Determination of lipoprotein(a): evaluation of three methods

Lipoprotein(a) (Lp(a)) is a strong independent risk factor for premature atherosclerosis. Structurally, Lp(a) closely resembles LDL. Its protein moiety contains apolipoprotein B-100 and apolipoprotein(a). We evaluated two commercial enzyme immunoassays (EIAs) and an immunoradiometric assay (IRMA) for Lp(a). The three assays differed in their design and they used different antibodies. In the immunoradiometric assay, two different monoclonal antibodies were used. In the first EIA, monoclonal anti-apolipoprotein(a) was bound to the solid phase and Lp(a) was detected with polyclonal anti-apolipoprotein B (Lp(a):B-EIA). In the second EIA, polyclonal anti-apolipoprotein(a) was used as capturing antibody and as detecting antibody (apo(a)-EIA). Ninety three plasma samples were assayed for Lp(a) with the three methods. The best correlation was obtained between the IRMA and the Lp(a):B-EIA (r = 0.971). Correlations between the apo(a)-EIA on the one hand and the IRMA or the Lp(a):B-EIA on the other hand were 0.889 and 0.836, respectively. The methods significantly differed in their calibration. This resulted in different mean Lp(a) concentrations. When tested against purified Lp(a), the apo(a)-EIA appeared accurately calibrated, whereas the IRMA and the Lp(a):B-EIA overestimated Lp(a) by approximately twofold. In the Lp(a):B-EIA, the detecting antibody is directed against apolipoprotein B. The Lp(a):B-EIA is, therefore, not affected by apolipoprotein(a) size polymorphism. This allows expression of the concentration of Lp(a):B complexes on a molar basis. In contrast, the polyclonal antibody-based apo(a)-EIA measures the concentration of apolipoprotein(a) antigen, and may, therefore, be susceptible to inter- and intra-individual polydispersity of apolipoprotein(a) and Lp(a) particles.(ABSTRACT TRUNCATED AT 250 WORDS)