Modification of the dextran-Mg²⁺ high-density lipoprotein cholesterol precipitation method for use with previously frozen plasma

Although dextran-Mg²⁺ precipitation produces accurate and precise results for high-density lipoprotein (HDL) cholesterol in fresh plasma and serum, precipitation of frozen specimens with triglycerides >2.26 mmol/L (>200 mg/dL) is difficult. We developed a modification that dilutes thawed samples by 35% and increases dextran-Mg²⁺ reagent to 15% of sample volume. Standard precipitations were performed on 62 fresh EDTA-treated plasma specimens; supernatant solutions were analyzed fresh and after freezing. Standard and modified methods were also performed on thawed, paired plasmas. In specimens with triglycerides ≤2.26 mmol/L, HDL cholesterol results for all methods were similar. For triglycerides >2.26 mmol/L, however, bias and precision were significantly affected by freezing, and 38.5% of samples with standard precipitation required additional procedures to produce clear supernatant solutions. HDL cholesterol concentrations for thawed samples with standard precipitation were significantly greater than for fresh samples (P <0.02), but those for the modified method were not different from fresh samples, and only one specimen required additional steps to produce a clear supernate.