Effect of sample storage on the measurement of lipoprotein[a], apolipoproteins B and A-IV, total and high density lipoprotein cholesterol and triglycerides.

This study investigated the influence of long-term storage, for periods up to 24 months, and multiple freezing and thawing on the measured values of lipoprotein[a] (Lp[a]), apolipoproteins B and A-IV, total and high density lipoprotein (HDL) cholesterol and triglycerides using plasma samples stored at -80 degrees C, -20 degrees C, and 4 degrees C. Samples stored at -80 degrees C or -20 degrees C showed significant changes in Lp[a] after 24 months, with a mean decrease of 7% and 13%, respectively (P < 0.01). The major part of the decrease occurred during the first freezing and thawing. In contrast, apolipoproteins B and A-IV decreased continuously over time (P < 0.05). The increase in plasma concentrations of total and HDL cholesterol and triglycerides was small but significant because of its uniformity. Multiple freezing and thawing influenced only the measured values of Lp[a] and apolipoprotein B. Comparison of samples stored at -80 degrees C and -20 degrees C showed no difference in any of the parameters at any time with the exception of Lp[a] after 18 and 24 months (P < 0.05). After a storage period of 24 months, immunoblotting with detection of apo[a] was possible from samples under each storage condition. ApoB and apoA-IV were detectable only in samples stored at -20 degrees C or -80 degrees C. These data, when compared to recent studies, suggest a critical role of the assay methodology in the reproducibility of measured Lp[a] and apolipoprotein plasma concentrations. We therefore recommend the examination of each system for measurement of long-term stored plasma samples.