Effect of storage at -70 degrees C on lipid, lipoprotein and apolipoprotein concentrations

We have investigated the effects on lipid, apolipoprotein and lipoprotein measurements of storing unfractionated serum from normolipidaemic and hyperlipidaemic subjects at -70 degrees C for 10 days, 3 months and 6 months. Total serum concentrations of lipids and apolipoproteins were stable except for triglyceride concentrations. These increased on storage although the change was < 2.0% after 6 months. Storage of serum before sequential flotation ultracentrifugation resulted in decreased free cholesterol and phospholipid concentrations in very low density lipoproteins. In low density lipoproteins, free cholesterol concentrations increased and protein concentrations decreased on storage, while esterified cholesterol and phospholipid concentrations fell after 10 days but did not differ from baseline concentrations after storage for 6 months. Within high density lipoproteins, there were decreases in triglyceride and protein concentrations. Although storage of serum at -70 degrees C for up to 6 months did not result in extensive changes in most lipoprotein fractions, separation of lipoprotein fractions from serum should, ideally, be performed as soon as possible after collection.