Lactate dehydrogenase activities in serum and plasma.

Abstract

We investigated lactate dehydrogenase activity in serum and plasma because of the conflicting data found in the literature. We assayed serum, platelet-rich, and platelet-poor plasma by two colorimetric endpoint methods and by an ultraviolet kinetic procedure. Platelet-poor plasma and serum had essentially the same activities by all three methods, whereas the activity in platelet-rich plasma plasma averaged fourfold that in platelet-poor plasma or serum when the assay was performed under conditions that result in lysis of platelets and release of their lactate dehydrogenase. When measurements were performed in platelet-rich plasma under conditions that prevented lysis of platelets, all three types of specimens gave the same results. This occurred when the osmolality of the reaction mixture was about 240 mOsm/kg of water. At an osmolality of about 120 mOsm, the activity of platelet-rich plasma was substantially lower than that of platelet-poor plasma or serum. If plasma must be used, the sample must first be centrifuged (3000 X g, 15 min) to provide a platelet-free plasma.