Determination of high density lipoprotein-cholesterol in human plasma stored at -70°C

We determined the effect of storage at -70°C on the determination of plasma high density lipoprotein (HDL)-cholesterol. Plasma from 106 subjects was stored for 1, 6, and 12 months, then treated with heparin and MnCl₂ to remove other lipoproteins, and HDL-cholesterol was measured. The mean HDL-cholesterol level decreased by 2.9% after 1 month, and by 5.1% after 12 months. The magnitude and direction of the changes were not constant, but were correlated primarily with HDL-cholesterol concentration. After 1 month, samples with HDL-cholesterol levels below 40 mg/dl tended to increase, and those above this value tended to decrease. By 12 months, only those samples with HDL-cholesterol below 22 mg/dl tended to increase. Linear regression analysis indicated changes of 0.9-1.5 mg/dl for each 10 mg/dl initial HDL-cholesterol concentration. Storage of heparin-MnCl₂ supernatants, rather than unfractionated plasma, minimized these changes. The mean HDL-cholesterol of stored heparin-MnCl₂ supernatants was 3.3% lower after 12 months, and the change was constant regardless of lipoprotein concentration. The findings suggest the possible occurrence of changes in the heparin-MnCl₂ precipitability of lipoproteins during storage, which produce errors in HDL-cholesterol analyses, and indicate that samples can be stored more satisfactory if the other lipoproteins are removed first.