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 degrees C), up to 14 days, 2) frozen (-20 degrees C), up to 14 days, and 3) frozen (-20 degrees 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 degrees C, -0.10 to +4.60 mg/dl; 1 hour-14 days, -20 degrees C, +0.80 to +1.92 mg/dl; 5-6 years -20 degrees C, -1.92 mg/dl. Values changed most rapidly during storage at 4 degrees 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.-Bachorik, P. S., R. Walker, K. D. Brownell, A. J. Stunkard, and P. O. Kwiterovich. Determination of high density lipoprotein-cholesterol in stored human plasma.