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Pleural effusion is broken down into two categories, transudates and exudates. A transudate is a clear fluid that forms not because the pleural surfaces are diseased, but because of an imbalance between the normal production and removal of the fluid. The most common cause of transudative fluid is congestive heart failure. An exudate, which is often cloudy and contains many cells and proteins, results from disease of the pleura itself, and is common to mesothelioma. To determine whether a fluid is a transudate or exudate, a diagnostic thoracentesis, in which a needle or catheter is used to obtain a fluid sample, may be conducted.