Enzyme-linked immunoassay for Lp[a].

Based on our findings that rabbit antisera raised against human Lp(a) or apo(a) have the potential to cross-react with plasminogen, and in some cases have nearly equal affinities for plasminogen and Lp(a), we have developed an assay for plasma Lp(a) based on a "sandwich" ELISA that is insensitive to the presence of plasminogen. This was accomplished through the use of anti-apo(a) as a capture antibody and quantitation of the bound Lp(a), Le., the apoB-lOO-apo(a) complex, with an anti-apoB anti- body Although apo(a) is heterogeneous in size, all Lp(a) particles tested, either in pure form or contained in whole plasma, gave parallel dose-response curves and were immunologicaily equiva- lent. However, when purified Lp(a) particles with different apo(a) isoforms were studied, those having larger isoforms were, on a weight basis, less reactive than those having a smaller size. Nearly equivalent reactivity was observed when protein concentration was expressed on a molar basis. The distribution of Lp(a) in a population of 84 subjects was skewed with one-third of the indi- viduals having less than 1 mg/dl Lp(a) protein. All subjects tested had measurable concentrations of Lp(a) with a lower limit of de- tection of 0.030 mg/dl Lp(a) protein. The mean level was 3.2 mg/d with a range of 0.045 to 13.3 mg/dl. I These studies demon- strate the successful development of an ELISA for Lp(a) protein that is insensitive to the presence of plasminogen; that hetero- geneity of Lp(a) and apo(a) are an important source of variation in the assay; and the need for an appropriate Lp(a) standard in order to minimize this variation.- Fless, G. M., M. L. Snyder, and A. M. Scanu. Enzyme-linked immunoassay for Lp(a). J Lipid Res. 1989. 30: 651-662.