Freeze-thaw and matrix effects in direct high-density lipoprotein cholesterol methods

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BackgroundThere is frequent discussion on the susceptibility of direct high-density lipoprotein cholesterol (HDL-C) methods to matrix effects. In Vitro Diagnostics manufacturers recognize this issue and regularly improve their HDL-C reagent formulations in subsequent generations.MethodsThe 3rd generation direct HDL-C assay from Roche was investigated for matrix effects in comparison to the former generation, a Beckman direct HDL-C method and a conventional phosphotungstate (PTA)/Mg2+ precipitation method. In addition, 235 heparin plasma samples were measured freshly and after a freeze-thaw cycle with the Roche 2nd and 3rd generation direct HDL-C. Biases, outliers, and intra-class correlation coefficients (ICCs) were calculated for both experiments. Multivariate analysis was used to investigate interference by matrix components.ResultsIn fresh samples, Roche 2nd and 3rd generation HDL-C methods averaged +0.15 mmol/L (95% CI: 0.13–0.16) and +0.08 mmol/L (95% CI: 0.07–0.09) higher compared to frozen samples. In frozen ali-quots, ICCs for Roche 2nd and 3rd generation and Beckman direct HDL-C as compared to PTA/MgCl2 were 0.963, 0.966, and 0.924, respectively. Predictors of outliers (defined as having an absolute difference >0.21 mmol/L) in comparisons of direct methods to the PTA/MgCl2 precipitation method were high tri-glyceride and low albumin levels.ConclusionsThe 3rd generation direct HDL-C from Roche has become insensitive to most matrix effects, bringing along more accurate results in hypoalbumi-nemic and hypertriglyceridemic samples. Surprisingly, Roche direct assays produced significantly higher HDL-C levels in fresh samples compared to frozen plasma samples. If confirmed by others, the latter finding has implications for patient management and necessitates further reagent optimization.

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