Effect of sample storage on the assay of lipoprotein(a) by commercially available radial immunodiffusion and enzyme-linked immunosorbent assay kits

Abstract

Lipoprotein(a) [Lp(a)] was measured by both a radial immunodiffusion (RID) kit from Immuno AG (Zurich, Switzerland) and a Tint Elize enzyme-linked immunosorbent assay (ELISA) kit from CytRx Biopool Ltd. (Umeå, Sweden) in serum samples that had been stored at -20 and -70 °C for six months. Storage temperature had no significant effect on the Lp(a) concentrations obtained by either method. After six months, mean Lp(a) degradation was 46% (95% confidence interval, 34-58%) with the RID kit; the ELISA data could not be compared between time points. In fresh sera, Lp(a) concentrations obtained by RID were 41% higher than by ELISA (because of differences in assay calibration materials), but in paired measurements of a set of 215 samples stored at -40 °C for an average of 10 years, Lp(a) concentrations were 62% lower by RID. This suggests that RID is more sensitive to the effects of long-term storage than is ELISA.