Stability of lactate dehydrogenase at different storage temperatures

The effect of storing human serum, cord blood serum or heparinized plasma at 25°C, 4°C & -20°C on the activity and isoenzyme distribution of lactate dehydrogenase (LD) was studied. Cellulose acetate and agarose electrophoresis, as well as an immunochemical inhibition technique, were used for isoenzyme quantification. In contrast to previous reports, cryo-instability was found only in specimens stored at 4°C. Serum specimens stored at 25°C and -20°C retained 74% and 87% of total activity after 45 days of storage. LD-1 was stable at all three temperatures, with a maximum loss of 10%. LD-2, LD-3, LD-4, & LD-5 were most labile at 4°C. Specimens that are to be analyzed for total LD or LD isoenzymes should be stored frozen or, if necessary, at room temperature, but not in a refrigerator. Thus, separate storage of specimens for cardiac isoenzymes (LD & creatine kinase) is not necessary. This may eliminate a possible source of falsely elevated LD-1/LD-2 ratios, as well as reducing the labor factor and the corresponding cost of cardiac isoenzyme determinations.