To the Editor.
—In the July issue of the Archives, Brown et al1 describe a patient with hypothyroidism and bilateral pleural effusions. However, their statement that the pleural fluid lactic acid dehydrogenase (LDH) level was normal is curious.There is no such thing as a "normal" or "abnormal" pleural fluid LDH level. Pleural fluid LDH is useful in distinguishing exudates from transudates. A pleural fluid/serum LDH ratio greater than 0.6 suggests an exudate, whereas the ratio is usually less than 0.6 in a transudate.2 Similarly, a pleural fluid LDH level greater than 67% of the upper limit of the normal level for serum LDH suggests an exudate. An LDH level greater than 1,000 IU/L in a parapneumonic effusion suggests that the effusion is infected.3