The criteria established by Light et al in 1972 have been used widely for the differential diagnosis of the pleural effusions in transudates and exsudates. However, in recent years, several reports have agreed that these criteria misclassified an important number of effusions. For this reason, different parameters have been proposed for differentiation the transudates from exudates. Nevertheless, all these alternative parameters have not been better than the past criteria of Light et al. In response the usefulness of two parameters for differentiation pleural transudate from exudates were evaluated : pleural fluid cholinesterase level and pleural fluid to serum cholinesterase ratio.

A total of forty-three patient with know causes of the pleura effusion by diagnostic thoracentesis were studied. The following criteria for differentiating the pleural effusions in transudates and exsudates 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.

The conditions of forty-three patients were diagnosed. Ten were classified as having transudates and thirty-three as exudates. The percentage of effusions misclassified by each parameter was as follows : Light's criteria, 9.3% ; pleural fluid cholesterol, 2.3% ; pleural fluid to serum cholesterol ratio, 2.3% ; pleural fluid cholinesterase, 4.7% ; and pleural fluid to serum cholinesterase ratio, 2.3%.

The pleural fluid to serum cholinesterase ratio is one of the accurate criteria for differentiating pleural transudates from exudates. If further studies confirm these results, the cholinesterase ratio could be used as the first step in the evaluation of pleural effusion and if evaluated together with the other criteria, the differentiation of pleural transudate from exsudates will become more accurate.