effusion (MPE). However, previous studies have primarily compared MPE with exudative effusions, excluding transudates. The aim of this study was to determine the diagnostic accuracy of CR in differentiating MPE from benign pleural effusions (BPE), including both exudates and transudates. An observational analysis was conducted on patients with pleural effusion (PE) evaluated in a Portuguese center between January 2017 and August 2025. Diagnostic accuracy was assessed using receiver operating characteristic curves and area under the curve (AUC) analysis. Of the 423 PE evaluated, 154 (36.4%) were MPE and 269 (63.6%) were BPE, including 82 transudates, 159 true benign exudates (TBEPE) and 28 PE misclassified as exudates. Significantly higher CR levels were observed in MPE than in BPE (p < 0.001). When comparing MPE with TBEPE, a CR threshold of 20 yielded an AUC of 0.90, with 76.0% sensitivity and 93.1% specificity. The inclusion of misclassified exudative BPE and transudates reduced diagnostic accuracy (AUC 0.86/0.73, respectively). In conclusion, a CR > 20 is highly predictive of malignancy etiology. Accurate pre-classification according to the Light's criteria is essential to preserve the diagnostic validity of this biomarker.