Pleural fluid to serum cholinesterase ratio for the separation of transudates and exudates

Study objective: To evaluate the usefulness of two new parameters for separating pleural transudates and exudates: pleural fluid cholinesterase level and pleural fluid to serum cholinesterase ratio, and to compare the results with the other well-established criteria. Design: Prospective evaluation of the patients referred for diagnostic thoracentesis. Setting: Pulmonary sections of a community hospital and a university hospital. Patients: One hundred ninety-three consecutive patients. Forty were excluded for different reasons. Measurements: The following criteria for separating the pleural effusions in transudates and exudates were analyzed: Light's criteria, the pleural fluid cholesterol level, the pleural fluid to serum cholesterol ratio, the pleural fluid cholinesterase level, and the pleural fluid to serum cholinesterase ratio. Results: One hundred fifty-three patients had conditions diagnosed. Thirty-five were classified as having transudates and 118 as exudates. The percentage of effusions misclassified by each parameter was as follows: Light's criteria, 7.8%; pleural fluid cholesterol, 7.8%; pleural fluid to serum cholesterol ratio, 6.5%; pleural fluid cholinesterase, 8.5%; and pleural fluid to serum cholinesterase ratio, 1.3%. Conclusions: The pleural fluid to serum cholinesterase ratio is the most accurate criterion for separating pleural transudates and exudates. If further studies confirm our results, the cholinesterase ratio could be used as the first step in the diagnosis of pleural effusions.