Determination of lipoprotein(a): enzyme immunoassay and immunoradiometric assay compared

Lipoprotein(a) (Lp(a)) concentration in plasma is a strong independent risk factor for pre-mature atherosclerosis. Lp(a) closely resembles LDL. Its protein moiety contains apolipoprotein (apo) B-100 and apo(a). Two enzyme immunoassays (EIAs) for Lp(a) have been developed. In both, polyclonal antibodies for apo(a) are used as capturing antibodies. In the first, Lp(a) is detected with anti-apo(a) (apo(a)-EIA). In the second, detection is carried out with anti-apo B (Lp(a):B-EIA). Neither plasminogen nor LDL cross-reacted in the assays. Lp(a) was also measured using a commercial sandwich immunoradiometric assay (IRMA). This assay uses two monoclonal antibodies for apo(a). One of them, the solid phase antibody, cross-reacted with plasminogen. However, at physiological plasminogen concentrations there was no competition for solid phase binding sites. A quantity of plasma samples (201) were assayed for Lp(a) with the three methods. The best correlation was obtained between the IRMA and the Lp(a):B-EIA (r = 0.909). Correlations between the apo(a)-EIA and the IRMA or the Lp(a):B-EIA were 0.763 and 0.695, respectively. As compared to the EIAs, the IRMA overestimated Lp(a) by about 30%. It is concluded that both the Lp(a):B-EIA and the IRMA reflect the concentration of Lp(a) particles in plasma. In contrast, the apo(a)-EIA measures apo(a) antigen and may therefore be susceptible to the size polymorphism of apo(a).