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The objective of present study was to evaluate the efficacy of pleural fluid alkaline phosphatase and its ratio to serum levels to classify pleural fluids. A total of 80 patients were divided in transudates and exudates on the basis of extensive clinical, radiological and biochemical evaluation. The efficacy of pleural fluid alkaline phosphatase (P ALP) and pleural fluid / serum alkaline phosphatase ratio (P/S ALP) assessment along with that of Light′s criteria to accurately classify transudates and exudates were analyzed. Up to 89% transudates misclassified by Light′s criteria were correctly classified by pleural fluid alkaline phosphatase (P ALP) and pleural fluid/serum alkaline phosphatase ratio (P/S ALP) evaluation. Similarly 92% exudates misclassified by Light′s criteria were correctly classified by pleural fluid alkaline phosphatase (P ALP) and pleural fluid/serum alkaline phosphatase ratio (P/S ALP) evaluation. By applying a cut off value of 40.0 IU for P ALP, a sensitivity of 85% and specificity of 75% was found. For P/S ALP, applying a cut off value of 0.25 a sensitivity of 85% and specificity of 80% was found. Both P ALP and P/S ALP had a PPV of 92%. However, their respective NPV were 63% and 70%.