Determination of high density lipoprotein-cholesterol in stored human plasma

This study was performed to determine the effect of frozen and unfrozen storage on plasma high density lipoprotein (HDL) cholesterol analysis. HDL-cholesterol was determined, following removal of the other lipoproteins by precipitation with heparin and MnCl ₂, in fresh plasma samples and in aliquots of the samples that had been stored 1) unfrozen (4°C), up to 14 days, 2) frozen (-20°C), up to 14 days, and 3) frozen (-20°C), 5-6 years. There were progressive changes in measured HDL-cholesterol values under all conditions of storage. The changes were significantly and most highly correlated with HDL-cholesterol level and they reached about 4 mg/dl for each 10 mg/dl initial HDL-cholesterol concentration. Samples with low HDL-cholesterol concentrations tended to increase with storage, and those with high HDL-cholesterol levels tended to decrease. Since the changes were in opposite directions, the group means tended to be preserved. The mean differences between fresh and stored samples were (stored-fresh): 1-14 days, 4°C, -0.10 to +4.60 mg/dl; 1 hour-14 days, -20°C, +0.80 to 1.92 mg/dl; 5-6 years -20°C, -1.92 mg/dl. Values changed most rapidly during storage at 4°C and were accompanied by changes in the precipitability of the lipoproteins; low density lipoproteins became progressively difficult to precipitate completely, and HDL became progressively easier to precipitate. The measured HDL-cholesterol level depends on which process predominates and can lead to grossly inaccurate measurements, particularly at the extremes of HDL-cholesterol concentration.