[Significance of lactate dehydrogenases in pleural effusions]

In order to determine the exact significance and origin of lactate dehydrogenases (LDH) present in pleural effusions, LDH and their enzymes were assayed in the serum and pleural fluid of 60 patients with wet pleurisy and in the main blood cells (haematocytes, polymorphonuclears mononuclears) found in pleural fluids. Our results showed that: a pleural fluid/serum LDH ratio above 0.7 indicates that the fluid is an exudate; however, in about 10% of inflammatory pleural effusions due to infection or cancer this ratio is comprised between 0.5 and 0.7; the enzyme profile of transudates only differs from that of normal serum by a slight increase in isoLDH 4 and 5; in exudates this profile is the reverse of the normal serum profile, with a decrease in isoLDH 1 and 2 and an increase in isoLDH 4 and 5; the enzyme profile provides little information on the origin of exudates, although a more than 30% rise in isoLDH 2 is in favour of a malignancy (mesothelioma excluded); polymorphonuclears contain more isoLDH 4 and 5 than mononuclears; activation of mononucleate cells is attended by a significant increase in intracellular isoLDH 4 and 5; the presence of red blood cells and/or haemolysis in an exudate cannot account for its high isoLDH 4 and isoLDH 5 content; the high content seems to be due, at least partly, to release of these enzymes by the polymorphonuclears and/or mononucleate cells involved in pleural inflammation.