Interconverting measurements of human lactate dehydrogenase activities in three buffers

The lactate-to-pyruvate reaction for serum lactate dehydrogenase (LD) is most frequently assayed in one of three buffers, pyrophosphate (PPi), tris(hydroxymethyl)amino-methane (Tris), or 2-amino-2-methyl-1-propanol (AMP). We described interconverting results for serum samples and for highly purified LD isoenzymes I (dissolved in one of these matrixes) assayed in these buffers under optimized reaction conditions. The equation for converting results obtained for sera in Tris (x) to those in PPi(y) (both at 30 degrees C) is y = 0.74x+10 (n = 98). Since AMP is used extensively in Technicon procedures, we determined the LD activity of sera with an SMA 12/60, at 37 degrees C. The equation for convering these AMP results to results obtained in PPi at 30 degrees C is y = 0.45x-16 (n = 90). Very different equations were obtained with highly purified LD isoenzyme I maintained in two different matrixes and with both isoenzymes assayed in the same matrix. The matrix in which LD is dissolved and the proportion of various LD isoenzymes affect the magnitude of difference in observed LD activity under various conditions. Therefore, in clinical laboratories that use more than one analytical method or when conversion equations are used in the comparison of interlaboratory results, it is important to define the LD source, isoenzyme content, and the matrix, as well as the reaction conditions, and to use many samples with a wide range of activities when determining the conversion equations. For any changes in reagent source, substrate concentration, or alteration in procedure, a new normal range and new conversion equations should be determined.